attrs: Classes Without Boilerplate

Release v22.1.0 (What's new?).

attrs is the Python package that will bring back the joy of writing classes by relieving you from the drudgery of implementing object protocols (aka dunder methods). Trusted by NASA for Mars missions since 2020!

Its main goal is to help you to write concise and correct software without slowing down your code.

Getting Started

attrs is a Python-only package hosted on PyPI. The recommended installation method is pip-installing into a virtualenv:

$ python -m pip install attrs

The next steps will get you up and running in no time:

• Overview will show you a simple example of attrs in action and introduce you to its philosophy. Afterwards, you can start writing your own classes and understand what drives attrs’s design.

• attrs by Example will give you a comprehensive tour of attrs’s features. After reading, you will know about our advanced features and how to use them.

• Why not… gives you a rundown of potential alternatives and why we think attrs is still worthwhile – depending on your needs even superior.

• If at any point you get confused by some terminology, please check out our Glossary.

If you need any help while getting started, feel free to use the python-attrs tag on Stack Overflow and someone will surely help you out!

Day-to-Day Usage

• Type Annotations help you to write correct and self-documenting code. attrs has first class support for them, yet keeps them optional if you’re not convinced!

• Instance initialization is one of attrs key feature areas. Our goal is to relieve you from writing as much code as possible. Initialization gives you an overview what attrs has to offer and explains some related philosophies we believe in.

• Comparing and ordering objects is a common task. Comparison shows you how attrs helps you with that and how you can customize it.

• If you want to put objects into sets or use them as keys in dictionaries, they have to be hashable. The simplest way to do that is to use frozen classes, but the topic is more complex than it seems and Hashing will give you a primer on what to look out for.

• Once you’re comfortable with the concepts, our API Reference contains all information you need to use attrs to its fullest.

• attrs is built for extension from the ground up. Extending will show you the affordances it offers and how to make it a building block of your own projects.

• Finally, if you’re confused by all the attr.s, attr.ib, attrs, attrib, define, frozen, and field, head over to On The Core API Names for a very short explanation, and optionally a quick history lesson.

Project Information

• License: MIT

• PyPI: https://pypi.org/project/attrs/

• Source Code: https://github.com/python-attrs/attrs

• Documentation: https://www.attrs.org/

• Changelog: https://www.attrs.org/en/stable/changelog.html

• Get Help: please use the python-attrs tag on StackOverflow

• Third-party Extensions: https://github.com/python-attrs/attrs/wiki/Extensions-to-attrs

• Supported Python Versions: 3.5 and later (last 2.7-compatible release is 21.4.0)

If you’d like to contribute to attrs you’re most welcome and we’ve written a little guide to get you started!

attrs for Enterprise

Available as part of the Tidelift Subscription.

The maintainers of attrs and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source packages you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact packages you use. Learn more.

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Full Table of Contents

Overview

In order to fulfill its ambitious goal of bringing back the joy to writing classes, it gives you a class decorator and a way to declaratively define the attributes on that class:

>>> from attrs import asdict, define, make_class, Factory

>>> @define
... class SomeClass:
...     a_number: int = 42
...     list_of_numbers: list[int] = Factory(list)
...
...     def hard_math(self, another_number):
...         return self.a_number + sum(self.list_of_numbers) * another_number


>>> sc = SomeClass(1, [1, 2, 3])
>>> sc
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])

>>> sc.hard_math(3)
19
>>> sc == SomeClass(1, [1, 2, 3])
True
>>> sc != SomeClass(2, [3, 2, 1])
True

>>> asdict(sc)
{'a_number': 1, 'list_of_numbers': [1, 2, 3]}

>>> SomeClass()
SomeClass(a_number=42, list_of_numbers=[])

>>> C = make_class("C", ["a", "b"])
>>> C("foo", "bar")
C(a='foo', b='bar')

After declaring your attributes, attrs gives you:

• a concise and explicit overview of the class’s attributes,

• a nice human-readable __repr__,

• equality-checking methods,

• an initializer,

• and much more,

without writing dull boilerplate code again and again and without runtime performance penalties.

Hate type annotations!? No problem! Types are entirely optional with attrs. Simply assign attrs.field() to the attributes instead of annotating them with types.

---

This example uses attrs’s modern APIs that have been introduced in version 20.1.0, and the attrs package import name that has been added in version 21.3.0. The classic APIs (@attr.s, attr.ib, plus their serious-business aliases) and the attr package import name will remain indefinitely.

Please check out On The Core API Names for a more in-depth explanation.

Data Classes

On the tin, attrs might remind you of dataclasses (and indeed, dataclasses are a descendant of attrs). In practice it does a lot more and is more flexible. For instance it allows you to define special handling of NumPy arrays for equality checks, or allows more ways to plug into the initialization process.

For more details, please refer to our comparison page.

Philosophy

It’s about regular classes.

attrs is for creating well-behaved classes with a type, attributes, methods, and everything that comes with a class. It can be used for data-only containers like namedtuples or types.SimpleNamespace but they’re just a sub-genre of what attrs is good for.

The class belongs to the users.

You define a class and attrs adds static methods to that class based on the attributes you declare. The end. It doesn’t add metaclasses. It doesn’t add classes you’ve never heard of to your inheritance tree. An attrs class in runtime is indistinguishable from a regular class: because it is a regular class with a few boilerplate-y methods attached.

Be light on API impact.

As convenient as it seems at first, attrs will not tack on any methods to your classes except for the dunder ones. Hence all the useful tools that come with attrs live in functions that operate on top of instances. Since they take an attrs instance as their first argument, you can attach them to your classes with one line of code.

Performance matters.

attrs runtime impact is very close to zero because all the work is done when the class is defined. Once you’re instantiating it, attrs is out of the picture completely.

No surprises.

attrs creates classes that arguably work the way a Python beginner would reasonably expect them to work. It doesn’t try to guess what you mean because explicit is better than implicit. It doesn’t try to be clever because software shouldn’t be clever.

Check out How Does It Work? if you’d like to know how it achieves all of the above.

What attrs Is Not

attrs does not invent some kind of magic system that pulls classes out of its hat using meta classes, runtime introspection, and shaky interdependencies.

All attrs does is:

1. Take your declaration,

2. write dunder methods based on that information,

3. and attach them to your class.

It does nothing dynamic at runtime, hence zero runtime overhead. It’s still your class. Do with it as you please.

Why not…

If you’d like third party’s account why attrs is great, have a look at Glyph’s The One Python Library Everyone Needs. It predates type annotations and hence Data Classes, but it masterfully illustrates the appeal of class-building packages.

…Data Classes?

PEP 557 added Data Classes to Python 3.7 that resemble attrs in many ways.

They are the result of the Python community’s wish to have an easier way to write classes in the standard library that doesn’t carry the problems of namedtuples. To that end, attrs and its developers were involved in the PEP process and while we may disagree with some minor decisions that have been made, it’s a fine library and if it stops you from abusing namedtuples, they are a huge win.

Nevertheless, there are still reasons to prefer attrs over Data Classes. Whether they’re relevant to you depends on your circumstances:

• Data Classes are intentionally less powerful than attrs. There is a long list of features that were sacrificed for the sake of simplicity and while the most obvious ones are validators, converters, equality customization, or extensibility in general, it permeates throughout all APIs.

  On the other hand, Data Classes currently do not offer any significant feature that attrs doesn’t already have.

• attrs supports all mainstream Python versions including PyPy.

• attrs doesn’t force type annotations on you if you don’t like them.

• But since it also supports typing, it’s the best way to embrace type hints gradually, too.

• While Data Classes are implementing features from attrs every now and then, their presence is dependent on the Python version, not the package version. For example, support for __slots__ has only been added in Python 3.10, but it doesn’t do cell rewriting and therefore doesn’t support bare calls to super(). This may or may not be fixed in later Python releases, but handling all these differences is especially painful for PyPI packages that support multiple Python versions. And of course, this includes possible implementation bugs.

• attrs can and will move faster. We are not bound to any release schedules and we have a clear deprecation policy.

  One of the reasons to not vendor attrs in the standard library was to not impede attrs’s future development.

One way to think about attrs vs Data Classes is that attrs is a fully-fledged toolkit to write powerful classes while Data Classes are an easy way to get a class with some attributes. Basically what attrs was in 2015.

…pydantic?

pydantic is first and foremost a data validation library. As such, it is a capable complement to class building libraries like attrs (or Data Classes!) for parsing and validating untrusted data.

However, as convenient as it might be, using it for your business or data layer is problematic in several ways: Is it really necessary to re-validate all your objects while reading them from a trusted database? In the parlance of Form, Command, and Model Validation, pydantic is the right tool for Commands.

Separation of concerns feels tedious at times, but it’s one of those things that you get to appreciate once you’ve shot your own foot often enough.

…namedtuples?

collections.namedtuples are tuples with names, not classes. [1] Since writing classes is tiresome in Python, every now and then someone discovers all the typing they could save and gets really excited. However, that convenience comes at a price.

The most obvious difference between namedtuples and attrs-based classes is that the latter are type-sensitive:

>>> import attr
>>> C1 = attr.make_class("C1", ["a"])
>>> C2 = attr.make_class("C2", ["a"])
>>> i1 = C1(1)
>>> i2 = C2(1)
>>> i1.a == i2.a
True
>>> i1 == i2
False

…while a namedtuple is intentionally behaving like a tuple which means the type of a tuple is ignored:

>>> from collections import namedtuple
>>> NT1 = namedtuple("NT1", "a")
>>> NT2 = namedtuple("NT2", "b")
>>> t1 = NT1(1)
>>> t2 = NT2(1)
>>> t1 == t2 == (1,)
True

Other often surprising behaviors include:

• Since they are a subclass of tuples, namedtuples have a length and are both iterable and indexable. That’s not what you’d expect from a class and is likely to shadow subtle typo bugs.

• Iterability also implies that it’s easy to accidentally unpack a namedtuple which leads to hard-to-find bugs. [3]

• namedtuples have their methods on your instances whether you like it or not. [2]

• namedtuples are always immutable. Not only does that mean that you can’t decide for yourself whether your instances should be immutable or not, it also means that if you want to influence your class’ initialization (validation? default values?), you have to implement __new__() which is a particularly hacky and error-prone requirement for a very common problem. [4]

• To attach methods to a namedtuple you have to subclass it. And if you follow the standard library documentation’s recommendation of:

  class Point(namedtuple('Point', ['x', 'y'])):
      # ...
  

  you end up with a class that has two Points in its __mro__: [<class 'point.Point'>, <class 'point.Point'>, <type 'tuple'>, <type 'object'>].

  That’s not only confusing, it also has very practical consequences: for example if you create documentation that includes class hierarchies like Sphinx’s autodoc with show-inheritance. Again: common problem, hacky solution with confusing fallout.

All these things make namedtuples a particularly poor choice for public APIs because all your objects are irrevocably tainted. With attrs your users won’t notice a difference because it creates regular, well-behaved classes.

Summary

If you want a tuple with names, by all means: go for a namedtuple. [5] But if you want a class with methods, you’re doing yourself a disservice by relying on a pile of hacks that requires you to employ even more hacks as your requirements expand.

Other than that, attrs also adds nifty features like validators, converters, and (mutable!) default values.

Footnotes

[1]

The word is that namedtuples were added to the Python standard library as a way to make tuples in return values more readable. And indeed that is something you see throughout the standard library.

Looking at what the makers of namedtuples use it for themselves is a good guideline for deciding on your own use cases.

[2]

attrs only adds a single attribute: __attrs_attrs__ for introspection. All helpers are functions in the attr package. Since they take the instance as first argument, you can easily attach them to your classes under a name of your own choice.

[3]

attr.astuple can be used to get that behavior in attrs on explicit demand.

[4]

attrs offers optional immutability through the frozen keyword.

[5]

Although attrs would serve you just as well! Since both employ the same method of writing and compiling Python code for you, the performance penalty is negligible at worst and in some cases attrs is even faster if you use slots=True (which is generally a good idea anyway).

…tuples?

Readability

What makes more sense while debugging:

Point(x=1, y=2)

or:

(1, 2)

?

Let’s add even more ambiguity:

Customer(id=42, reseller=23, first_name="Jane", last_name="John")

or:

(42, 23, "Jane", "John")

?

Why would you want to write customer[2] instead of customer.first_name?

Don’t get me started when you add nesting. If you’ve never run into mysterious tuples you had no idea what the hell they meant while debugging, you’re much smarter than yours truly.

Using proper classes with names and types makes program code much more readable and comprehensible. Especially when trying to grok a new piece of software or returning to old code after several months.

Extendability

Imagine you have a function that takes or returns a tuple. Especially if you use tuple unpacking (eg. x, y = get_point()), adding additional data means that you have to change the invocation of that function everywhere.

Adding an attribute to a class concerns only those who actually care about that attribute.

…dicts?

Dictionaries are not for fixed fields.

If you have a dict, it maps something to something else. You should be able to add and remove values.

attrs lets you be specific about those expectations; a dictionary does not. It gives you a named entity (the class) in your code, which lets you explain in other places whether you take a parameter of that class or return a value of that class.

In other words: if your dict has a fixed and known set of keys, it is an object, not a hash. So if you never iterate over the keys of a dict, you should use a proper class.

…hand-written classes?

While we’re fans of all things artisanal, writing the same nine methods again and again doesn’t qualify. I usually manage to get some typos inside and there’s simply more code that can break and thus has to be tested.

To bring it into perspective, the equivalent of

>>> @attr.s
... class SmartClass:
...    a = attr.ib()
...    b = attr.ib()
>>> SmartClass(1, 2)
SmartClass(a=1, b=2)

is roughly

>>> class ArtisanalClass:
...     def __init__(self, a, b):
...         self.a = a
...         self.b = b
...
...     def __repr__(self):
...         return "ArtisanalClass(a={}, b={})".format(self.a, self.b)
...
...     def __eq__(self, other):
...         if other.__class__ is self.__class__:
...             return (self.a, self.b) == (other.a, other.b)
...         else:
...             return NotImplemented
...
...     def __ne__(self, other):
...         result = self.__eq__(other)
...         if result is NotImplemented:
...             return NotImplemented
...         else:
...             return not result
...
...     def __lt__(self, other):
...         if other.__class__ is self.__class__:
...             return (self.a, self.b) < (other.a, other.b)
...         else:
...             return NotImplemented
...
...     def __le__(self, other):
...         if other.__class__ is self.__class__:
...             return (self.a, self.b) <= (other.a, other.b)
...         else:
...             return NotImplemented
...
...     def __gt__(self, other):
...         if other.__class__ is self.__class__:
...             return (self.a, self.b) > (other.a, other.b)
...         else:
...             return NotImplemented
...
...     def __ge__(self, other):
...         if other.__class__ is self.__class__:
...             return (self.a, self.b) >= (other.a, other.b)
...         else:
...             return NotImplemented
...
...     def __hash__(self):
...         return hash((self.__class__, self.a, self.b))
>>> ArtisanalClass(a=1, b=2)
ArtisanalClass(a=1, b=2)

which is quite a mouthful and it doesn’t even use any of attrs’s more advanced features like validators or defaults values. Also: no tests whatsoever. And who will guarantee you, that you don’t accidentally flip the < in your tenth implementation of __gt__?

It also should be noted that attrs is not an all-or-nothing solution. You can freely choose which features you want and disable those that you want more control over:

>>> @attr.s(repr=False)
... class SmartClass:
...    a = attr.ib()
...    b = attr.ib()
...
...    def __repr__(self):
...        return "<SmartClass(a=%d)>" % (self.a,)
>>> SmartClass(1, 2)
<SmartClass(a=1)>

Summary

If you don’t care and like typing, we’re not gonna stop you.

However it takes a lot of bias and determined rationalization to claim that attrs raises the mental burden on a project given how difficult it is to find the important bits in a hand-written class and how annoying it is to ensure you’ve copy-pasted your code correctly over all your classes.

In any case, if you ever get sick of the repetitiveness and drowning important code in a sea of boilerplate, attrs will be waiting for you.

attrs by Example

Basics

The simplest possible usage is:

>>> from attrs import define, field
>>> @define
... class Empty:
...     pass
>>> Empty()
Empty()
>>> Empty() == Empty()
True
>>> Empty() is Empty()
False

So in other words: attrs is useful even without actual attributes!

But you’ll usually want some data on your classes, so let’s add some:

>>> @define
... class Coordinates:
...     x: int
...     y: int

By default, all features are added, so you immediately have a fully functional data class with a nice repr string and comparison methods.

>>> c1 = Coordinates(1, 2)
>>> c1
Coordinates(x=1, y=2)
>>> c2 = Coordinates(x=2, y=1)
>>> c2
Coordinates(x=2, y=1)
>>> c1 == c2
False

As shown, the generated __init__ method allows for both positional and keyword arguments.

For private attributes, attrs will strip the leading underscores for keyword arguments:

>>> @define
... class C:
...     _x: int
>>> C(x=1)
C(_x=1)

If you want to initialize your private attributes yourself, you can do that too:

>>> @define
... class C:
...     _x: int = field(init=False, default=42)
>>> C()
C(_x=42)
>>> C(23)
Traceback (most recent call last):
   ...
TypeError: __init__() takes exactly 1 argument (2 given)

An additional way of defining attributes is supported too. This is useful in times when you want to enhance classes that are not yours (nice __repr__ for Django models anyone?):

>>> class SomethingFromSomeoneElse:
...     def __init__(self, x):
...         self.x = x
>>> SomethingFromSomeoneElse = define(
...     these={
...         "x": field()
...     }, init=False)(SomethingFromSomeoneElse)
>>> SomethingFromSomeoneElse(1)
SomethingFromSomeoneElse(x=1)

Subclassing is bad for you, but attrs will still do what you’d hope for:

>>> @define(slots=False)
... class A:
...     a: int
...     def get_a(self):
...         return self.a
>>> @define(slots=False)
... class B:
...     b: int
>>> @define(slots=False)
... class C(B, A):
...     c: int
>>> i = C(1, 2, 3)
>>> i
C(a=1, b=2, c=3)
>>> i == C(1, 2, 3)
True
>>> i.get_a()
1

Slotted classes, which are the default for the new APIs, don’t play well with multiple inheritance so we don’t use them in the example.

The order of the attributes is defined by the MRO.

Keyword-only Attributes

You can also add keyword-only attributes:

>>> @define
... class A:
...     a: int = field(kw_only=True)
>>> A()
Traceback (most recent call last):
  ...
TypeError: A() missing 1 required keyword-only argument: 'a'
>>> A(a=1)
A(a=1)

kw_only may also be specified at via define, and will apply to all attributes:

>>> @define(kw_only=True)
... class A:
...     a: int
...     b: int
>>> A(1, 2)
Traceback (most recent call last):
  ...
TypeError: __init__() takes 1 positional argument but 3 were given
>>> A(a=1, b=2)
A(a=1, b=2)

If you create an attribute with init=False, the kw_only argument is ignored.

Keyword-only attributes allow subclasses to add attributes without default values, even if the base class defines attributes with default values:

>>> @define
... class A:
...     a: int = 0
>>> @define
... class B(A):
...     b: int = field(kw_only=True)
>>> B(b=1)
B(a=0, b=1)
>>> B()
Traceback (most recent call last):
  ...
TypeError: B() missing 1 required keyword-only argument: 'b'

If you don’t set kw_only=True, then there is no valid attribute ordering, and you’ll get an error:

>>> @define
... class A:
...     a: int = 0
>>> @define
... class B(A):
...     b: int
Traceback (most recent call last):
  ...
ValueError: No mandatory attributes allowed after an attribute with a default value or factory.  Attribute in question: Attribute(name='b', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, converter=None, metadata=mappingproxy({}), type=int, kw_only=False)

Converting to Collections Types

When you have a class with data, it often is very convenient to transform that class into a dict (for example if you want to serialize it to JSON):

>>> from attrs import asdict

>>> asdict(Coordinates(x=1, y=2))
{'x': 1, 'y': 2}

Some fields cannot or should not be transformed. For that, attrs.asdict offers a callback that decides whether an attribute should be included:

>>> @define
... class User:
...     email: str
...     password: str

>>> @define
... class UserList:
...     users: list[User]

>>> asdict(UserList([User("jane@doe.invalid", "s33kred"),
...                  User("joe@doe.invalid", "p4ssw0rd")]),
...        filter=lambda attr, value: attr.name != "password")
{'users': [{'email': 'jane@doe.invalid'}, {'email': 'joe@doe.invalid'}]}

For the common case where you want to include or exclude certain types or attributes, attrs ships with a few helpers:

>>> from attrs import asdict, filters, fields

>>> @define
... class User:
...     login: str
...     password: str
...     id: int

>>> asdict(
...     User("jane", "s33kred", 42),
...     filter=filters.exclude(fields(User).password, int))
{'login': 'jane'}

>>> @define
... class C:
...     x: str
...     y: str
...     z: int

>>> asdict(C("foo", "2", 3),
...        filter=filters.include(int, fields(C).x))
{'x': 'foo', 'z': 3}

Other times, all you want is a tuple and attrs won’t let you down:

>>> import sqlite3
>>> from attrs import astuple

>>> @define
... class Foo:
...    a: int
...    b: int

>>> foo = Foo(2, 3)
>>> with sqlite3.connect(":memory:") as conn:
...    c = conn.cursor()
...    c.execute("CREATE TABLE foo (x INTEGER PRIMARY KEY ASC, y)") 
...    c.execute("INSERT INTO foo VALUES (?, ?)", astuple(foo)) 
...    foo2 = Foo(*c.execute("SELECT x, y FROM foo").fetchone())
<sqlite3.Cursor object at ...>
<sqlite3.Cursor object at ...>
>>> foo == foo2
True

For more advanced transformations and conversions, we recommend you look at a companion library (such as cattrs).

Defaults

Sometimes you want to have default values for your initializer. And sometimes you even want mutable objects as default values (ever accidentally used def f(arg=[])?). attrs has you covered in both cases:

>>> import collections

>>> @define
... class Connection:
...     socket: int
...     @classmethod
...     def connect(cls, db_string):
...        # ... connect somehow to db_string ...
...        return cls(socket=42)

>>> @define
... class ConnectionPool:
...     db_string: str
...     pool: collections.deque = Factory(collections.deque)
...     debug: bool = False
...     def get_connection(self):
...         try:
...             return self.pool.pop()
...         except IndexError:
...             if self.debug:
...                 print("New connection!")
...             return Connection.connect(self.db_string)
...     def free_connection(self, conn):
...         if self.debug:
...             print("Connection returned!")
...         self.pool.appendleft(conn)
...
>>> cp = ConnectionPool("postgres://localhost")
>>> cp
ConnectionPool(db_string='postgres://localhost', pool=deque([]), debug=False)
>>> conn = cp.get_connection()
>>> conn
Connection(socket=42)
>>> cp.free_connection(conn)
>>> cp
ConnectionPool(db_string='postgres://localhost', pool=deque([Connection(socket=42)]), debug=False)

More information on why class methods for constructing objects are awesome can be found in this insightful blog post.

Default factories can also be set using the factory argument to field, and using a decorator. The method receives the partially initialized instance which enables you to base a default value on other attributes:

>>> @define
... class C:
...     x: int = 1
...     y: int = field()
...     @y.default
...     def _any_name_except_a_name_of_an_attribute(self):
...         return self.x + 1
...     z: list = field(factory=list)
>>> C()
C(x=1, y=2, z=[])

Please keep in mind that the decorator approach only works if the attribute in question has a field assigned to it. As a result, annotating an attribute with a type is not enough if you use @default.

Validators

Although your initializers should do as little as possible (ideally: just initialize your instance according to the arguments!), it can come in handy to do some kind of validation on the arguments.

attrs offers two ways to define validators for each attribute and it’s up to you to choose which one suits your style and project better.

You can use a decorator:

>>> @define
... class C:
...     x: int = field()
...     @x.validator
...     def check(self, attribute, value):
...         if value > 42:
...             raise ValueError("x must be smaller or equal to 42")
>>> C(42)
C(x=42)
>>> C(43)
Traceback (most recent call last):
   ...
ValueError: x must be smaller or equal to 42

…or a callable…

>>> from attrs import validators

>>> def x_smaller_than_y(instance, attribute, value):
...     if value >= instance.y:
...         raise ValueError("'x' has to be smaller than 'y'!")
>>> @define
... class C:
...     x: int = field(validator=[validators.instance_of(int),
...                               x_smaller_than_y])
...     y: int
>>> C(x=3, y=4)
C(x=3, y=4)
>>> C(x=4, y=3)
Traceback (most recent call last):
   ...
ValueError: 'x' has to be smaller than 'y'!

…or both at once:

>>> @define
... class C:
...     x: int = field(validator=validators.instance_of(int))
...     @x.validator
...     def fits_byte(self, attribute, value):
...         if not 0 <= value < 256:
...             raise ValueError("value out of bounds")
>>> C(128)
C(x=128)
>>> C("128")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=int, converter=None, kw_only=False), <class 'int'>, '128')
>>> C(256)
Traceback (most recent call last):
   ...
ValueError: value out of bounds

Please note that the decorator approach only works if – and only if! – the attribute in question has a field assigned. Therefore if you use @validator, it is not enough to annotate said attribute with a type.

attrs ships with a bunch of validators, make sure to check them out before writing your own:

>>> @define
... class C:
...     x: int = field(validator=validators.instance_of(int))
>>> C(42)
C(x=42)
>>> C("42")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, factory=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None, kw_only=False), <type 'int'>, '42')

Please note that if you use attr.s (and not attrs.define) to define your class, validators only run on initialization by default. This behavior can be changed using the on_setattr argument.

Check out Validators for more details.

Conversion

Attributes can have a converter function specified, which will be called with the attribute’s passed-in value to get a new value to use. This can be useful for doing type-conversions on values that you don’t want to force your callers to do.

>>> @define
... class C:
...     x: int = field(converter=int)
>>> o = C("1")
>>> o.x
1

Please note that converters only run on initialization.

Check out Converters for more details.

Metadata

All attrs attributes may include arbitrary metadata in the form of a read-only dictionary.

>>> from attrs import fields

>>> @define
... class C:
...    x = field(metadata={'my_metadata': 1})
>>> fields(C).x.metadata
mappingproxy({'my_metadata': 1})
>>> fields(C).x.metadata['my_metadata']
1

Metadata is not used by attrs, and is meant to enable rich functionality in third-party libraries. The metadata dictionary follows the normal dictionary rules: keys need to be hashable, and both keys and values are recommended to be immutable.

If you’re the author of a third-party library with attrs integration, please see Extending Metadata.

Types

attrs also allows you to associate a type with an attribute using either the type argument to attr.ib or – as of Python 3.6 – using PEP 526-annotations:

>>> from attrs import fields

>>> @define
... class C:
...     x: int
>>> fields(C).x.type
<class 'int'>

>>> import attr
>>> @attr.s
... class C:
...     x = attr.ib(type=int)
>>> fields(C).x.type
<class 'int'>

If you don’t mind annotating all attributes, you can even drop the attrs.field and assign default values instead:

>>> import typing
>>> from attrs import fields

>>> @define
... class AutoC:
...     cls_var: typing.ClassVar[int] = 5  # this one is ignored
...     l: list[int] = Factory(list)
...     x: int = 1
...     foo: str = "every attrib needs a type if auto_attribs=True"
...     bar: typing.Any = None
>>> fields(AutoC).l.type
list[int]
>>> fields(AutoC).x.type
<class 'int'>
>>> fields(AutoC).foo.type
<class 'str'>
>>> fields(AutoC).bar.type
typing.Any
>>> AutoC()
AutoC(l=[], x=1, foo='every attrib needs a type if auto_attribs=True', bar=None)
>>> AutoC.cls_var
5

The generated __init__ method will have an attribute called __annotations__ that contains this type information.

If your annotations contain strings (e.g. forward references), you can resolve these after all references have been defined by using attrs.resolve_types(). This will replace the type attribute in the respective fields.

>>> from attrs import fields, resolve_types

>>> @define
... class A:
...     a: 'list[A]'
...     b: 'B'
...
>>> @define
... class B:
...     a: A
...
>>> fields(A).a.type
'list[A]'
>>> fields(A).b.type
'B'
>>> resolve_types(A, globals(), locals())
<class 'A'>
>>> fields(A).a.type
list[A]
>>> fields(A).b.type
<class 'B'>

Note

If you find yourself using string type annotations to handle forward references, wrap the entire type annotation in quotes instead of only the type you need a forward reference to (so 'list[A]' instead of list['A']). This is a limitation of the Python typing system.

Warning

attrs itself doesn’t have any features that work on top of type metadata yet. However it’s useful for writing your own validators or serialization frameworks.

Slots

Slotted classes have several advantages on CPython. Defining __slots__ by hand is tedious, in attrs it’s just a matter of using attrs.define or passing slots=True to attr.s:

>>> import attr

>>> @attr.s(slots=True)
... class Coordinates:
...     x: int
...     y: int

Immutability

Sometimes you have instances that shouldn’t be changed after instantiation. Immutability is especially popular in functional programming and is generally a very good thing. If you’d like to enforce it, attrs will try to help:

>>> @frozen
... class C:
...     x: int
>>> i = C(1)
>>> i.x = 2
Traceback (most recent call last):
   ...
attr.exceptions.FrozenInstanceError: can't set attribute
>>> i.x
1

Please note that true immutability is impossible in Python but it will get you 99% there. By themselves, immutable classes are useful for long-lived objects that should never change; like configurations for example.

In order to use them in regular program flow, you’ll need a way to easily create new instances with changed attributes. In Clojure that function is called assoc and attrs shamelessly imitates it: attr.evolve:

>>> from attrs import evolve, frozen

>>> @frozen
... class C:
...     x: int
...     y: int
>>> i1 = C(1, 2)
>>> i1
C(x=1, y=2)
>>> i2 = evolve(i1, y=3)
>>> i2
C(x=1, y=3)
>>> i1 == i2
False

Other Goodies

Sometimes you may want to create a class programmatically. attrs won’t let you down and gives you attrs.make_class :

>>> from attrs import fields, make_class
>>> @define
... class C1:
...     x = field()
...     y = field()
>>> C2 = make_class("C2", ["x", "y"])
>>> fields(C1) == fields(C2)
True

You can still have power over the attributes if you pass a dictionary of name: field mappings and can pass arguments to @attr.s:

>>> from attrs import make_class

>>> C = make_class("C", {"x": field(default=42),
...                      "y": field(default=Factory(list))},
...                repr=False)
>>> i = C()
>>> i  # no repr added!
<__main__.C object at ...>
>>> i.x
42
>>> i.y
[]

If you need to dynamically make a class with attrs.make_class and it needs to be a subclass of something else than object, use the bases argument:

>>> from attrs import make_class

>>> class D:
...    def __eq__(self, other):
...        return True  # arbitrary example
>>> C = make_class("C", {}, bases=(D,), cmp=False)
>>> isinstance(C(), D)
True

Sometimes, you want to have your class’s __init__ method do more than just the initialization, validation, etc. that gets done for you automatically when using @define. To do this, just define a __attrs_post_init__ method in your class. It will get called at the end of the generated __init__ method.

>>> @define
... class C:
...     x: int
...     y: int
...     z: int = field(init=False)
...
...     def __attrs_post_init__(self):
...         self.z = self.x + self.y
>>> obj = C(x=1, y=2)
>>> obj
C(x=1, y=2, z=3)

You can exclude single attributes from certain methods:

>>> @define
... class C:
...     user: str
...     password: str = field(repr=False)
>>> C("me", "s3kr3t")
C(user='me')

Alternatively, to influence how the generated __repr__() method formats a specific attribute, specify a custom callable to be used instead of the repr() built-in function:

>>> @define
... class C:
...     user: str
...     password: str = field(repr=lambda value: '***')
>>> C("me", "s3kr3t")
C(user='me', password=***)

Type Annotations

attrs comes with first class support for type annotations for both Python 3.6 (PEP 526) and legacy syntax.

However they will forever remain optional, therefore the example from the README could also be written as:

>>> from attrs import define, field

>>> @define
... class SomeClass:
...     a_number = field(default=42)
...     list_of_numbers = field(factory=list)

>>> sc = SomeClass(1, [1, 2, 3])
>>> sc
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])

You can choose freely between the approaches, but please remember that if you choose to use type annotations, you must annotate all attributes!

---

Even when going all-in on type annotations, you will need attr.field for some advanced features though.

One of those features are the decorator-based features like defaults. It’s important to remember that attrs doesn’t do any magic behind your back. All the decorators are implemented using an object that is returned by the call to attrs.field.

Attributes that only carry a class annotation do not have that object so trying to call a method on it will inevitably fail.

---

Please note that types – however added – are only metadata that can be queried from the class and they aren’t used for anything out of the box!

Because Python does not allow references to a class object before the class is defined, types may be defined as string literals, so-called forward references (PEP 526). You can enable this automatically for a whole module by using from __future__ import annotations (PEP 563) as of Python 3.7. In this case attrs simply puts these string literals into the type attributes. If you need to resolve these to real types, you can call attrs.resolve_types which will update the attribute in place.

In practice though, types show their biggest usefulness in combination with tools like mypy, pytype, or pyright that have dedicated support for attrs classes.

The addition of static types is certainly one of the most exciting features in the Python ecosystem and helps you write correct and verified self-documenting code.

If you don’t know where to start, Carl Meyer gave a great talk on Type-checked Python in the Real World at PyCon US 2018 that will help you to get started in no time.

mypy

While having a nice syntax for type metadata is great, it’s even greater that mypy as of 0.570 ships with a dedicated attrs plugin which allows you to statically check your code.

Imagine you add another line that tries to instantiate the defined class using SomeClass("23"). Mypy will catch that error for you:

$ mypy t.py
t.py:12: error: Argument 1 to "SomeClass" has incompatible type "str"; expected "int"

This happens without running your code!

And it also works with both Python 2-style annotation styles. To mypy, this code is equivalent to the one above:

@attr.s
class SomeClass:
    a_number = attr.ib(default=42)  # type: int
    list_of_numbers = attr.ib(factory=list, type=list[int])

pyright

attrs provides support for pyright though the dataclass_transform specification. This provides static type inference for a subset of attrs equivalent to standard-library dataclasses, and requires explicit type annotations using the attrs.define or @attr.s(auto_attribs=True) API.

Given the following definition, pyright will generate static type signatures for SomeClass attribute access, __init__, __eq__, and comparison methods:

@attr.define
class SomeClass:
    a_number: int = 42
    list_of_numbers: list[int] = attr.field(factory=list)

Warning

The pyright inferred types are a subset of those supported by mypy, including:

• The generated __init__ signature only includes the attribute type annotations. It currently does not include attribute converter types.

• The attr.frozen decorator is not typed with frozen attributes, which are properly typed via attr.define(frozen=True).

  A full list of limitations and incompatibilities can be found in pyright’s repository.

Your constructive feedback is welcome in both attrs#795 and pyright#1782. Generally speaking, the decision on improving attrs support in pyright is entirely Microsoft’s prerogative though.

Initialization

In Python, instance initialization happens in the __init__ method. Generally speaking, you should keep as little logic as possible in it, and you should think about what the class needs and not how it is going to be instantiated.

Passing complex objects into __init__ and then using them to derive data for the class unnecessarily couples your new class with the old class which makes it harder to test and also will cause problems later.

So assuming you use an ORM and want to extract 2D points from a row object, do not write code like this:

class Point:
    def __init__(self, database_row):
        self.x = database_row.x
        self.y = database_row.y

pt = Point(row)

Instead, write a classmethod that will extract it for you:

@define
class Point:
    x: float
    y: float

    @classmethod
    def from_row(cls, row):
        return cls(row.x, row.y)

pt = Point.from_row(row)

Now you can instantiate Points without creating fake row objects in your tests and you can have as many smart creation helpers as you want, in case more data sources appear.

For similar reasons, we strongly discourage from patterns like:

pt = Point(**row.attributes)

which couples your classes to the database data model. Try to design your classes in a way that is clean and convenient to use – not based on your database format. The database format can change anytime and you’re stuck with a bad class design that is hard to change. Embrace functions and classmethods as a filter between reality and what’s best for you to work with.

If you look for object serialization, there’s a bunch of projects listed on our attrs extensions Wiki page. Some of them even support nested schemas.

Private Attributes

One thing people tend to find confusing is the treatment of private attributes that start with an underscore. attrs follows the doctrine that there is no such thing as a private argument and strips the underscores from the name when writing the __init__ method signature:

>>> import inspect, attr, attrs
>>> from attr import define
>>> @define
... class C:
...    _x: int
>>> inspect.signature(C.__init__)
<Signature (self, x: int) -> None>

There really isn’t a right or wrong, it’s a matter of taste. But it’s important to be aware of it because it can lead to surprising syntax errors:

>>> @define
... class C:
...    _1: int
Traceback (most recent call last):
   ...
SyntaxError: invalid syntax

In this case a valid attribute name _1 got transformed into an invalid argument name 1.

Defaults

Sometimes you don’t want to pass all attribute values to a class. And sometimes, certain attributes aren’t even intended to be passed but you want to allow for customization anyways for easier testing.

This is when default values come into play:

>>> from attr import define, field, Factory

>>> @define
... class C:
...     a: int = 42
...     b: list = field(factory=list)
...     c: list = Factory(list)  # syntactic sugar for above
...     d: dict = field()
...     @d.default
...     def _any_name_except_a_name_of_an_attribute(self):
...        return {}
>>> C()
C(a=42, b=[], c=[], d={})

It’s important that the decorated method – or any other method or property! – doesn’t have the same name as the attribute, otherwise it would overwrite the attribute definition.

Please note that as with function and method signatures, default=[] will not do what you may think it might do:

>>> @define
... class C:
...     x = []
>>> i = C()
>>> k = C()
>>> i.x.append(42)
>>> k.x
[42]

This is why attrs comes with factory options.

Warning

Please note that the decorator based defaults have one gotcha: they are executed when the attribute is set, that means depending on the order of attributes, the self object may not be fully initialized when they’re called.

Therefore you should use self as little as possible.

Even the smartest of us can get confused by what happens if you pass partially initialized objects around.

Validators

Another thing that definitely does belong in __init__ is checking the resulting instance for invariants. This is why attrs has the concept of validators.

Decorator

The most straightforward way is using the attribute’s validator method as a decorator.

The method has to accept three arguments:

1. the instance that’s being validated (aka self),

2. the attribute that it’s validating, and finally

3. the value that is passed for it.

These values are passed as positional arguments, therefore their names don’t matter.

If the value does not pass the validator’s standards, it just raises an appropriate exception.

>>> @define
... class C:
...     x: int = field()
...     @x.validator
...     def _check_x(self, attribute, value):
...         if value > 42:
...             raise ValueError("x must be smaller or equal to 42")
>>> C(42)
C(x=42)
>>> C(43)
Traceback (most recent call last):
   ...
ValueError: x must be smaller or equal to 42

Again, it’s important that the decorated method doesn’t have the same name as the attribute and that the attrs.field() helper is used.

Callables

If you want to re-use your validators, you should have a look at the validator argument to attrs.field.

It takes either a callable or a list of callables (usually functions) and treats them as validators that receive the same arguments as with the decorator approach. Also as with the decorator approach, they are passed as positional arguments so you can name them however you want.

Since the validators run after the instance is initialized, you can refer to other attributes while validating:

>>> def x_smaller_than_y(instance, attribute, value):
...     if value >= instance.y:
...         raise ValueError("'x' has to be smaller than 'y'!")
>>> @define
... class C:
...     x = field(validator=[attrs.validators.instance_of(int),
...                          x_smaller_than_y])
...     y = field()
>>> C(x=3, y=4)
C(x=3, y=4)
>>> C(x=4, y=3)
Traceback (most recent call last):
   ...
ValueError: 'x' has to be smaller than 'y'!

This example also shows of some syntactic sugar for using the attrs.validators.and_ validator: if you pass a list, all validators have to pass.

attrs won’t intercept your changes to those attributes but you can always call attrs.validate on any instance to verify that it’s still valid: When using attrs.define or attrs.frozen, attrs will run the validators even when setting the attribute.

>>> i = C(4, 5)
>>> i.x = 5
Traceback (most recent call last):
   ...
ValueError: 'x' has to be smaller than 'y'!

attrs ships with a bunch of validators, make sure to check them out before writing your own:

>>> @define
... class C:
...     x = field(validator=attrs.validators.instance_of(int))
>>> C(42)
C(x=42)
>>> C("42")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, factory=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None), <type 'int'>, '42')

Of course you can mix and match the two approaches at your convenience. If you define validators both ways for an attribute, they are both ran:

>>> @define
... class C:
...     x = field(validator=attrs.validators.instance_of(int))
...     @x.validator
...     def fits_byte(self, attribute, value):
...         if not 0 <= value < 256:
...             raise ValueError("value out of bounds")
>>> C(128)
C(x=128)
>>> C("128")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=one), <class 'int'>, '128')
>>> C(256)
Traceback (most recent call last):
   ...
ValueError: value out of bounds

And finally you can disable validators globally:

>>> attrs.validators.set_disabled(True)
>>> C("128")
C(x='128')
>>> attrs.validators.set_disabled(False)
>>> C("128")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=None), <class 'int'>, '128')

You can achieve the same by using the context manager:

>>> with attrs.validators.disabled():
...     C("128")
C(x='128')
>>> C("128")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <class 'int'> (got '128' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=[<instance_of validator for type <class 'int'>>, <function fits_byte at 0x10fd7a0d0>], repr=True, cmp=True, hash=True, init=True, metadata=mappingproxy({}), type=None, converter=None), <class 'int'>, '128')

Converters

Finally, sometimes you may want to normalize the values coming in. For that attrs comes with converters.

Attributes can have a converter function specified, which will be called with the attribute’s passed-in value to get a new value to use. This can be useful for doing type-conversions on values that you don’t want to force your callers to do.

>>> @define
... class C:
...     x = field(converter=int)
>>> o = C("1")
>>> o.x
1

Converters are run before validators, so you can use validators to check the final form of the value.

>>> def validate_x(instance, attribute, value):
...     if value < 0:
...         raise ValueError("x must be at least 0.")
>>> @define
... class C:
...     x = field(converter=int, validator=validate_x)
>>> o = C("0")
>>> o.x
0
>>> C("-1")
Traceback (most recent call last):
    ...
ValueError: x must be at least 0.

Arguably, you can abuse converters as one-argument validators:

>>> C("x")
Traceback (most recent call last):
    ...
ValueError: invalid literal for int() with base 10: 'x'

If a converter’s first argument has a type annotation, that type will appear in the signature for __init__. A converter will override an explicit type annotation or type argument.

>>> def str2int(x: str) -> int:
...     return int(x)
>>> @define
... class C:
...     x = field(converter=str2int)
>>> C.__init__.__annotations__
{'return': None, 'x': <class 'str'>}

Hooking Yourself Into Initialization

Generally speaking, the moment you think that you need finer control over how your class is instantiated than what attrs offers, it’s usually best to use a classmethod factory or to apply the builder pattern.

However, sometimes you need to do that one quick thing before or after your class is initialized. And for that attrs offers three means:

• __attrs_pre_init__ is automatically detected and run before attrs starts initializing. This is useful if you need to inject a call to super().__init__().

• __attrs_post_init__ is automatically detected and run after attrs is done initializing your instance. This is useful if you want to derive some attribute from others or perform some kind of validation over the whole instance.

• __attrs_init__ is written and attached to your class instead of __init__, if attrs is told to not write one (i.e. init=False or a combination of auto_detect=True and a custom __init__). This is useful if you want full control over the initialization process, but don’t want to set the attributes by hand.

Pre Init

The sole reason for the existence of __attrs_pre_init__ is to give users the chance to call super().__init__(), because some subclassing-based APIs require that.

>>> @define
... class C:
...     x: int
...     def __attrs_pre_init__(self):
...         super().__init__()
>>> C(42)
C(x=42)

If you need more control, use the custom init approach described next.

Custom Init

If you tell attrs to not write an __init__, it will write an __attrs_init__ instead, with the same code that it would have used for __init__. You have full control over the initialization, but also have to type out the types of your arguments etc. Here’s an example of a manual default value:

>>> @define
... class C:
...     x: int
...
...     def __init__(self, x: int = 42):
...         self.__attrs_init__(x)
>>> C()
C(x=42)

Post Init

>>> @define
... class C:
...     x: int
...     y: int = field(init=False)
...     def __attrs_post_init__(self):
...         self.y = self.x + 1
>>> C(1)
C(x=1, y=2)

Please note that you can’t directly set attributes on frozen classes:

>>> @frozen
... class FrozenBroken:
...     x: int
...     y: int = field(init=False)
...     def __attrs_post_init__(self):
...         self.y = self.x + 1
>>> FrozenBroken(1)
Traceback (most recent call last):
   ...
attrs.exceptions.FrozenInstanceError: can't set attribute

If you need to set attributes on a frozen class, you’ll have to resort to the same trick as attrs and use object.__setattr__():

>>> @define
... class Frozen:
...     x: int
...     y: int = field(init=False)
...     def __attrs_post_init__(self):
...         object.__setattr__(self, "y", self.x + 1)
>>> Frozen(1)
Frozen(x=1, y=2)

Note that you must not access the hash code of the object in __attrs_post_init__ if cache_hash=True.

Order of Execution

If present, the hooks are executed in the following order:

1. __attrs_pre_init__ (if present on current class)

2. For each attribute, in the order it was declared:

   1. default factory

   2. converter

3. all validators

4. __attrs_post_init__ (if present on current class)

Notably this means, that you can access all attributes from within your validators, but your converters have to deal with invalid values and have to return a valid value.

Derived Attributes

One of the most common attrs questions on Stack Overflow is how to have attributes that depend on other attributes. For example if you have an API token and want to instantiate a web client that uses it for authentication. Based on the previous sections, there are two approaches.

The simpler one is using __attrs_post_init__:

@define
class APIClient:
    token: str
    client: WebClient = field(init=False)

    def __attrs_post_init__(self):
        self.client = WebClient(self.token)

The second one is using a decorator-based default:

@define
class APIClient:
    token: str
    client: WebClient = field()  # needed! attr.ib works too

    @client.default
    def _client_factory(self):
        return WebClient(self.token)

That said, and as pointed out in the beginning of the chapter, a better approach would be to have a factory class method:

@define
class APIClient:
    client: WebClient

    @classmethod
    def from_token(cls, token: str) -> "APIClient":
        return cls(client=WebClient(token))

This makes the class more testable.

Comparison

By default, two instances of attrs classes are equal if all their fields are equal. For that, attrs writes __eq__ and __ne__ methods for you.

Additionally, if you pass order=True (which is the default if you use the attr.s decorator), attrs will also create a full set of ordering methods that are based on the defined fields: __le__, __lt__, __ge__, and __gt__.

Customization

As with other features, you can exclude fields from being involved in comparison operations:

>>> from attr import define, field

>>> @define
... class C:
...     x: int
...     y: int = field(eq=False)

>>> C(1, 2) == C(1, 3)
True

Additionally you can also pass a callable instead of a bool to both eq and order. It is then used as a key function like you may know from sorted:

>>> from attr import define, field

>>> @define
... class S:
...     x: str = field(eq=str.lower)

>>> S("foo") == S("FOO")
True

>>> @define(order=True)
... class C:
...     x: str = field(order=int)

>>> C("10") > C("2")
True

This is especially useful when you have fields with objects that have atypical comparison properties. Common examples of such objects are NumPy arrays.

To save you unnecessary boilerplate, attrs comes with the attrs.cmp_using helper to create such functions. For NumPy arrays it would look like this:

import numpy

@define(order=False)
class C:
   an_array = field(eq=attr.cmp_using(eq=numpy.array_equal))

Warning

Please note that eq and order are set independently, because order is False by default in attrs.define (but not in attr.s). You can set both at once by using the cmp argument that we’ve undeprecated just for this use-case.

Hashing

Hash Method Generation

Warning

The overarching theme is to never set the @attr.s(hash=X) parameter yourself. Leave it at None which means that attrs will do the right thing for you, depending on the other parameters:

• If you want to make objects hashable by value: use @attr.s(frozen=True).

• If you want hashing and equality by object identity: use @attr.s(eq=False)

Setting hash yourself can have unexpected consequences so we recommend to tinker with it only if you know exactly what you’re doing.

Under certain circumstances, it’s necessary for objects to be hashable. For example if you want to put them into a set or if you want to use them as keys in a dict.

The hash of an object is an integer that represents the contents of an object. It can be obtained by calling hash on an object and is implemented by writing a __hash__ method for your class.

attrs will happily write a __hash__ method for you [1], however it will not do so by default. Because according to the definition from the official Python docs, the returned hash has to fulfill certain constraints:

1. Two objects that are equal, must have the same hash. This means that if x == y, it must follow that hash(x) == hash(y).

   By default, Python classes are compared and hashed by their id. That means that every instance of a class has a different hash, no matter what attributes it carries.

   It follows that the moment you (or attrs) change the way equality is handled by implementing __eq__ which is based on attribute values, this constraint is broken. For that reason Python 3 will make a class that has customized equality unhashable. Python 2 on the other hand will happily let you shoot your foot off. Unfortunately, attrs still mimics (otherwise unsupported) Python 2’s behavior for backward compatibility reasons if you set hash=False.

   The correct way to achieve hashing by id is to set @attr.s(eq=False). Setting @attr.s(hash=False) (which implies eq=True) is almost certainly a bug.

   Warning

   Be careful when subclassing! Setting eq=False on a class whose base class has a non-default __hash__ method will not make attrs remove that __hash__ for you.

   It is part of attrs’s philosophy to only add to classes so you have the freedom to customize your classes as you wish. So if you want to get rid of methods, you’ll have to do it by hand.

   The easiest way to reset __hash__ on a class is adding __hash__ = object.__hash__ in the class body.

2. If two objects are not equal, their hash should be different.

   While this isn’t a requirement from a standpoint of correctness, sets and dicts become less effective if there are a lot of identical hashes. The worst case is when all objects have the same hash which turns a set into a list.

3. The hash of an object must not change.

   If you create a class with @attr.s(frozen=True) this is fulfilled by definition, therefore attrs will write a __hash__ function for you automatically. You can also force it to write one with hash=True but then it’s your responsibility to make sure that the object is not mutated.

   This point is the reason why mutable structures like lists, dictionaries, or sets aren’t hashable while immutable ones like tuples or frozensets are: point 1 and 2 require that the hash changes with the contents but point 3 forbids it.

For a more thorough explanation of this topic, please refer to this blog post: Python Hashes and Equality.

Hashing and Mutability

Changing any field involved in hash code computation after the first call to __hash__ (typically this would be after its insertion into a hash-based collection) can result in silent bugs. Therefore, it is strongly recommended that hashable classes be frozen. Beware, however, that this is not a complete guarantee of safety: if a field points to an object and that object is mutated, the hash code may change, but frozen will not protect you.

Hash Code Caching

Some objects have hash codes which are expensive to compute. If such objects are to be stored in hash-based collections, it can be useful to compute the hash codes only once and then store the result on the object to make future hash code requests fast. To enable caching of hash codes, pass cache_hash=True to @attrs. This may only be done if attrs is already generating a hash function for the object.

[1]

The hash is computed by hashing a tuple that consists of a unique id for the class plus all attribute values.

API Reference

attrs works by decorating a class using attrs.define or attr.s and then optionally defining attributes on the class using attrs.field, attr.ib, or a type annotation.

If you’re confused by the many names, please check out On The Core API Names for clarification.

What follows is the API explanation, if you’d like a more hands-on introduction, have a look at attrs by Example.

As of version 21.3.0, attrs consists of two top-level package names:

• The classic attr that powered the venerable attr.s and attr.ib

• The modern attrs that only contains most modern APIs and relies on attrs.define and attrs.field to define your classes. Additionally it offers some attr APIs with nicer defaults (e.g. attrs.asdict). Using this namespace requires Python 3.6 or later.

The attrs namespace is built on top of attr which will never go away.

Core

Note

Please note that the attrs namespace has been added in version 21.3.0. Most of the objects are simply re-imported from attr. Therefore if a class, method, or function claims that it has been added in an older version, it is only available in the attr namespace.

attrs.NOTHING = NOTHING

Sentinel class to indicate the lack of a value when None is ambiguous.

_Nothing is a singleton. There is only ever one of it.

Changed in version 21.1.0: bool(NOTHING) is now False.

attrs.define(maybe_cls=None, *, these=None, repr=None, hash=None, init=None, slots=True, frozen=False, weakref_slot=True, str=False, auto_attribs=None, kw_only=False, cache_hash=False, auto_exc=True, eq=None, order=False, auto_detect=True, getstate_setstate=None, on_setattr=None, field_transformer=None, match_args=True)

Define an attrs class.

Differences to the classic attr.s that it uses underneath:

• Automatically detect whether or not auto_attribs should be True (c.f. auto_attribs parameter).

• If frozen is False, run converters and validators when setting an attribute by default.

• slots=True

  Caution

  Usually this has only upsides and few visible effects in everyday programming. But it can lead to some suprising behaviors, so please make sure to read slotted classes.

• auto_exc=True

• auto_detect=True

• order=False

• Some options that were only relevant on Python 2 or were kept around for backwards-compatibility have been removed.

Please note that these are all defaults and you can change them as you wish.

Parameters:

auto_attribs (Optional[bool]) –

If set to True or False, it behaves exactly like attr.s. If left None, attr.s will try to guess:

1. If any attributes are annotated and no unannotated attrs.fieldss are found, it assumes auto_attribs=True.

2. Otherwise it assumes auto_attribs=False and tries to collect attrs.fieldss.

For now, please refer to attr.s for the rest of the parameters.

New in version 20.1.0.

Changed in version 21.3.0: Converters are also run on_setattr.

attrs.mutable(same_as_define)

Alias for attrs.define.

New in version 20.1.0.

attrs.frozen(same_as_define)

Behaves the same as attrs.define but sets frozen=True and on_setattr=None.

New in version 20.1.0.

attrs.field(*, default=NOTHING, validator=None, repr=True, hash=None, init=True, metadata=None, converter=None, factory=None, kw_only=False, eq=None, order=None, on_setattr=None)

Identical to attr.ib, except keyword-only and with some arguments removed.

New in version 20.1.0.

attr.define()

Old import path for attrs.define.

attr.mutable()

Old import path for attrs.mutable.

attr.frozen()

Old import path for attrs.frozen.

attr.field()

Old import path for attrs.field.

class attrs.Attribute(name, default, validator, repr, cmp, hash, init, inherited, metadata=None, type=None, converter=None, kw_only=False, eq=None, eq_key=None, order=None, order_key=None, on_setattr=None)

Read-only representation of an attribute.

The class has all arguments of attr.ib (except for factory which is only syntactic sugar for default=Factory(...) plus the following:

• name (str): The name of the attribute.

• inherited (bool): Whether or not that attribute has been inherited from a base class.

• eq_key and order_key (typing.Callable or None): The callables that are used for comparing and ordering objects by this attribute, respectively. These are set by passing a callable to attr.ib’s eq, order, or cmp arguments. See also comparison customization.

Instances of this class are frequently used for introspection purposes like:

• fields returns a tuple of them.

• Validators get them passed as the first argument.

• The field transformer hook receives a list of them.

New in version 20.1.0: inherited

New in version 20.1.0: on_setattr

Changed in version 20.2.0: inherited is not taken into account for equality checks and hashing anymore.

New in version 21.1.0: eq_key and order_key

For the full version history of the fields, see attr.ib.

For example:

>>> import attr
>>> @attr.s
... class C:
...     x = attr.ib()
>>> attr.fields(C).x
Attribute(name='x', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None)

evolve(**changes)

Copy self and apply changes.

This works similarly to attr.evolve but that function does not work with Attribute.

It is mainly meant to be used for Automatic Field Transformation and Modification.

New in version 20.3.0.

attrs.make_class(name, attrs, bases=(<class 'object'>, ), **attributes_arguments)

A quick way to create a new class called name with attrs.

Parameters:

• name (str) – The name for the new class.

• attrs (list or dict) –

  A list of names or a dictionary of mappings of names to attributes.

  If attrs is a list or an ordered dict (dict on Python 3.6+, collections.OrderedDict otherwise), the order is deduced from the order of the names or attributes inside attrs. Otherwise the order of the definition of the attributes is used.

• bases (tuple) – Classes that the new class will subclass.

• attributes_arguments – Passed unmodified to attr.s.

Returns:

A new class with attrs.

Return type:

type

New in version 17.1.0: bases

Changed in version 18.1.0: If attrs is ordered, the order is retained.

This is handy if you want to programmatically create classes.

For example:

>>> C1 = attr.make_class("C1", ["x", "y"])
>>> C1(1, 2)
C1(x=1, y=2)
>>> C2 = attr.make_class("C2", {"x": attr.ib(default=42),
...                             "y": attr.ib(default=attr.Factory(list))})
>>> C2()
C2(x=42, y=[])

class attrs.Factory(factory, takes_self=False)

Stores a factory callable.

If passed as the default value to attrs.field, the factory is used to generate a new value.

Parameters:

• factory (callable) – A callable that takes either none or exactly one mandatory positional argument depending on takes_self.

• takes_self (bool) – Pass the partially initialized instance that is being initialized as a positional argument.

New in version 17.1.0: takes_self

For example:

>>> @attr.s
... class C:
...     x = attr.ib(default=attr.Factory(list))
...     y = attr.ib(default=attr.Factory(
...         lambda self: set(self.x),
...         takes_self=True)
...     )
>>> C()
C(x=[], y=set())
>>> C([1, 2, 3])
C(x=[1, 2, 3], y={1, 2, 3})

Classic

attr.NOTHING

Same as attrs.NOTHING.

attr.s(these=None, repr_ns=None, repr=None, cmp=None, hash=None, init=None, slots=False, frozen=False, weakref_slot=True, str=False, auto_attribs=False, kw_only=False, cache_hash=False, auto_exc=False, eq=None, order=None, auto_detect=False, collect_by_mro=False, getstate_setstate=None, on_setattr=None, field_transformer=None, match_args=True)

A class decorator that adds dunder-methods according to the specified attributes using attr.ib or the these argument.

Parameters:

• these (dict of str to attr.ib) –

  A dictionary of name to attr.ib mappings. This is useful to avoid the definition of your attributes within the class body because you can’t (e.g. if you want to add __repr__ methods to Django models) or don’t want to.

  If these is not None, attrs will not search the class body for attributes and will not remove any attributes from it.

  If these is an ordered dict (dict on Python 3.6+, collections.OrderedDict otherwise), the order is deduced from the order of the attributes inside these. Otherwise the order of the definition of the attributes is used.

• repr_ns (str) – When using nested classes, there’s no way in Python 2 to automatically detect that. Therefore it’s possible to set the namespace explicitly for a more meaningful repr output.

• auto_detect (bool) –

  Instead of setting the init, repr, eq, order, and hash arguments explicitly, assume they are set to True unless any of the involved methods for one of the arguments is implemented in the current class (i.e. it is not inherited from some base class).

  So for example by implementing __eq__ on a class yourself, attrs will deduce eq=False and will create neither __eq__ nor __ne__ (but Python classes come with a sensible __ne__ by default, so it should be enough to only implement __eq__ in most cases).

  Warning

  If you prevent attrs from creating the ordering methods for you (order=False, e.g. by implementing __le__), it becomes your responsibility to make sure its ordering is sound. The best way is to use the functools.total_ordering decorator.

  Passing True or False to init, repr, eq, order, cmp, or hash overrides whatever auto_detect would determine.

  auto_detect requires Python 3. Setting it True on Python 2 raises an attrs.exceptions.PythonTooOldError.

• repr (bool) – Create a __repr__ method with a human readable representation of attrs attributes..

• str (bool) – Create a __str__ method that is identical to __repr__. This is usually not necessary except for Exceptions.

• eq (Optional[bool]) –

  If True or None (default), add __eq__ and __ne__ methods that check two instances for equality.

  They compare the instances as if they were tuples of their attrs attributes if and only if the types of both classes are identical!

• order (Optional[bool]) – If True, add __lt__, __le__, __gt__, and __ge__ methods that behave like eq above and allow instances to be ordered. If None (default) mirror value of eq.

• cmp (Optional[bool]) – Setting cmp is equivalent to setting eq and order to the same value. Must not be mixed with eq or order.

• hash (Optional[bool]) –

  If None (default), the __hash__ method is generated according how eq and frozen are set.

  1. If both are True, attrs will generate a __hash__ for you.

  2. If eq is True and frozen is False, __hash__ will be set to None, marking it unhashable (which it is).

  3. If eq is False, __hash__ will be left untouched meaning the __hash__ method of the base class will be used (if base class is object, this means it will fall back to id-based hashing.).

  Although not recommended, you can decide for yourself and force attrs to create one (e.g. if the class is immutable even though you didn’t freeze it programmatically) by passing True or not. Both of these cases are rather special and should be used carefully.

  See our documentation on Hashing, Python’s documentation on object.__hash__, and the GitHub issue that led to the default behavior for more details.

• init (bool) –

  Create a __init__ method that initializes the attrs attributes. Leading underscores are stripped for the argument name. If a __attrs_pre_init__ method exists on the class, it will be called before the class is initialized. If a __attrs_post_init__ method exists on the class, it will be called after the class is fully initialized.

  If init is False, an __attrs_init__ method will be injected instead. This allows you to define a custom __init__ method that can do pre-init work such as super().__init__(), and then call __attrs_init__() and __attrs_post_init__().

• slots (bool) – Create a slotted class that’s more memory-efficient. Slotted classes are generally superior to the default dict classes, but have some gotchas you should know about, so we encourage you to read the glossary entry.

• frozen (bool) –

  Make instances immutable after initialization. If someone attempts to modify a frozen instance, attr.exceptions.FrozenInstanceError is raised.

  Note

  1. This is achieved by installing a custom __setattr__ method on your class, so you can’t implement your own.

  2. True immutability is impossible in Python.

  3. This does have a minor a runtime performance impact when initializing new instances. In other words: __init__ is slightly slower with frozen=True.

  4. If a class is frozen, you cannot modify self in __attrs_post_init__ or a self-written __init__. You can circumvent that limitation by using object.__setattr__(self, "attribute_name", value).

  5. Subclasses of a frozen class are frozen too.

• weakref_slot (bool) – Make instances weak-referenceable. This has no effect unless slots is also enabled.

• auto_attribs (bool) –

  If True, collect PEP 526-annotated attributes (Python 3.6 and later only) from the class body.

  In this case, you must annotate every field. If attrs encounters a field that is set to an attr.ib but lacks a type annotation, an attr.exceptions.UnannotatedAttributeError is raised. Use field_name: typing.Any = attr.ib(...) if you don’t want to set a type.

  If you assign a value to those attributes (e.g. x: int = 42), that value becomes the default value like if it were passed using attr.ib(default=42). Passing an instance of attrs.Factory also works as expected in most cases (see warning below).

  Attributes annotated as typing.ClassVar, and attributes that are neither annotated nor set to an attr.ib are ignored.

  Warning

  For features that use the attribute name to create decorators (e.g. validators), you still must assign attr.ib to them. Otherwise Python will either not find the name or try to use the default value to call e.g. validator on it.

  These errors can be quite confusing and probably the most common bug report on our bug tracker.

• kw_only (bool) – Make all attributes keyword-only (Python 3+) in the generated __init__ (if init is False, this parameter is ignored).

• cache_hash (bool) – Ensure that the object’s hash code is computed only once and stored on the object. If this is set to True, hashing must be either explicitly or implicitly enabled for this class. If the hash code is cached, avoid any reassignments of fields involved in hash code computation or mutations of the objects those fields point to after object creation. If such changes occur, the behavior of the object’s hash code is undefined.

• auto_exc (bool) –

  If the class subclasses BaseException (which implicitly includes any subclass of any exception), the following happens to behave like a well-behaved Python exceptions class:

  • the values for eq, order, and hash are ignored and the instances compare and hash by the instance’s ids (N.B. attrs will not remove existing implementations of __hash__ or the equality methods. It just won’t add own ones.),

  • all attributes that are either passed into __init__ or have a default value are additionally available as a tuple in the args attribute,

  • the value of str is ignored leaving __str__ to base classes.

• collect_by_mro (bool) –

  Setting this to True fixes the way attrs collects attributes from base classes. The default behavior is incorrect in certain cases of multiple inheritance. It should be on by default but is kept off for backward-compatibility.

  See issue #428 for more details.

• getstate_setstate (Optional[bool]) –

  Note

  This is usually only interesting for slotted classes and you should probably just set auto_detect to True.

  If True, __getstate__ and __setstate__ are generated and attached to the class. This is necessary for slotted classes to be pickleable. If left None, it’s True by default for slotted classes and False for dict classes.

  If auto_detect is True, and getstate_setstate is left None, and either __getstate__ or __setstate__ is detected directly on the class (i.e. not inherited), it is set to False (this is usually what you want).

• on_setattr (callable, or a list of callables, or None, or attrs.setters.NO_OP) –

  A callable that is run whenever the user attempts to set an attribute (either by assignment like i.x = 42 or by using setattr like setattr(i, "x", 42)). It receives the same arguments as validators: the instance, the attribute that is being modified, and the new value.

  If no exception is raised, the attribute is set to the return value of the callable.

  If a list of callables is passed, they’re automatically wrapped in an attrs.setters.pipe.

• field_transformer (Optional[callable]) – A function that is called with the original class object and all fields right before attrs finalizes the class. You can use this, e.g., to automatically add converters or validators to fields based on their types. See Automatic Field Transformation and Modification for more details.

• match_args (bool) – If True (default), set __match_args__ on the class to support PEP 634 (Structural Pattern Matching). It is a tuple of all non-keyword-only __init__ parameter names on Python 3.10 and later. Ignored on older Python versions.

New in version 16.0.0: slots

New in version 16.1.0: frozen

New in version 16.3.0: str

New in version 16.3.0: Support for __attrs_post_init__.

Changed in version 17.1.0: hash supports None as value which is also the default now.

New in version 17.3.0: auto_attribs

Changed in version 18.1.0: If these is passed, no attributes are deleted from the class body.

Changed in version 18.1.0: If these is ordered, the order is retained.

New in version 18.2.0: weakref_slot

Deprecated since version 18.2.0: __lt__, __le__, __gt__, and __ge__ now raise a DeprecationWarning if the classes compared are subclasses of each other. __eq and __ne__ never tried to compared subclasses to each other.

Changed in version 19.2.0: __lt__, __le__, __gt__, and __ge__ now do not consider subclasses comparable anymore.

New in version 18.2.0: kw_only

New in version 18.2.0: cache_hash

New in version 19.1.0: auto_exc

Deprecated since version 19.2.0: cmp Removal on or after 2021-06-01.

New in version 19.2.0: eq and order

New in version 20.1.0: auto_detect

New in version 20.1.0: collect_by_mro

New in version 20.1.0: getstate_setstate

New in version 20.1.0: on_setattr

New in version 20.3.0: field_transformer

Changed in version 21.1.0: init=False injects __attrs_init__

Changed in version 21.1.0: Support for __attrs_pre_init__

Changed in version 21.1.0: cmp undeprecated

New in version 21.3.0: match_args

Note

attrs also comes with a serious-business alias attr.attrs.

For example:

>>> import attr
>>> @attr.s
... class C:
...     _private = attr.ib()
>>> C(private=42)
C(_private=42)
>>> class D:
...     def __init__(self, x):
...         self.x = x
>>> D(1)
<D object at ...>
>>> D = attr.s(these={"x": attr.ib()}, init=False)(D)
>>> D(1)
D(x=1)
>>> @attr.s(auto_exc=True)
... class Error(Exception):
...     x = attr.ib()
...     y = attr.ib(default=42, init=False)
>>> Error("foo")
Error(x='foo', y=42)
>>> raise Error("foo")
Traceback (most recent call last):
   ...
Error: ('foo', 42)
>>> raise ValueError("foo", 42)   # for comparison
Traceback (most recent call last):
   ...
ValueError: ('foo', 42)

attr.ib(default=NOTHING, validator=None, repr=True, cmp=None, hash=None, init=True, metadata=None, type=None, converter=None, factory=None, kw_only=False, eq=None, order=None, on_setattr=None)

Create a new attribute on a class.

Warning

Does not do anything unless the class is also decorated with attr.s!

Parameters:

• default (Any value) –

  A value that is used if an attrs-generated __init__ is used and no value is passed while instantiating or the attribute is excluded using init=False.

  If the value is an instance of attrs.Factory, its callable will be used to construct a new value (useful for mutable data types like lists or dicts).

  If a default is not set (or set manually to attrs.NOTHING), a value must be supplied when instantiating; otherwise a TypeError will be raised.

  The default can also be set using decorator notation as shown below.

• factory (callable) – Syntactic sugar for default=attr.Factory(factory).

• validator (callable or a list of callables.) –

  callable that is called by attrs-generated __init__ methods after the instance has been initialized. They receive the initialized instance, the Attribute(), and the passed value.

  The return value is not inspected so the validator has to throw an exception itself.

  If a list is passed, its items are treated as validators and must all pass.

  Validators can be globally disabled and re-enabled using get_run_validators.

  The validator can also be set using decorator notation as shown below.

• repr (a bool or a callable to use a custom function.) – Include this attribute in the generated __repr__ method. If True, include the attribute; if False, omit it. By default, the built-in repr() function is used. To override how the attribute value is formatted, pass a callable that takes a single value and returns a string. Note that the resulting string is used as-is, i.e. it will be used directly instead of calling repr() (the default).

• eq (a bool or a callable.) – If True (default), include this attribute in the generated __eq__ and __ne__ methods that check two instances for equality. To override how the attribute value is compared, pass a callable that takes a single value and returns the value to be compared.

• order (a bool or a callable.) – If True (default), include this attributes in the generated __lt__, __le__, __gt__ and __ge__ methods. To override how the attribute value is ordered, pass a callable that takes a single value and returns the value to be ordered.

• cmp (a bool or a callable.) – Setting cmp is equivalent to setting eq and order to the same value. Must not be mixed with eq or order.

• hash (Optional[bool]) – Include this attribute in the generated __hash__ method. If None (default), mirror eq’s value. This is the correct behavior according the Python spec. Setting this value to anything else than None is discouraged.

• init (bool) – Include this attribute in the generated __init__ method. It is possible to set this to False and set a default value. In that case this attributed is unconditionally initialized with the specified default value or factory.

• converter (callable) – callable that is called by attrs-generated __init__ methods to convert attribute’s value to the desired format. It is given the passed-in value, and the returned value will be used as the new value of the attribute. The value is converted before being passed to the validator, if any.

• metadata – An arbitrary mapping, to be used by third-party components. See Metadata.

• type –

  The type of the attribute. In Python 3.6 or greater, the preferred method to specify the type is using a variable annotation (see PEP 526). This argument is provided for backward compatibility. Regardless of the approach used, the type will be stored on Attribute.type.

  Please note that attrs doesn’t do anything with this metadata by itself. You can use it as part of your own code or for static type checking.

• kw_only – Make this attribute keyword-only (Python 3+) in the generated __init__ (if init is False, this parameter is ignored).

• on_setattr (callable, or a list of callables, or None, or attrs.setters.NO_OP) – Allows to overwrite the on_setattr setting from attr.s. If left None, the on_setattr value from attr.s is used. Set to attrs.setters.NO_OP to run no setattr hooks for this attribute – regardless of the setting in attr.s.

New in version 15.2.0: convert

New in version 16.3.0: metadata

Changed in version 17.1.0: validator can be a list now.

Changed in version 17.1.0: hash is None and therefore mirrors eq by default.

New in version 17.3.0: type

Deprecated since version 17.4.0: convert

New in version 17.4.0: converter as a replacement for the deprecated convert to achieve consistency with other noun-based arguments.

New in version 18.1.0: factory=f is syntactic sugar for default=attr.Factory(f).

New in version 18.2.0: kw_only

Changed in version 19.2.0: convert keyword argument removed.

Changed in version 19.2.0: repr also accepts a custom callable.

Deprecated since version 19.2.0: cmp Removal on or after 2021-06-01.

New in version 19.2.0: eq and order

New in version 20.1.0: on_setattr

Changed in version 20.3.0: kw_only backported to Python 2

Changed in version 21.1.0: eq, order, and cmp also accept a custom callable

Changed in version 21.1.0: cmp undeprecated

Note

attrs also comes with a serious-business alias attr.attrib.

The object returned by attr.ib also allows for setting the default and the validator using decorators:

>>> @attr.s
... class C:
...     x = attr.ib()
...     y = attr.ib()
...     @x.validator
...     def _any_name_except_a_name_of_an_attribute(self, attribute, value):
...         if value < 0:
...             raise ValueError("x must be positive")
...     @y.default
...     def _any_name_except_a_name_of_an_attribute(self):
...         return self.x + 1
>>> C(1)
C(x=1, y=2)
>>> C(-1)
Traceback (most recent call last):
    ...
ValueError: x must be positive

Exceptions

All exceptions are available from both attr.exceptions and attrs.exceptions and are the same thing. That means that it doesn’t matter from from which namespace they’ve been raised and/or caught:

>>> import attrs, attr
>>> try:
...     raise attrs.exceptions.FrozenError()
... except attr.exceptions.FrozenError:
...     print("this works!")
this works!

exception attrs.exceptions.PythonTooOldError

It was attempted to use an attrs feature that requires a newer Python version.

New in version 18.2.0.

exception attrs.exceptions.FrozenError

A frozen/immutable instance or attribute have been attempted to be modified.

It mirrors the behavior of namedtuples by using the same error message and subclassing AttributeError.

New in version 20.1.0.

exception attrs.exceptions.FrozenInstanceError

A frozen instance has been attempted to be modified.

New in version 16.1.0.

exception attrs.exceptions.FrozenAttributeError

A frozen attribute has been attempted to be modified.

New in version 20.1.0.

exception attrs.exceptions.AttrsAttributeNotFoundError

An attrs function couldn’t find an attribute that the user asked for.

New in version 16.2.0.

exception attrs.exceptions.NotAnAttrsClassError

A non-attrs class has been passed into an attrs function.

New in version 16.2.0.

exception attrs.exceptions.DefaultAlreadySetError

A default has been set using attr.ib() and is attempted to be reset using the decorator.

New in version 17.1.0.

exception attrs.exceptions.UnannotatedAttributeError

A class with auto_attribs=True has an attr.ib() without a type annotation.

New in version 17.3.0.

exception attrs.exceptions.NotCallableError(msg, value)

A attr.ib() requiring a callable has been set with a value that is not callable.

New in version 19.2.0.

For example:

@attr.s(auto_attribs=True)
class C:
    x: int
    y = attr.ib()  # <- ERROR!

Helpers

attrs comes with a bunch of helper methods that make working with it easier:

attrs.cmp_using(eq=None, lt=None, le=None, gt=None, ge=None, require_same_type=True, class_name='Comparable')

Create a class that can be passed into attr.ib’s eq, order, and cmp arguments to customize field comparison.

The resulting class will have a full set of ordering methods if at least one of {lt, le, gt, ge} and eq are provided.

Parameters:

• eq (Optional[callable]) – callable used to evaluate equality of two objects.

• lt (Optional[callable]) – callable used to evaluate whether one object is less than another object.

• le (Optional[callable]) – callable used to evaluate whether one object is less than or equal to another object.

• gt (Optional[callable]) – callable used to evaluate whether one object is greater than another object.

• ge (Optional[callable]) – callable used to evaluate whether one object is greater than or equal to another object.

• require_same_type (bool) – When True, equality and ordering methods will return NotImplemented if objects are not of the same type.

• class_name (Optional[str]) – Name of class. Defaults to ‘Comparable’.

See Comparison for more details.

New in version 21.1.0.

attr.cmp_using()

Same as attrs.cmp_using.

attrs.fields(cls)

Return the tuple of attrs attributes for a class.

The tuple also allows accessing the fields by their names (see below for examples).

Parameters:

cls (type) – Class to introspect.

Raises:

• TypeError – If cls is not a class.

• attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

Return type:

tuple (with name accessors) of attrs.Attribute

Changed in version 16.2.0: Returned tuple allows accessing the fields by name.

For example:

>>> @attr.s
... class C:
...     x = attr.ib()
...     y = attr.ib()
>>> attrs.fields(C)
(Attribute(name='x', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None), Attribute(name='y', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None))
>>> attrs.fields(C)[1]
Attribute(name='y', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None)
>>> attrs.fields(C).y is attrs.fields(C)[1]
True

attr.fields()

Same as attrs.fields.

attrs.fields_dict(cls)

Return an ordered dictionary of attrs attributes for a class, whose keys are the attribute names.

Parameters:

cls (type) – Class to introspect.

Raises:

• TypeError – If cls is not a class.

• attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

Return type:

an ordered dict where keys are attribute names and values are attrs.Attributes. This will be a dict if it’s naturally ordered like on Python 3.6+ or an OrderedDict otherwise.

New in version 18.1.0.

For example:

>>> @attr.s
... class C:
...     x = attr.ib()
...     y = attr.ib()
>>> attrs.fields_dict(C)
{'x': Attribute(name='x', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None), 'y': Attribute(name='y', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None)}
>>> attr.fields_dict(C)['y']
Attribute(name='y', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None)
>>> attrs.fields_dict(C)['y'] is attrs.fields(C).y
True

attr.fields_dict()

Same as attrs.fields_dict.

attrs.has(cls)

Check whether cls is a class with attrs attributes.

Parameters:

cls (type) – Class to introspect.

Raises:

TypeError – If cls is not a class.

Return type:

bool

For example:

>>> @attr.s
... class C:
...     pass
>>> attr.has(C)
True
>>> attr.has(object)
False

attr.has()

Same as attrs.has.

attrs.resolve_types(cls, globalns=None, localns=None, attribs=None)

Resolve any strings and forward annotations in type annotations.

This is only required if you need concrete types in Attribute’s type field. In other words, you don’t need to resolve your types if you only use them for static type checking.

With no arguments, names will be looked up in the module in which the class was created. If this is not what you want, e.g. if the name only exists inside a method, you may pass globalns or localns to specify other dictionaries in which to look up these names. See the docs of typing.get_type_hints for more details.

Parameters:

• cls (type) – Class to resolve.

• globalns (Optional[dict]) – Dictionary containing global variables.

• localns (Optional[dict]) – Dictionary containing local variables.

• attribs (Optional[list]) – List of attribs for the given class. This is necessary when calling from inside a field_transformer since cls is not an attrs class yet.

Raises:

• TypeError – If cls is not a class.

• attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class and you didn’t pass any attribs.

• NameError – If types cannot be resolved because of missing variables.

Returns:

cls so you can use this function also as a class decorator. Please note that you have to apply it after attrs.define. That means the decorator has to come in the line before attrs.define.

New in version 20.1.0.

New in version 21.1.0: attribs

For example:

>>> import typing
>>> @attrs.define
... class A:
...     a: typing.List['A']
...     b: 'B'
...
>>> @attrs.define
... class B:
...     a: A
...
>>> attrs.fields(A).a.type
typing.List[ForwardRef('A')]
>>> attrs.fields(A).b.type
'B'
>>> attrs.resolve_types(A, globals(), locals())
<class 'A'>
>>> attrs.fields(A).a.type
typing.List[A]
>>> attrs.fields(A).b.type
<class 'B'>

attr.resolve_types()

Same as attrs.resolve_types.

attrs.asdict(inst, *, recurse=True, filter=None, value_serializer=None)

Same as attr.asdict, except that collections types are always retained and dict is always used as dict_factory.

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x: int
...     y: int
>>> attrs.asdict(C(1, C(2, 3)))
{'x': 1, 'y': {'x': 2, 'y': 3}}

attr.asdict(inst, recurse=True, filter=None, dict_factory=<class 'dict'>, retain_collection_types=False, value_serializer=None)

Return the attrs attribute values of inst as a dict.

Optionally recurse into other attrs-decorated classes.

Parameters:

• inst – Instance of an attrs-decorated class.

• recurse (bool) – Recurse into classes that are also attrs-decorated.

• filter (callable) – A callable whose return code determines whether an attribute or element is included (True) or dropped (False). Is called with the attrs.Attribute as the first argument and the value as the second argument.

• dict_factory (callable) – A callable to produce dictionaries from. For example, to produce ordered dictionaries instead of normal Python dictionaries, pass in collections.OrderedDict.

• retain_collection_types (bool) – Do not convert to list when encountering an attribute whose type is tuple or set. Only meaningful if recurse is True.

• value_serializer (Optional[callable]) – A hook that is called for every attribute or dict key/value. It receives the current instance, field and value and must return the (updated) value. The hook is run after the optional filter has been applied.

Return type:

return type of dict_factory

Raises:

attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

New in version 16.0.0: dict_factory

New in version 16.1.0: retain_collection_types

New in version 20.3.0: value_serializer

New in version 21.3.0: If a dict has a collection for a key, it is serialized as a tuple.

attrs.astuple(inst, *, recurse=True, filter=None)

Same as attr.astuple, except that collections types are always retained and tuple is always used as the tuple_factory.

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attr.field()
...     y = attr.field()
>>> attrs.astuple(C(1,2))
(1, 2)

attr.astuple(inst, recurse=True, filter=None, tuple_factory=<class 'tuple'>, retain_collection_types=False)

Return the attrs attribute values of inst as a tuple.

Optionally recurse into other attrs-decorated classes.

Parameters:

• inst – Instance of an attrs-decorated class.

• recurse (bool) – Recurse into classes that are also attrs-decorated.

• filter (callable) – A callable whose return code determines whether an attribute or element is included (True) or dropped (False). Is called with the attrs.Attribute as the first argument and the value as the second argument.

• tuple_factory (callable) – A callable to produce tuples from. For example, to produce lists instead of tuples.

• retain_collection_types (bool) – Do not convert to list or dict when encountering an attribute which type is tuple, dict or set. Only meaningful if recurse is True.

Return type:

return type of tuple_factory

Raises:

attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

New in version 16.2.0.

attrs includes some handy helpers for filtering the attributes in attrs.asdict and attrs.astuple:

attrs.filters.include(*what)

Include what.

Parameters:

what (list of type or attrs.Attributes) – What to include.

Return type:

callable

attrs.filters.exclude(*what)

Exclude what.

Parameters:

what (list of classes or attrs.Attributes.) – What to exclude.

Return type:

callable

attr.filters.include()

Same as attrs.filters.include.

attr.filters.exclude()

Same as attrs.filters.exclude.

See attrs.asdict() for examples.

All objects from attrs.filters are also available from attr.filters.

---

attrs.evolve(inst, **changes)

Create a new instance, based on inst with changes applied.

Parameters:

• inst – Instance of a class with attrs attributes.

• changes – Keyword changes in the new copy.

Returns:

A copy of inst with changes incorporated.

Raises:

• TypeError – If attr_name couldn’t be found in the class __init__.

• attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

New in version 17.1.0.

For example:

>>> @attrs.define
... class C:
...     x: int
...     y: int
>>> i1 = C(1, 2)
>>> i1
C(x=1, y=2)
>>> i2 = attrs.evolve(i1, y=3)
>>> i2
C(x=1, y=3)
>>> i1 == i2
False

evolve creates a new instance using __init__. This fact has several implications:

• private attributes should be specified without the leading underscore, just like in __init__.

• attributes with init=False can’t be set with evolve.

• the usual __init__ validators will validate the new values.

attr.evolve()

Same as attrs.evolve.

attrs.validate(inst)

Validate all attributes on inst that have a validator.

Leaves all exceptions through.

Parameters:

inst – Instance of a class with attrs attributes.

For example:

>>> @attrs.define(on_setattr=attrs.setters.NO_OP)
... class C:
...     x = attrs.field(validator=attrs.validators.instance_of(int))
>>> i = C(1)
>>> i.x = "1"
>>> attrs.validate(i)
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <class 'int'> (got '1' that is a <class 'str'>).", ...)

attr.validate()

Same as attrs.validate.

Validators can be globally disabled if you want to run them only in development and tests but not in production because you fear their performance impact:

attr.set_run_validators(run)

Set whether or not validators are run. By default, they are run.

Deprecated since version 21.3.0: It will not be removed, but it also will not be moved to new attrs namespace. Use attrs.validators.set_disabled() instead.

attr.get_run_validators()

Return whether or not validators are run.

Deprecated since version 21.3.0: It will not be removed, but it also will not be moved to new attrs namespace. Use attrs.validators.get_disabled() instead.

Validators

attrs comes with some common validators in the attrs.validators module. All objects from attrs.validators are also available from attr.validators.

attrs.validators.lt(val)

A validator that raises ValueError if the initializer is called with a number larger or equal to val.

Parameters:

val – Exclusive upper bound for values

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.lt(42))
>>> C(41)
C(x=41)
>>> C(42)
Traceback (most recent call last):
   ...
ValueError: ("'x' must be < 42: 42")

attrs.validators.le(val)

A validator that raises ValueError if the initializer is called with a number greater than val.

Parameters:

val – Inclusive upper bound for values

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attr.validators.le(42))
>>> C(42)
C(x=42)
>>> C(43)
Traceback (most recent call last):
   ...
ValueError: ("'x' must be <= 42: 43")

attrs.validators.ge(val)

A validator that raises ValueError if the initializer is called with a number smaller than val.

Parameters:

val – Inclusive lower bound for values

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.ge(42))
>>> C(42)
C(x=42)
>>> C(41)
Traceback (most recent call last):
   ...
ValueError: ("'x' must be => 42: 41")

attrs.validators.gt(val)

A validator that raises ValueError if the initializer is called with a number smaller or equal to val.

Parameters:

val – Exclusive lower bound for values

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attr.field(validator=attrs.validators.gt(42))
>>> C(43)
C(x=43)
>>> C(42)
Traceback (most recent call last):
   ...
ValueError: ("'x' must be > 42: 42")

attrs.validators.max_len(length)

A validator that raises ValueError if the initializer is called with a string or iterable that is longer than length.

Parameters:

length (int) – Maximum length of the string or iterable

New in version 21.3.0.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.max_len(4))
>>> C("spam")
C(x='spam')
>>> C("bacon")
Traceback (most recent call last):
   ...
ValueError: ("Length of 'x' must be <= 4: 5")

attrs.validators.min_len(length)

A validator that raises ValueError if the initializer is called with a string or iterable that is shorter than length.

Parameters:

length (int) – Minimum length of the string or iterable

New in version 22.1.0.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.min_len(1))
>>> C("bacon")
C(x='bacon')
>>> C("")
Traceback (most recent call last):
   ...
ValueError: ("Length of 'x' must be => 1: 0")

attrs.validators.instance_of(type)

A validator that raises a TypeError if the initializer is called with a wrong type for this particular attribute (checks are performed using isinstance therefore it’s also valid to pass a tuple of types).

Parameters:

type (type or tuple of types) – The type to check for.

Raises:

TypeError – With a human readable error message, the attribute (of type attrs.Attribute), the expected type, and the value it got.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.instance_of(int))
>>> C(42)
C(x=42)
>>> C("42")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None, kw_only=False), <type 'int'>, '42')
>>> C(None)
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got None that is a <type 'NoneType'>).", Attribute(name='x', default=NOTHING, validator=<instance_of validator for type <type 'int'>>, repr=True, cmp=True, hash=None, init=True, type=None, kw_only=False), <type 'int'>, None)

attrs.validators.in_(options)

A validator that raises a ValueError if the initializer is called with a value that does not belong in the options provided. The check is performed using value in options.

Parameters:

options (list, tuple, enum.Enum, …) – Allowed options.

Raises:

ValueError – With a human readable error message, the attribute (of type attrs.Attribute), the expected options, and the value it got.

New in version 17.1.0.

Changed in version 22.1.0: The ValueError was incomplete until now and only contained the human readable error message. Now it contains all the information that has been promised since 17.1.0.

For example:

>>> import enum
>>> class State(enum.Enum):
...     ON = "on"
...     OFF = "off"
>>> @attrs.define
... class C:
...     state = attrs.field(validator=attrs.validators.in_(State))
...     val = attrs.field(validator=attrs.validators.in_([1, 2, 3]))
>>> C(State.ON, 1)
C(state=<State.ON: 'on'>, val=1)
>>> C("on", 1)
Traceback (most recent call last):
   ...
ValueError: 'state' must be in <enum 'State'> (got 'on'), Attribute(name='state', default=NOTHING, validator=<in_ validator with options <enum 'State'>>, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None), <enum 'State'>, 'on')
>>> C(State.ON, 4)
Traceback (most recent call last):
...
ValueError: 'val' must be in [1, 2, 3] (got 4), Attribute(name='val', default=NOTHING, validator=<in_ validator with options [1, 2, 3]>, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False, inherited=False, on_setattr=None), [1, 2, 3], 4)

attrs.validators.provides(interface)

A validator that raises a TypeError if the initializer is called with an object that does not provide the requested interface (checks are performed using interface.providedBy(value) (see zope.interface).

Parameters:

interface (zope.interface.Interface) – The interface to check for.

Raises:

TypeError – With a human readable error message, the attribute (of type attrs.Attribute), the expected interface, and the value it got.

attrs.validators.and_(*validators)

A validator that composes multiple validators into one.

When called on a value, it runs all wrapped validators.

Parameters:

validators (callables) – Arbitrary number of validators.

New in version 17.1.0.

For convenience, it’s also possible to pass a list to attrs.field’s validator argument.

Thus the following two statements are equivalent:

x = attrs.field(validator=attrs.validators.and_(v1, v2, v3))
x = attrs.field(validator=[v1, v2, v3])

attrs.validators.optional(validator)

A validator that makes an attribute optional. An optional attribute is one which can be set to None in addition to satisfying the requirements of the sub-validator.

Parameters:

validator (callable or list of callables.) – A validator (or a list of validators) that is used for non-None values.

New in version 15.1.0.

Changed in version 17.1.0: validator can be a list of validators.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.optional(attr.validators.instance_of(int)))
>>> C(42)
C(x=42)
>>> C("42")
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, validator=<instance_of validator for type <type 'int'>>, type=None, kw_only=False), <type 'int'>, '42')
>>> C(None)
C(x=None)

attrs.validators.is_callable()

A validator that raises a attr.exceptions.NotCallableError if the initializer is called with a value for this particular attribute that is not callable.

New in version 19.1.0.

Raises:

attr.exceptions.NotCallableError – With a human readable error message containing the attribute (attrs.Attribute) name, and the value it got.

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.is_callable())
>>> C(isinstance)
C(x=<built-in function isinstance>)
>>> C("not a callable")
Traceback (most recent call last):
    ...
attr.exceptions.NotCallableError: 'x' must be callable (got 'not a callable' that is a <class 'str'>).

attrs.validators.matches_re(regex, flags=0, func=None)

A validator that raises ValueError if the initializer is called with a string that doesn’t match regex.

Parameters:

• regex – a regex string or precompiled pattern to match against

• flags (int) – flags that will be passed to the underlying re function (default 0)

• func (callable) – which underlying re function to call. Valid options are re.fullmatch, re.search, and re.match; the default None means re.fullmatch. For performance reasons, the pattern is always precompiled using re.compile.

New in version 19.2.0.

Changed in version 21.3.0: regex can be a pre-compiled pattern.

For example:

>>> @attrs.define
... class User:
...     email = attrs.field(validator=attrs.validators.matches_re(
...         "(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$)"))
>>> User(email="user@example.com")
User(email='user@example.com')
>>> User(email="user@example.com@test.com")
Traceback (most recent call last):
    ...
ValueError: ("'email' must match regex '(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\\\.[a-zA-Z0-9-.]+$)' ('user@example.com@test.com' doesn't)", Attribute(name='email', default=NOTHING, validator=<matches_re validator for pattern re.compile('(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\.[a-zA-Z0-9-.]+$)')>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), re.compile('(^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\\.[a-zA-Z0-9-.]+$)'), 'user@example.com@test.com')

attrs.validators.deep_iterable(member_validator, iterable_validator=None)

A validator that performs deep validation of an iterable.

Parameters:

• member_validator – Validator(s) to apply to iterable members

• iterable_validator – Validator to apply to iterable itself (optional)

New in version 19.1.0.

Raises:

TypeError – if any sub-validators fail

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.deep_iterable(
...     member_validator=attrs.validators.instance_of(int),
...     iterable_validator=attrs.validators.instance_of(list)
...     ))
>>> C(x=[1, 2, 3])
C(x=[1, 2, 3])
>>> C(x=set([1, 2, 3]))
Traceback (most recent call last):
    ...
TypeError: ("'x' must be <class 'list'> (got {1, 2, 3} that is a <class 'set'>).", Attribute(name='x', default=NOTHING, validator=<deep_iterable validator for <instance_of validator for type <class 'list'>> iterables of <instance_of validator for type <class 'int'>>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <class 'list'>, {1, 2, 3})
>>> C(x=[1, 2, "3"])
Traceback (most recent call last):
    ...
TypeError: ("'x' must be <class 'int'> (got '3' that is a <class 'str'>).", Attribute(name='x', default=NOTHING, validator=<deep_iterable validator for <instance_of validator for type <class 'list'>> iterables of <instance_of validator for type <class 'int'>>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <class 'int'>, '3')

attrs.validators.deep_mapping(key_validator, value_validator, mapping_validator=None)

A validator that performs deep validation of a dictionary.

Parameters:

• key_validator – Validator to apply to dictionary keys

• value_validator – Validator to apply to dictionary values

• mapping_validator – Validator to apply to top-level mapping attribute (optional)

New in version 19.1.0.

Raises:

TypeError – if any sub-validators fail

For example:

>>> @attrs.define
... class C:
...     x = attrs.field(validator=attrs.validators.deep_mapping(
...         key_validator=attrs.validators.instance_of(str),
...         value_validator=attrs.validators.instance_of(int),
...         mapping_validator=attrs.validators.instance_of(dict)
...     ))
>>> C(x={"a": 1, "b": 2})
C(x={'a': 1, 'b': 2})
>>> C(x=None)
Traceback (most recent call last):
    ...
TypeError: ("'x' must be <class 'dict'> (got None that is a <class 'NoneType'>).", Attribute(name='x', default=NOTHING, validator=<deep_mapping validator for objects mapping <instance_of validator for type <class 'str'>> to <instance_of validator for type <class 'int'>>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <class 'dict'>, None)
>>> C(x={"a": 1.0, "b": 2})
Traceback (most recent call last):
    ...
TypeError: ("'x' must be <class 'int'> (got 1.0 that is a <class 'float'>).", Attribute(name='x', default=NOTHING, validator=<deep_mapping validator for objects mapping <instance_of validator for type <class 'str'>> to <instance_of validator for type <class 'int'>>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <class 'int'>, 1.0)
>>> C(x={"a": 1, 7: 2})
Traceback (most recent call last):
    ...
TypeError: ("'x' must be <class 'str'> (got 7 that is a <class 'int'>).", Attribute(name='x', default=NOTHING, validator=<deep_mapping validator for objects mapping <instance_of validator for type <class 'str'>> to <instance_of validator for type <class 'int'>>>, repr=True, cmp=True, hash=None, init=True, metadata=mappingproxy({}), type=None, converter=None, kw_only=False), <class 'str'>, 7)

Validators can be both globally and locally disabled:

attrs.validators.set_disabled(disabled)

Globally disable or enable running validators.

By default, they are run.

Parameters:

disabled (bool) – If True, disable running all validators.

Warning

This function is not thread-safe!

New in version 21.3.0.

attrs.validators.get_disabled()

Return a bool indicating whether validators are currently disabled or not.

Returns:

True if validators are currently disabled.

Return type:

bool

New in version 21.3.0.

attrs.validators.disabled()

Context manager that disables running validators within its context.

Warning

This context manager is not thread-safe!

New in version 21.3.0.

Converters

All objects from attrs.converters are also available from attr.converters.

attrs.converters.pipe(*converters)

A converter that composes multiple converters into one.

When called on a value, it runs all wrapped converters, returning the last value.

Type annotations will be inferred from the wrapped converters’, if they have any.

Parameters:

converters (callables) – Arbitrary number of converters.

New in version 20.1.0.

For convenience, it’s also possible to pass a list to attr.ib’s converter argument.

Thus the following two statements are equivalent:

x = attr.ib(converter=attr.converter.pipe(c1, c2, c3))
x = attr.ib(converter=[c1, c2, c3])

attrs.converters.optional(converter)

A converter that allows an attribute to be optional. An optional attribute is one which can be set to None.

Type annotations will be inferred from the wrapped converter’s, if it has any.

Parameters:

converter (callable) – the converter that is used for non-None values.

New in version 17.1.0.

For example:

>>> @attr.s
... class C:
...     x = attr.ib(converter=attr.converters.optional(int))
>>> C(None)
C(x=None)
>>> C(42)
C(x=42)

attrs.converters.default_if_none(default=NOTHING, factory=None)

A converter that allows to replace None values by default or the result of factory.

Parameters:

• default – Value to be used if None is passed. Passing an instance of attrs.Factory is supported, however the takes_self option is not.

• factory (callable) – A callable that takes no parameters whose result is used if None is passed.

Raises:

• TypeError – If neither default or factory is passed.

• TypeError – If both default and factory are passed.

• ValueError – If an instance of attrs.Factory is passed with takes_self=True.

New in version 18.2.0.

For example:

>>> @attr.s
... class C:
...     x = attr.ib(
...         converter=attr.converters.default_if_none("")
...     )
>>> C(None)
C(x='')

attrs.converters.to_bool(val)

Convert “boolean” strings (e.g., from env. vars.) to real booleans.

Values mapping to True:

• True

• "true" / "t"

• "yes" / "y"

• "on"

• "1"

• 1

Values mapping to False:

• False

• "false" / "f"

• "no" / "n"

• "off"

• "0"

• 0

Raises:

ValueError – for any other value.

New in version 21.3.0.

For example:

>>> @attr.s
... class C:
...     x = attr.ib(
...         converter=attr.converters.to_bool
...     )
>>> C("yes")
C(x=True)
>>> C(0)
C(x=False)
>>> C("foo")
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
ValueError: Cannot convert value to bool: foo

Setters

These are helpers that you can use together with attrs.define’s and attrs.fields’s on_setattr arguments. All setters in attrs.setters are also available from attr.setters.

attrs.setters.frozen(_, __, ___)

Prevent an attribute to be modified.

New in version 20.1.0.

attrs.setters.validate(instance, attrib, new_value)

Run attrib’s validator on new_value if it has one.

New in version 20.1.0.

attrs.setters.convert(instance, attrib, new_value)

Run attrib’s converter – if it has one – on new_value and return the result.

New in version 20.1.0.

attrs.setters.pipe(*setters)

Run all setters and return the return value of the last one.

New in version 20.1.0.

attrs.setters.NO_OP

Sentinel for disabling class-wide on_setattr hooks for certain attributes.

Does not work in attrs.setters.pipe or within lists.

New in version 20.1.0.

For example, only x is frozen here:

>>> @attrs.define(on_setattr=attr.setters.frozen)
... class C:
...     x = attr.field()
...     y = attr.field(on_setattr=attr.setters.NO_OP)
>>> c = C(1, 2)
>>> c.y = 3
>>> c.y
3
>>> c.x = 4
Traceback (most recent call last):
    ...
attrs.exceptions.FrozenAttributeError: ()

N.B. Please use attrs.define’s frozen argument (or attrs.frozen) to freeze whole classes; it is more efficient.

Deprecated APIs

To help you write backward compatible code that doesn’t throw warnings on modern releases, the attr module has an __version_info__ attribute as of version 19.2.0. It behaves similarly to sys.version_info and is an instance of VersionInfo:

class attr.VersionInfo(year: int, minor: int, micro: int, releaselevel: str)

A version object that can be compared to tuple of length 1–4:

>>> attr.VersionInfo(19, 1, 0, "final")  <= (19, 2)
True
>>> attr.VersionInfo(19, 1, 0, "final") < (19, 1, 1)
True
>>> vi = attr.VersionInfo(19, 2, 0, "final")
>>> vi < (19, 1, 1)
False
>>> vi < (19,)
False
>>> vi == (19, 2,)
True
>>> vi == (19, 2, 1)
False

New in version 19.2.

With its help you can write code like this:

>>> if getattr(attr, "__version_info__", (0,)) >= (19, 2):
...     cmp_off = {"eq": False}
... else:
...     cmp_off = {"cmp": False}
>>> cmp_off == {"eq":  False}
True
>>> @attr.s(**cmp_off)
... class C:
...     pass

---

The serious-business aliases used to be called attr.attributes and attr.attr. There are no plans to remove them but they shouldn’t be used in new code.

attr.assoc(inst, **changes)

Copy inst and apply changes.

Parameters:

• inst – Instance of a class with attrs attributes.

• changes – Keyword changes in the new copy.

Returns:

A copy of inst with changes incorporated.

Raises:

• attr.exceptions.AttrsAttributeNotFoundError – If attr_name couldn’t be found on cls.

• attr.exceptions.NotAnAttrsClassError – If cls is not an attrs class.

Deprecated since version 17.1.0: Use attrs.evolve instead if you can. This function will not be removed du to the slightly different approach compared to attrs.evolve.

Extending

Each attrs-decorated class has a __attrs_attrs__ class attribute. It’s a tuple of attrs.Attribute carrying metadata about each attribute.

So it is fairly simple to build your own decorators on top of attrs:

>>> from attr import define
>>> def print_attrs(cls):
...     print(cls.__attrs_attrs__)
...     return cls
>>> @print_attrs
... @define
... class C:
...     a: int
(Attribute(name='a', default=NOTHING, validator=None, repr=True, eq=True, eq_key=None, order=True, order_key=None, hash=None, init=True, metadata=mappingproxy({}), type=<class 'int'>, converter=None, kw_only=False, inherited=False, on_setattr=None),)

Warning

The attrs.define/attr.s decorator must be applied first because it puts __attrs_attrs__ in place! That means that is has to come after your decorator because:

@a
@b
def f():
   pass

is just syntactic sugar for:

def original_f():
   pass

f = a(b(original_f))

Wrapping the Decorator

A more elegant way can be to wrap attrs altogether and build a class DSL on top of it.

An example for that is the package environ-config that uses attrs under the hood to define environment-based configurations declaratively without exposing attrs APIs at all.

Another common use case is to overwrite attrs’s defaults.

Mypy

Unfortunately, decorator wrapping currently confuses mypy’s attrs plugin. At the moment, the best workaround is to hold your nose, write a fake mypy plugin, and mutate a bunch of global variables:

from mypy.plugin import Plugin
from mypy.plugins.attrs import (
   attr_attrib_makers,
   attr_class_makers,
   attr_dataclass_makers,
)

# These work just like `attr.dataclass`.
attr_dataclass_makers.add("my_module.method_looks_like_attr_dataclass")

# This works just like `attr.s`.
attr_class_makers.add("my_module.method_looks_like_attr_s")

# These are our `attr.ib` makers.
attr_attrib_makers.add("my_module.method_looks_like_attrib")

class MyPlugin(Plugin):
    # Our plugin does nothing but it has to exist so this file gets loaded.
    pass


def plugin(version):
    return MyPlugin

Then tell mypy about your plugin using your project’s mypy.ini:

[mypy]
plugins=<path to file>

Warning

Please note that it is currently impossible to let mypy know that you’ve changed defaults like eq or order. You can only use this trick to tell mypy that a class is actually an attrs class.

Pyright

Generic decorator wrapping is supported in pyright via their dataclass_transform specification.

For a custom wrapping of the form:

def custom_define(f):
    return attr.define(f)

This is implemented via a __dataclass_transform__ type decorator in the custom extension’s .pyi of the form:

def __dataclass_transform__(
    *,
    eq_default: bool = True,
    order_default: bool = False,
    kw_only_default: bool = False,
    field_descriptors: Tuple[Union[type, Callable[..., Any]], ...] = (()),
) -> Callable[[_T], _T]: ...

@__dataclass_transform__(field_descriptors=(attr.attrib, attr.field))
def custom_define(f): ...

Warning

dataclass_transform is supported provisionally as of pyright 1.1.135.

Both the pyright dataclass_transform specification and attrs implementation may change in future versions.

Types

attrs offers two ways of attaching type information to attributes:

• PEP 526 annotations on Python 3.6 and later,

• and the type argument to attr.ib.

This information is available to you:

>>> from attr import attrib, define, field, fields
>>> @define
... class C:
...     x: int = field()
...     y = attrib(type=str)
>>> fields(C).x.type
<class 'int'>
>>> fields(C).y.type
<class 'str'>

Currently, attrs doesn’t do anything with this information but it’s very useful if you’d like to write your own validators or serializers!

Metadata

If you’re the author of a third-party library with attrs integration, you may want to take advantage of attribute metadata.

Here are some tips for effective use of metadata:

• Try making your metadata keys and values immutable. This keeps the entire Attribute instances immutable too.

• To avoid metadata key collisions, consider exposing your metadata keys from your modules.:

  from mylib import MY_METADATA_KEY
  
  @define
  class C:
    x = field(metadata={MY_METADATA_KEY: 1})
  

  Metadata should be composable, so consider supporting this approach even if you decide implementing your metadata in one of the following ways.

• Expose field wrappers for your specific metadata. This is a more graceful approach if your users don’t require metadata from other libraries.

  >>> from attr import fields, NOTHING
  >>> MY_TYPE_METADATA = '__my_type_metadata'
  >>>
  >>> def typed(
  ...     cls, default=NOTHING, validator=None, repr=True,
  ...     eq=True, order=None, hash=None, init=True, metadata=None,
  ...     converter=None
  ... ):
  ...     metadata = metadata or {}
  ...     metadata[MY_TYPE_METADATA] = cls
  ...     return field(
  ...         default=default, validator=validator, repr=repr,
  ...         eq=eq, order=order, hash=hash, init=init,
  ...         metadata=metadata, converter=converter
  ...     )
  >>>
  >>> @define
  ... class C:
  ...     x: int = typed(int, default=1, init=False)
  >>> fields(C).x.metadata[MY_TYPE_METADATA]
  <class 'int'>
  

Automatic Field Transformation and Modification

attrs allows you to automatically modify or transform the class’ fields while the class is being created. You do this by passing a field_transformer hook to attr.define (and its friends). Its main purpose is to automatically add converters to attributes based on their type to aid the development of API clients and other typed data loaders.

This hook must have the following signature:

your_hook(cls: type, fields: list[attrs.Attribute]) → list[attrs.Attribute]

• cls is your class right before it is being converted into an attrs class. This means it does not yet have the __attrs_attrs__ attribute.

• fields is a list of all attrs.Attribute instances that will later be set to __attrs_attrs__. You can modify these attributes any way you want: You can add converters, change types, and even remove attributes completely or create new ones!

For example, let’s assume that you really don’t like floats:

>>> def drop_floats(cls, fields):
...     return [f for f in fields if f.type not in {float, 'float'}]
...
>>> @frozen(field_transformer=drop_floats)
... class Data:
...     a: int
...     b: float
...     c: str
...
>>> Data(42, "spam")
Data(a=42, c='spam')

A more realistic example would be to automatically convert data that you, e.g., load from JSON:

>>> from datetime import datetime
>>>
>>> def auto_convert(cls, fields):
...     results = []
...     for field in fields:
...         if field.converter is not None:
...             results.append(field)
...             continue
...         if field.type in {datetime, 'datetime'}:
...             converter = (lambda d: datetime.fromisoformat(d) if isinstance(d, str) else d)
...         else:
...             converter = None
...         results.append(field.evolve(converter=converter))
...     return results
...
>>> @frozen(field_transformer=auto_convert)
... class Data:
...     a: int
...     b: str
...     c: datetime
...
>>> from_json = {"a": 3, "b": "spam", "c": "2020-05-04T13:37:00"}
>>> Data(**from_json)  # ****
Data(a=3, b='spam', c=datetime.datetime(2020, 5, 4, 13, 37))

Customize Value Serialization in asdict()

attrs allows you to serialize instances of attrs classes to dicts using the attrs.asdict function. However, the result can not always be serialized since most data types will remain as they are:

>>> import json
>>> import datetime
>>> from attrs import asdict
>>>
>>> @frozen
... class Data:
...    dt: datetime.datetime
...
>>> data = asdict(Data(datetime.datetime(2020, 5, 4, 13, 37)))
>>> data
{'dt': datetime.datetime(2020, 5, 4, 13, 37)}
>>> json.dumps(data)
Traceback (most recent call last):
  ...
TypeError: Object of type datetime is not JSON serializable

To help you with this, attr.asdict allows you to pass a value_serializer hook. It has the signature

your_hook(inst: type, field: attrs.Attribute, value: Any) → Any

>>> from attr import asdict
>>> def serialize(inst, field, value):
...     if isinstance(value, datetime.datetime):
...         return value.isoformat()
...     return value
...
>>> data = asdict(
...     Data(datetime.datetime(2020, 5, 4, 13, 37)),
...     value_serializer=serialize,
... )
>>> data
{'dt': '2020-05-04T13:37:00'}
>>> json.dumps(data)
'{"dt": "2020-05-04T13:37:00"}'

---

How Does It Work?

Boilerplate

attrs certainly isn’t the first library that aims to simplify class definition in Python. But its declarative approach combined with no runtime overhead lets it stand out.

Once you apply the @attrs.define (or @attr.s) decorator to a class, attrs searches the class object for instances of attr.ibs. Internally they’re a representation of the data passed into attr.ib along with a counter to preserve the order of the attributes. Alternatively, it’s possible to define them using Type Annotations.

In order to ensure that subclassing works as you’d expect it to work, attrs also walks the class hierarchy and collects the attributes of all base classes. Please note that attrs does not call super() ever. It will write dunder methods to work on all of those attributes which also has performance benefits due to fewer function calls.

Once attrs knows what attributes it has to work on, it writes the requested dunder methods and – depending on whether you wish to have a dict or slotted class – creates a new class for you (slots=True) or attaches them to the original class (slots=False). While creating new classes is more elegant, we’ve run into several edge cases surrounding metaclasses that make it impossible to go this route unconditionally.

To be very clear: if you define a class with a single attribute without a default value, the generated __init__ will look exactly how you’d expect:

>>> import inspect
>>> from attr import define
>>> @define
... class C:
...     x: int
>>> print(inspect.getsource(C.__init__))
def __init__(self, x):
    self.x = x

No magic, no meta programming, no expensive introspection at runtime.

---

Everything until this point happens exactly once when the class is defined. As soon as a class is done, it’s done. And it’s just a regular Python class like any other, except for a single __attrs_attrs__ attribute that attrs uses internally. Much of the information is accessible via attrs.fields and other functions which can be used for introspection or for writing your own tools and decorators on top of attrs (like attrs.asdict).

And once you start instantiating your classes, attrs is out of your way completely.

This static approach was very much a design goal of attrs and what I strongly believe makes it distinct.

Immutability

In order to give you immutability, attrs will attach a __setattr__ method to your class that raises an attrs.exceptions.FrozenInstanceError whenever anyone tries to set an attribute.

The same is true if you choose to freeze individual attributes using the attrs.setters.frozen on_setattr hook – except that the exception becomes attrs.exceptions.FrozenAttributeError.

Both errors subclass attrs.exceptions.FrozenError.

---

Depending on whether a class is a dict class or a slotted class, attrs uses a different technique to circumvent that limitation in the __init__ method.

Once constructed, frozen instances don’t differ in any way from regular ones except that you cannot change its attributes.

Dict Classes

Dict classes – i.e. regular classes – simply assign the value directly into the class’ eponymous __dict__ (and there’s nothing we can do to stop the user to do the same).

The performance impact is negligible.

Slotted Classes

Slotted classes are more complicated. Here it uses (an aggressively cached) object.__setattr__() to set your attributes. This is (still) slower than a plain assignment:

$ pyperf timeit --rigorous \
      -s "import attr; C = attr.make_class('C', ['x', 'y', 'z'], slots=True)" \
      "C(1, 2, 3)"
.........................................
Mean +- std dev: 228 ns +- 18 ns

$ pyperf timeit --rigorous \
      -s "import attr; C = attr.make_class('C', ['x', 'y', 'z'], slots=True, frozen=True)" \
      "C(1, 2, 3)"
.........................................
Mean +- std dev: 450 ns +- 26 ns

So on a laptop computer the difference is about 230 nanoseconds (1 second is 1,000,000,000 nanoseconds). It’s certainly something you’ll feel in a hot loop but shouldn’t matter in normal code. Pick what’s more important to you.

Summary

You should avoid instantiating lots of frozen slotted classes (i.e. @frozen) in performance-critical code.

Frozen dict classes have barely a performance impact, unfrozen slotted classes are even faster than unfrozen dict classes (i.e. regular classes).

On The Core API Names

You may be surprised seeing attrs classes being created using attrs.define and with type annotated fields, instead of attr.s and attr.ib().

Or, you wonder why the web and talks are full of this weird attr.s and attr.ib – including people having strong opinions about it and using attr.attrs and attr.attrib instead.

And what even is attr.dataclass that’s not documented but commonly used!?

TL;DR

We recommend our modern APIs for new code:

• attrs.define() to define a new class,

• attrs.mutable() is an alias for attrs.define(),

• attrs.frozen() is an alias for define(frozen=True)

• and attrs.field() to define an attribute.

They have been added in attrs 20.1.0, they are expressive, and they have modern defaults like slots and type annotation awareness switched on by default. They are only available in Python 3.6 and later. Sometimes they’re referred to as next-generation or NG APIs. As of attrs 21.3.0 you can also import them from the attrs package namespace.

The traditional APIs attr.s / attr.ib, their serious-business aliases attr.attrs / attr.attrib, and the never-documented, but popular attr.dataclass easter egg will stay forever.

attrs will never force you to use type annotations.

A Short History Lesson

At this point, attrs is an old project. It had its first release in April 2015 – back when most Python code was on Python 2.7 and Python 3.4 was the first Python 3 release that showed promise. attrs was always Python 3-first, but type annotations came only into Python 3.5 that was released in September 2015 and were largely ignored until years later.

At this time, if you didn’t want to implement all the dunder methods, the most common way to create a class with some attributes on it was to subclass a collections.namedtuple, or one of the many hacks that allowed you to access dictionary keys using attribute lookup.

But attrs history goes even a bit further back, to the now-forgotten characteristic that came out in May 2014 and already used a class decorator, but was overall too unergonomic.

In the wake of all of that, glyph and Hynek came together on IRC and brainstormed how to take the good ideas of characteristic, but make them easier to use and read. At this point the plan was not to make attrs what it is now – a flexible class building kit. All we wanted was an ergonomic little library to succinctly define classes with attributes.

Under the impression of the unwieldy characteristic name, we went to the other side and decided to make the package name part of the API, and keep the API functions very short. This led to the infamous attr.s and attr.ib which some found confusing and pronounced it as “attr dot s” or used a singular @s as the decorator. But it was really just a way to say attrs and attrib[1].

Some people hated this cutey API from day one, which is why we added aliases for them that we called serious business: @attr.attrs and attr.attrib(). Fans of them usually imported the names and didn’t use the package name in the first place. Unfortunately, the attr package name started creaking the moment we added attr.Factory, since it couldn’t be morphed into something meaningful in any way. A problem that grew worse over time, as more APIs and even modules were added.

But overall, attrs in this shape was a huge success – especially after glyph’s blog post The One Python Library Everyone Needs in August 2016 and pytest adopting it.

Being able to just write:

@attr.s
class Point:
    x = attr.ib()
    y = attr.ib()

was a big step for those who wanted to write small, focused classes.

Dataclasses Enter The Arena

A big change happened in May 2017 when Hynek sat down with Guido van Rossum and Eric V. Smith at PyCon US 2017.

Type annotations for class attributes have just landed in Python 3.6 and Guido felt like it would be a good mechanic to introduce something similar to attrs to the Python standard library. The result, of course, was PEP 557[2] which eventually became the dataclasses module in Python 3.7.

attrs at this point was lucky to have several people on board who were also very excited about type annotations and helped implement it; including a Mypy plugin. And so it happened that attrs shipped the new method of defining classes more than half a year before Python 3.7 – and thus dataclasses – were released.

---

Due to backward-compatibility concerns, this feature is off by default in the attr.s decorator and has to be activated using @attr.s(auto_attribs=True), though. As a little easter egg and to save ourselves some typing, we’ve also added an alias called attr.dataclasses that just set auto_attribs=True. It was never documented, but people found it and used it and loved it.

Over the next months and years it became clear that type annotations have become the popular way to define classes and their attributes. However, it has also become clear that some people viscerally hate type annotations. We’re determined to serve both.

attrs TNG

Over its existence, attrs never stood still. But since we also greatly care about backward compatibility and not breaking our users’ code, many features and niceties have to be manually activated.

That is not only annoying, it also leads to the problem that many of attrs’s users don’t even know what it can do for them. We’ve spent years alone explaining that defining attributes using type annotations is in no way unique to dataclasses.

Finally we’ve decided to take the Go route: instead of fiddling with the old APIs – whose names felt anachronistic anyway – we’d define new ones, with better defaults. So in July 2018, we looked for better names and came up with attr.define, attr.field, and friends. Then in January 2019, we started looking for inconvenient defaults that we now could fix without any repercussions.

These APIs proved to be very popular, so we’ve finally changed the documentation to them in November of 2021.

All of this took way too long, of course. One reason is the COVID-19 pandemic, but also our fear to fumble this historic chance to fix our APIs.

Finally, in December 2021, we’ve added the attrs package namespace.

We hope you like the result:

from attrs import define

@define
class Point:
    x: int
    y: int

[1]

We considered calling the PyPI package just attr too, but the name was already taken by an ostensibly inactive package on PyPI.

[2]

The highly readable PEP also explains why attrs wasn’t just added to the standard library. Don’t believe the myths and rumors.

Glossary

dunder methods

“Dunder” is a contraction of “double underscore”.

It’s methods like __init__ or __eq__ that are sometimes also called magic methods or it’s said that they implement an object protocol.

In spoken form, you’d call __init__ just “dunder init”.

Its first documented use is a mailing list posting by Mark Jackson from 2002.

dict classes

A regular class whose attributes are stored in the object.__dict__ attribute of every single instance. This is quite wasteful especially for objects with very few data attributes and the space consumption can become significant when creating large numbers of instances.

This is the type of class you get by default both with and without attrs (except with the next APIs attrs.define(), attrs.mutable(), and attrs.frozen()).

slotted classes

A class whose instances have no object.__dict__ attribute and define their attributes in a object.__slots__ attribute instead. In attrs, they are created by passing slots=True to @attr.s (and are on by default in attrs.define()/attrs.mutable()/attrs.frozen()).

Their main advantage is that they use less memory on CPython [1] and are slightly faster.

However they also come with several possibly surprising gotchas:

• Slotted classes don’t allow for any other attribute to be set except for those defined in one of the class’ hierarchies __slots__:

  >>> from attr import define
  >>> @define
  ... class Coordinates:
  ...     x: int
  ...     y: int
  ...
  >>> c = Coordinates(x=1, y=2)
  >>> c.z = 3
  Traceback (most recent call last):
      ...
  AttributeError: 'Coordinates' object has no attribute 'z'
  

• Slotted classes can inherit from other classes just like non-slotted classes, but some of the benefits of slotted classes are lost if you do that. If you must inherit from other classes, try to inherit only from other slotted classes.

• However, it’s not possible to inherit from more than one class that has attributes in __slots__ (you will get an TypeError: multiple bases have instance lay-out conflict).

• It’s not possible to monkeypatch methods on slotted classes. This can feel limiting in test code, however the need to monkeypatch your own classes is usually a design smell.

  If you really need to monkeypatch an instance in your tests, but don’t want to give up on the advantages of slotted classes in production code, you can always subclass a slotted class as a dict class with no further changes and all the limitations go away:

  >>> import attr, unittest.mock
  >>> @define
  ... class Slotted:
  ...     x: int
  ...
  ...     def method(self):
  ...         return self.x
  >>> s = Slotted(42)
  >>> s.method()
  42
  >>> with unittest.mock.patch.object(s, "method", return_value=23):
  ...     pass
  Traceback (most recent call last):
     ...
  AttributeError: 'Slotted' object attribute 'method' is read-only
  >>> @define(slots=False)
  ... class Dicted(Slotted):
  ...     pass
  >>> d = Dicted(42)
  >>> d.method()
  42
  >>> with unittest.mock.patch.object(d, "method", return_value=23):
  ...     assert 23 == d.method()
  

• Slotted classes must implement __getstate__ and __setstate__ to be serializable with pickle protocol 0 and 1. Therefore, attrs creates these methods automatically for slots=True classes.

  Note

  If the @attr.s(slots=True) decorated class already implements the __getstate__ and __setstate__ methods, they will be overwritten by attrs autogenerated implementation by default.

  This can be avoided by setting @attr.s(getstate_setstate=False) or by setting @attr.s(auto_detect=True).

  Also, think twice before using pickle.

• Slotted classes are weak-referenceable by default. This can be disabled in CPython by passing weakref_slot=False to @attr.s [2].

• Since it’s currently impossible to make a class slotted after it’s been created, attrs has to replace your class with a new one. While it tries to do that as graciously as possible, certain metaclass features like object.__init_subclass__ do not work with slotted classes.

• The class.__subclasses__ attribute needs a garbage collection run (which can be manually triggered using gc.collect), for the original class to be removed. See issue #407 for more details.

[1]

On PyPy, there is no memory advantage in using slotted classes.

[2]

On PyPy, slotted classes are naturally weak-referenceable so weakref_slot=False has no effect.

License and Credits

attrs is licensed under the MIT license. The full license text can be also found in the source code repository.

Credits

attrs is written and maintained by Hynek Schlawack.

The development is kindly supported by Variomedia AG.

A full list of contributors can be found in GitHub’s overview.

It’s the spiritual successor of characteristic and aspires to fix some of it clunkiness and unfortunate decisions. Both were inspired by Twisted’s FancyEqMixin but both are implemented using class decorators because subclassing is bad for you, m’kay?

Changelog

Versions follow CalVer with a strict backwards-compatibility policy.

The first number of the version is the year. The second number is incremented with each release, starting at 1 for each year. The third number is when we need to start branches for older releases (only for emergencies).

Put simply, you shouldn’t ever be afraid to upgrade attrs if you’re only using its public APIs. Whenever there is a need to break compatibility, it is announced here in the changelog, and raises a DeprecationWarning for a year (if possible) before it’s finally really broken.

Warning

The structure of the attrs.Attribute class is exempt from this rule. It will change in the future, but since it should be considered read-only, that shouldn’t matter.

However if you intend to build extensions on top of attrs you have to anticipate that.

22.1.0 (2022-07-28)

Backwards-incompatible Changes

• Python 2.7 is not supported anymore.

  Dealing with Python 2.7 tooling has become too difficult for a volunteer-run project.

  We have supported Python 2 more than 2 years after it was officially discontinued and feel that we have paid our dues. All version up to 21.4.0 from December 2021 remain fully functional, of course. #936

• The deprecated cmp attribute of attrs.Attribute has been removed. This does not affect the cmp argument to attr.s that can be used as a shortcut to set eq and order at the same time. #939

Changes

• Instantiation of frozen slotted classes is now faster. #898

• If an eq key is defined, it is also used before hashing the attribute. #909

• Added attrs.validators.min_len(). #916

• attrs.validators.deep_iterable()’s member_validator argument now also accepts a list of validators and wraps them in an attrs.validators.and_(). #925

• Added missing type stub re-imports for attrs.converters and attrs.filters. #931

• Added missing stub for attr(s).cmp_using(). #949

• attrs.validators._in()’s ValueError is not missing the attribute, expected options, and the value it got anymore. #951

• Python 3.11 is now officially supported. #969

---

21.4.0 (2021-12-29)

Changes

• Fixed the test suite on PyPy3.8 where cloudpickle does not work. #892

• Fixed coverage report for projects that use attrs and don’t set a --source. #895, #896

---

21.3.0 (2021-12-28)

Backward-incompatible Changes

• When using @define, converters are now run by default when setting an attribute on an instance – additionally to validators. I.e. the new default is on_setattr=[attrs.setters.convert, attrs.setters.validate].

  This is unfortunately a breaking change, but it was an oversight, impossible to raise a DeprecationWarning about, and it’s better to fix it now while the APIs are very fresh with few users. #835, #886

• import attrs has finally landed! As of this release, you can finally import attrs using its proper name.

  Not all names from the attr namespace have been transferred; most notably attr.s and attr.ib are missing. See attrs.define and attrs.field if you haven’t seen our next-generation APIs yet. A more elaborate explanation can be found On The Core API Names

  This feature is at least for one release provisional. We don’t plan on changing anything, but such a big change is unlikely to go perfectly on the first strike.

  The API docs have been mostly updated, but it will be an ongoing effort to change everything to the new APIs. Please note that we have not moved – or even removed – anything from attr!

  Please do report any bugs or documentation inconsistencies! #887

Changes

• attr.asdict(retain_collection_types=False) (default) dumps collection-esque keys as tuples. #646, #888

• __match_args__ are now generated to support Python 3.10’s Structural Pattern Matching. This can be controlled by the match_args argument to the class decorators on Python 3.10 and later. On older versions, it is never added and the argument is ignored. #815

• If the class-level on_setattr is set to attrs.setters.validate (default in @define and @mutable) but no field defines a validator, pretend that it’s not set. #817

• The generated __repr__ is significantly faster on Pythons with f-strings. #819

• Attributes transformed via field_transformer are wrapped with AttrsClass again. #824

• Generated source code is now cached more efficiently for identical classes. #828

• Added attrs.converters.to_bool(). #830

• attrs.resolve_types() now resolves types of subclasses after the parents are resolved. #842 #843

• Added new validators: lt(val) (< val), le(va) (≤ val), ge(val) (≥ val), gt(val) (> val), and maxlen(n). #845

• attrs classes are now fully compatible with cloudpickle (no need to disable repr anymore). #857

• Added new context manager attrs.validators.disabled() and functions attrs.validators.(set|get)_disabled(). They deprecate attrs.(set|get)_run_validators(). All functions are interoperable and modify the same internal state. They are not – and never were – thread-safe, though. #859

• attrs.validators.matches_re() now accepts pre-compiled regular expressions in addition to pattern strings. #877

---

21.2.0 (2021-05-07)

Backward-incompatible Changes

• We had to revert the recursive feature for attr.evolve() because it broke some use-cases – sorry! #806

• Python 3.4 is now blocked using packaging metadata because attrs can’t be imported on it anymore. To ensure that 3.4 users can keep installing attrs easily, we will yank 21.1.0 from PyPI. This has no consequences if you pin attrs to 21.1.0. #807

---

21.1.0 (2021-05-06)

Deprecations

• The long-awaited, much-talked-about, little-delivered import attrs is finally upon us!

  Since the NG APIs have now been proclaimed stable, the next release of attrs will allow you to actually import attrs. We’re taking this opportunity to replace some defaults in our APIs that made sense in 2015, but don’t in 2021.

  So please, if you have any pet peeves about defaults in attrs’s APIs, now is the time to air your grievances in #487! We’re not gonna get such a chance for a second time, without breaking our backward-compatibility guarantees, or long deprecation cycles. Therefore, speak now or forever hold you peace! #487

• The cmp argument to attr.s() and attr.ib() has been undeprecated It will continue to be supported as syntactic sugar to set eq and order in one go.

  I’m terribly sorry for the hassle around this argument! The reason we’re bringing it back is it’s usefulness regarding customization of equality/ordering.

  The cmp attribute and argument on attr.Attribute remains deprecated and will be removed later this year. #773

Changes

• It’s now possible to customize the behavior of eq and order by passing in a callable. #435, #627

• The instant favorite next-generation APIs are not provisional anymore!

  They are also officially supported by Mypy as of their 0.800 release.

  We hope the next release will already contain an (additional) importable package called attrs. #668, #786

• If an attribute defines a converter, the type of its parameter is used as type annotation for its corresponding __init__ parameter.

  If an attr.converters.pipe is used, the first one’s is used. #710

• Fixed the creation of an extra slot for an attr.ib when the parent class already has a slot with the same name. #718

• __attrs__init__() will now be injected if init=False, or if auto_detect=True and a user-defined __init__() exists.

  This enables users to do “pre-init” work in their __init__() (such as super().__init__()).

  __init__() can then delegate constructor argument processing to self.__attrs_init__(*args, **kwargs). #731

• bool(attr.NOTHING) is now False. #732

• It’s now possible to use super() inside of properties of slotted classes. #747

• Allow for a __attrs_pre_init__() method that – if defined – will get called at the beginning of the attrs-generated __init__() method. #750

• Added forgotten attr.Attribute.evolve() to type stubs. #752

• attrs.evolve() now works recursively with nested attrs classes. #759

• Python 3.10 is now officially supported. #763

• attr.resolve_types() now takes an optional attrib argument to work inside a field_transformer. #774

• ClassVars are now also detected if they come from typing-extensions. #782

• To make it easier to customize attribute comparison (#435), we have added the attr.cmp_with() helper.

  See the new docs on comparison for more details. #787

• Added provisional support for static typing in pyright via the dataclass_transforms specification. Both the pyright specification and attrs implementation may change in future versions of both projects.

  Your constructive feedback is welcome in both attrs#795 and pyright#1782. #796

---

20.3.0 (2020-11-05)

Backward-incompatible Changes

• attr.define(), attr.frozen(), attr.mutable(), and attr.field() remain provisional.

  This release does not change anything about them and they are already used widely in production though.

  If you wish to use them together with mypy, you can simply drop this plugin into your project.

  Feel free to provide feedback to them in the linked issue #668.

  We will release the attrs namespace once we have the feeling that the APIs have properly settled. #668

Changes

• attr.s() now has a field_transformer hook that is called for all Attributes and returns a (modified or updated) list of Attribute instances. attr.asdict() has a value_serializer hook that can change the way values are converted. Both hooks are meant to help with data (de-)serialization workflows. #653

• kw_only=True now works on Python 2. #700

• raise from now works on frozen classes on PyPy. #703, #712

• attr.asdict() and attr.astuple() now treat frozensets like sets with regards to the retain_collection_types argument. #704

• The type stubs for attr.s() and attr.make_class() are not missing the collect_by_mro argument anymore. #711

---

20.2.0 (2020-09-05)

Backward-incompatible Changes

• attr.define(), attr.frozen(), attr.mutable(), and attr.field() remain provisional.

  This release fixes a bunch of bugs and ergonomics but they remain mostly unchanged.

  If you wish to use them together with mypy, you can simply drop this plugin into your project.

  Feel free to provide feedback to them in the linked issue #668.

  We will release the attrs namespace once we have the feeling that the APIs have properly settled. #668

Changes

• attr.define() et al now correct detect __eq__ and __ne__. #671

• attr.define() et al’s hybrid behavior now also works correctly when arguments are passed. #675

• It’s possible to define custom __setattr__ methods on slotted classes again. #681

• In 20.1.0 we introduced the inherited attribute on the attr.Attribute class to differentiate attributes that have been inherited and those that have been defined directly on the class.

  It has shown to be problematic to involve that attribute when comparing instances of attr.Attribute though, because when sub-classing, attributes from base classes are suddenly not equal to themselves in a super class.

  Therefore the inherited attribute will now be ignored when hashing and comparing instances of attr.Attribute. #684

• zope.interface is now a “soft dependency” when running the test suite; if zope.interface is not installed when running the test suite, the interface-related tests will be automatically skipped. #685

• The ergonomics of creating frozen classes using @define(frozen=True) and sub-classing frozen classes has been improved: you don’t have to set on_setattr=None anymore. #687

---

20.1.0 (2020-08-20)

Backward-incompatible Changes

• Python 3.4 is not supported anymore. It has been unsupported by the Python core team for a while now, its PyPI downloads are negligible, and our CI provider removed it as a supported option.

  It’s very unlikely that attrs will break under 3.4 anytime soon, which is why we do not block its installation on Python 3.4. But we don’t test it anymore and will block it once someone reports breakage. #608

Deprecations

• Less of a deprecation and more of a heads up: the next release of attrs will introduce an attrs namespace. That means that you’ll finally be able to run import attrs with new functions that aren’t cute abbreviations and that will carry better defaults.

  This should not break any of your code, because project-local packages have priority before installed ones. If this is a problem for you for some reason, please report it to our bug tracker and we’ll figure something out.

  The old attr namespace isn’t going anywhere and its defaults are not changing – this is a purely additive measure. Please check out the linked issue for more details.

  These new APIs have been added provisionally as part of #666 so you can try them out today and provide feedback. Learn more in the API docs. #408

Changes

• Added attr.resolve_types(). It ensures that all forward-references and types in string form are resolved into concrete types.

  You need this only if you need concrete types at runtime. That means that if you only use types for static type checking, you do not need this function. #288, #302

• Added @attr.s(collect_by_mro=False) argument that if set to True fixes the collection of attributes from base classes.

  It’s only necessary for certain cases of multiple-inheritance but is kept off for now for backward-compatibility reasons. It will be turned on by default in the future.

  As a side-effect, attr.Attribute now always has an inherited attribute indicating whether an attribute on a class was directly defined or inherited. #428, #635

• On Python 3, all generated methods now have a docstring explaining that they have been created by attrs. #506

• It is now possible to prevent attrs from auto-generating the __setstate__ and __getstate__ methods that are required for pickling of slotted classes.

  Either pass @attr.s(getstate_setstate=False) or pass @attr.s(auto_detect=True) and implement them yourself: if attrs finds either of the two methods directly on the decorated class, it assumes implicitly getstate_setstate=False (and implements neither).

  This option works with dict classes but should never be necessary. #512, #513, #642

• Fixed a ValueError: Cell is empty bug that could happen in some rare edge cases. #590

• attrs can now automatically detect your own implementations and infer init=False, repr=False, eq=False, order=False, and hash=False if you set @attr.s(auto_detect=True). attrs will ignore inherited methods. If the argument implies more than one method (e.g. eq=True creates both __eq__ and __ne__), it’s enough for one of them to exist and attrs will create neither.

  This feature requires Python 3. #607

• Added attr.converters.pipe(). The feature allows combining multiple conversion callbacks into one by piping the value through all of them, and retuning the last result.

  As part of this feature, we had to relax the type information for converter callables. #618

• Fixed serialization behavior of non-slots classes with cache_hash=True. The hash cache will be cleared on operations which make “deep copies” of instances of classes with hash caching, though the cache will not be cleared with shallow copies like those made by copy.copy().

  Previously, copy.deepcopy() or serialization and deserialization with pickle would result in an un-initialized object.

  This change also allows the creation of cache_hash=True classes with a custom __setstate__, which was previously forbidden (#494). #620

• It is now possible to specify hooks that are called whenever an attribute is set after a class has been instantiated.

  You can pass on_setattr both to @attr.s() to set the default for all attributes on a class, and to @attr.ib() to overwrite it for individual attributes.

  attrs also comes with a new module attr.setters that brings helpers that run validators, converters, or allow to freeze a subset of attributes. #645, #660

• Provisional APIs called attr.define(), attr.mutable(), and attr.frozen() have been added.

  They are only available on Python 3.6 and later, and call attr.s() with different default values.

  If nothing comes up, they will become the official way for creating classes in 20.2.0 (see above).

  Please note that it may take some time until mypy – and other tools that have dedicated support for attrs – recognize these new APIs. Please do not open issues on our bug tracker, there is nothing we can do about it. #666

• We have also provisionally added attr.field() that supplants attr.ib(). It also requires at least Python 3.6 and is keyword-only. Other than that, it only dropped a few arguments, but changed no defaults.

  As with attr.s(): attr.ib() is not going anywhere. #669

---

19.3.0 (2019-10-15)

Changes

• Fixed auto_attribs usage when default values cannot be compared directly with ==, such as numpy arrays. #585

---

19.2.0 (2019-10-01)

Backward-incompatible Changes

• Removed deprecated Attribute attribute convert per scheduled removal on 2019/1. This planned deprecation is tracked in issue #307. #504

• __lt__, __le__, __gt__, and __ge__ do not consider subclasses comparable anymore.

  This has been deprecated since 18.2.0 and was raising a DeprecationWarning for over a year. #570

Deprecations

• The cmp argument to attr.s() and attr.ib() is now deprecated.

  Please use eq to add equality methods (__eq__ and __ne__) and order to add ordering methods (__lt__, __le__, __gt__, and __ge__) instead – just like with dataclasses.

  Both are effectively True by default but it’s enough to set eq=False to disable both at once. Passing eq=False, order=True explicitly will raise a ValueError though.

  Since this is arguably a deeper backward-compatibility break, it will have an extended deprecation period until 2021-06-01. After that day, the cmp argument will be removed.

  attr.Attribute also isn’t orderable anymore. #574

Changes

• Updated attr.validators.__all__ to include new validators added in #425. #517

• Slotted classes now use a pure Python mechanism to rewrite the __class__ cell when rebuilding the class, so super() works even on environments where ctypes is not installed. #522

• When collecting attributes using @attr.s(auto_attribs=True), attributes with a default of None are now deleted too. #523, #556

• Fixed attr.validators.deep_iterable() and attr.validators.deep_mapping() type stubs. #533

• attr.validators.is_callable() validator now raises an exception attr.exceptions.NotCallableError, a subclass of TypeError, informing the received value. #536

• @attr.s(auto_exc=True) now generates classes that are hashable by ID, as the documentation always claimed it would. #543, #563

• Added attr.validators.matches_re() that checks string attributes whether they match a regular expression. #552

• Keyword-only attributes (kw_only=True) and attributes that are excluded from the attrs’s __init__ (init=False) now can appear before mandatory attributes. #559

• The fake filename for generated methods is now more stable. It won’t change when you restart the process. #560

• The value passed to @attr.ib(repr=…) can now be either a boolean (as before) or a callable. That callable must return a string and is then used for formatting the attribute by the generated __repr__() method. #568

• Added attr.__version_info__ that can be used to reliably check the version of attrs and write forward- and backward-compatible code. Please check out the section on deprecated APIs on how to use it. #580

---

19.1.0 (2019-03-03)

Backward-incompatible Changes

• Fixed a bug where deserialized objects with cache_hash=True could have incorrect hash code values. This change breaks classes with cache_hash=True when a custom __setstate__ is present. An exception will be thrown when applying the attrs annotation to such a class. This limitation is tracked in issue #494. #482

Changes

• Add is_callable, deep_iterable, and deep_mapping validators.

  • is_callable: validates that a value is callable

  • deep_iterable: Allows recursion down into an iterable, applying another validator to every member in the iterable as well as applying an optional validator to the iterable itself.

  • deep_mapping: Allows recursion down into the items in a mapping object, applying a key validator and a value validator to the key and value in every item. Also applies an optional validator to the mapping object itself.

  You can find them in the attr.validators package. #425

• Fixed stub files to prevent errors raised by mypy’s disallow_any_generics = True option. #443

• Attributes with init=False now can follow after kw_only=True attributes. #450

• attrs now has first class support for defining exception classes.

  If you define a class using @attr.s(auto_exc=True) and subclass an exception, the class will behave like a well-behaved exception class including an appropriate __str__ method, and all attributes additionally available in an args attribute. #500

• Clarified documentation for hashing to warn that hashable objects should be deeply immutable (in their usage, even if this is not enforced). #503

---

18.2.0 (2018-09-01)

Deprecations

• Comparing subclasses using <, >, <=, and >= is now deprecated. The docs always claimed that instances are only compared if the types are identical, so this is a first step to conform to the docs.

  Equality operators (== and !=) were always strict in this regard. #394

Changes

• attrs now ships its own PEP 484 type hints. Together with mypy’s attrs plugin, you’ve got all you need for writing statically typed code in both Python 2 and 3!

  At that occasion, we’ve also added narrative docs about type annotations in attrs. #238

• Added kw_only arguments to attr.ib and attr.s, and a corresponding kw_only attribute to attr.Attribute. This change makes it possible to have a generated __init__ with keyword-only arguments on Python 3, relaxing the required ordering of default and non-default valued attributes. #281, #411

• The test suite now runs with hypothesis.HealthCheck.too_slow disabled to prevent CI breakage on slower computers. #364, #396

• attr.validators.in_() now raises a ValueError with a useful message even if the options are a string and the value is not a string. #383

• attr.asdict() now properly handles deeply nested lists and dictionaries. #395

• Added attr.converters.default_if_none() that allows to replace None values in attributes. For example attr.ib(converter=default_if_none("")) replaces None by empty strings. #400, #414

• Fixed a reference leak where the original class would remain live after being replaced when slots=True is set. #407

• Slotted classes can now be made weakly referenceable by passing @attr.s(weakref_slot=True). #420

• Added cache_hash option to @attr.s which causes the hash code to be computed once and stored on the object. #426

• Attributes can be named property and itemgetter now. #430

• It is now possible to override a base class’ class variable using only class annotations. #431

---

18.1.0 (2018-05-03)

Changes

• x=X(); x.cycle = x; repr(x) will no longer raise a RecursionError, and will instead show as X(x=...).

  #95

• attr.ib(factory=f) is now syntactic sugar for the common case of attr.ib(default=attr.Factory(f)).

  #178, #356

• Added attr.field_dict() to return an ordered dictionary of attrs attributes for a class, whose keys are the attribute names.

  #290, #349

• The order of attributes that are passed into attr.make_class() or the these argument of @attr.s() is now retained if the dictionary is ordered (i.e. dict on Python 3.6 and later, collections.OrderedDict otherwise).

  Before, the order was always determined by the order in which the attributes have been defined which may not be desirable when creating classes programmatically.

  #300, #339, #343

• In slotted classes, __getstate__ and __setstate__ now ignore the __weakref__ attribute.

  #311, #326

• Setting the cell type is now completely best effort. This fixes attrs on Jython.

  We cannot make any guarantees regarding Jython though, because our test suite cannot run due to dependency incompatibilities.

  #321, #334

• If attr.s is passed a these argument, it will no longer attempt to remove attributes with the same name from the class body.

  #322, #323

• The hash of attr.NOTHING is now vegan and faster on 32bit Python builds.

  #331, #332

• The overhead of instantiating frozen dict classes is virtually eliminated. #336

• Generated __init__ methods now have an __annotations__ attribute derived from the types of the fields.

  #363

• We have restructured the documentation a bit to account for attrs’ growth in scope. Instead of putting everything into the examples page, we have started to extract narrative chapters.

  So far, we’ve added chapters on initialization and hashing.

  Expect more to come!

  #369, #370

---

17.4.0 (2017-12-30)

Backward-incompatible Changes

• The traversal of MROs when using multiple inheritance was backward: If you defined a class C that subclasses A and B like C(A, B), attrs would have collected the attributes from B before those of A.

  This is now fixed and means that in classes that employ multiple inheritance, the output of __repr__ and the order of positional arguments in __init__ changes. Because of the nature of this bug, a proper deprecation cycle was unfortunately impossible.

  Generally speaking, it’s advisable to prefer kwargs-based initialization anyways – especially if you employ multiple inheritance and diamond-shaped hierarchies.

  #298, #299, #304

• The __repr__ set by attrs no longer produces an AttributeError when the instance is missing some of the specified attributes (either through deleting or after using init=False on some attributes).

  This can break code that relied on repr(attr_cls_instance) raising AttributeError to check if any attrs-specified members were unset.

  If you were using this, you can implement a custom method for checking this:

  def has_unset_members(self):
      for field in attr.fields(type(self)):
          try:
              getattr(self, field.name)
          except AttributeError:
              return True
      return False
  

  #308

Deprecations

• The attr.ib(convert=callable) option is now deprecated in favor of attr.ib(converter=callable).

  This is done to achieve consistency with other noun-based arguments like validator.

  convert will keep working until at least January 2019 while raising a DeprecationWarning.

  #307

Changes

• Generated __hash__ methods now hash the class type along with the attribute values. Until now the hashes of two classes with the same values were identical which was a bug.

  The generated method is also much faster now.

  #261, #295, #296

• attr.ib’s metadata argument now defaults to a unique empty dict instance instead of sharing a common empty dict for all. The singleton empty dict is still enforced.

  #280

• ctypes is optional now however if it’s missing, a bare super() will not work in slotted classes. This should only happen in special environments like Google App Engine.

  #284, #286

• The attribute redefinition feature introduced in 17.3.0 now takes into account if an attribute is redefined via multiple inheritance. In that case, the definition that is closer to the base of the class hierarchy wins.

  #285, #287

• Subclasses of auto_attribs=True can be empty now.

  #291, #292

• Equality tests are much faster now.

  #306

• All generated methods now have correct __module__, __name__, and (on Python 3) __qualname__ attributes.

  #309

---

17.3.0 (2017-11-08)

Backward-incompatible Changes

• Attributes are no longer defined on the class body.

  This means that if you define a class C with an attribute x, the class will not have an attribute x for introspection. Instead of C.x, use attr.fields(C).x or look at C.__attrs_attrs__. The old behavior has been deprecated since version 16.1. (#253)

Changes

• super() and __class__ now work with slotted classes on Python 3. (#102, #226, #269, #270, #272)

• Added type argument to attr.ib() and corresponding type attribute to attr.Attribute.

  This change paves the way for automatic type checking and serialization (though as of this release attrs does not make use of it). In Python 3.6 or higher, the value of attr.Attribute.type can alternately be set using variable type annotations (see PEP 526). (#151, #214, #215, #239)

• The combination of str=True and slots=True now works on Python 2. (#198)

• attr.Factory is hashable again. (#204)

• Subclasses now can overwrite attribute definitions of their base classes.

  That means that you can – for example – change the default value for an attribute by redefining it. (#221, #229)

• Added new option auto_attribs to @attr.s that allows to collect annotated fields without setting them to attr.ib().

  Setting a field to an attr.ib() is still possible to supply options like validators. Setting it to any other value is treated like it was passed as attr.ib(default=value) – passing an instance of attr.Factory also works as expected. (#262, #277)

• Instances of classes created using attr.make_class() can now be pickled. (#282)

---

17.2.0 (2017-05-24)

Changes:

• Validators are hashable again. Note that validators may become frozen in the future, pending availability of no-overhead frozen classes. #192

---

17.1.0 (2017-05-16)

To encourage more participation, the project has also been moved into a dedicated GitHub organization and everyone is most welcome to join!

attrs also has a logo now!

Backward-incompatible Changes:

• attrs will set the __hash__() method to None by default now. The way hashes were handled before was in conflict with Python’s specification. This may break some software although this breakage is most likely just surfacing of latent bugs. You can always make attrs create the __hash__() method using @attr.s(hash=True). See #136 for the rationale of this change.

  Warning

  Please do not upgrade blindly and do test your software! Especially if you use instances as dict keys or put them into sets!

• Correspondingly, attr.ib’s hash argument is None by default too and mirrors the cmp argument as it should.

Deprecations:

• attr.assoc() is now deprecated in favor of attr.evolve() and will stop working in 2018.

Changes:

• Fix default hashing behavior. Now hash mirrors the value of cmp and classes are unhashable by default. #136 #142

• Added attr.evolve() that, given an instance of an attrs class and field changes as keyword arguments, will instantiate a copy of the given instance with the changes applied. evolve() replaces assoc(), which is now deprecated. evolve() is significantly faster than assoc(), and requires the class have an initializer that can take the field values as keyword arguments (like attrs itself can generate). #116 #124 #135

• FrozenInstanceError is now raised when trying to delete an attribute from a frozen class. #118

• Frozen-ness of classes is now inherited. #128

• __attrs_post_init__() is now run if validation is disabled. #130

• Added attr.validators.in_(options) that, given the allowed options, checks whether the attribute value is in it. This can be used to check constants, enums, mappings, etc. #181

• Added attr.validators.and_() that composes multiple validators into one. #161

• For convenience, the validator argument of @attr.s now can take a list of validators that are wrapped using and_(). #138

• Accordingly, attr.validators.optional() now can take a list of validators too. #161

• Validators can now be defined conveniently inline by using the attribute as a decorator. Check out the validator examples to see it in action! #143

• attr.Factory() now has a takes_self argument that makes the initializer to pass the partially initialized instance into the factory. In other words you can define attribute defaults based on other attributes. #165 #189

• Default factories can now also be defined inline using decorators. They are always passed the partially initialized instance. #165

• Conversion can now be made optional using attr.converters.optional(). #105 #173

• attr.make_class() now accepts the keyword argument bases which allows for subclassing. #152

• Metaclasses are now preserved with slots=True. #155

---

16.3.0 (2016-11-24)

Changes:

• Attributes now can have user-defined metadata which greatly improves attrs’s extensibility. #96

• Allow for a __attrs_post_init__() method that – if defined – will get called at the end of the attrs-generated __init__() method. #111

• Added @attr.s(str=True) that will optionally create a __str__() method that is identical to __repr__(). This is mainly useful with Exceptions and other classes that rely on a useful __str__() implementation but overwrite the default one through a poor own one. Default Python class behavior is to use __repr__() as __str__() anyways.

  If you tried using attrs with Exceptions and were puzzled by the tracebacks: this option is for you.

• __name__ is no longer overwritten with __qualname__ for attr.s(slots=True) classes. #99

---

16.2.0 (2016-09-17)

Changes:

• Added attr.astuple() that – similarly to attr.asdict() – returns the instance as a tuple. #77

• Converters now work with frozen classes. #76

• Instantiation of attrs classes with converters is now significantly faster. #80

• Pickling now works with slotted classes. #81

• attr.assoc() now works with slotted classes. #84

• The tuple returned by attr.fields() now also allows to access the Attribute instances by name. Yes, we’ve subclassed tuple so you don’t have to! Therefore attr.fields(C).x is equivalent to the deprecated C.x and works with slotted classes. #88

---

16.1.0 (2016-08-30)

Backward-incompatible Changes:

• All instances where function arguments were called cl have been changed to the more Pythonic cls. Since it was always the first argument, it’s doubtful anyone ever called those function with in the keyword form. If so, sorry for any breakage but there’s no practical deprecation path to solve this ugly wart.

Deprecations:

• Accessing Attribute instances on class objects is now deprecated and will stop working in 2017. If you need introspection please use the __attrs_attrs__ attribute or the attr.fields() function that carry them too. In the future, the attributes that are defined on the class body and are usually overwritten in your __init__ method are simply removed after @attr.s has been applied.

  This will remove the confusing error message if you write your own __init__ and forget to initialize some attribute. Instead you will get a straightforward AttributeError. In other words: decorated classes will work more like plain Python classes which was always attrs’s goal.

• The serious-business aliases attr.attributes and attr.attr have been deprecated in favor of attr.attrs and attr.attrib which are much more consistent and frankly obvious in hindsight. They will be purged from documentation immediately but there are no plans to actually remove them.

Changes:

• attr.asdict()‘s dict_factory arguments is now propagated on recursion. #45

• attr.asdict(), attr.has() and attr.fields() are significantly faster. #48 #51

• Add attr.attrs and attr.attrib as a more consistent aliases for attr.s and attr.ib.

• Add frozen option to attr.s that will make instances best-effort immutable. #60

• attr.asdict() now takes retain_collection_types as an argument. If True, it does not convert attributes of type tuple or set to list. #69

---

16.0.0 (2016-05-23)

Backward-incompatible Changes:

• Python 3.3 and 2.6 are no longer supported. They may work by chance but any effort to keep them working has ceased.

  The last Python 2.6 release was on October 29, 2013 and is no longer supported by the CPython core team. Major Python packages like Django and Twisted dropped Python 2.6 a while ago already.

  Python 3.3 never had a significant user base and wasn’t part of any distribution’s LTS release.

Changes:

• __slots__ have arrived! Classes now can automatically be slotted-style (and save your precious memory) just by passing slots=True. #35

• Allow the case of initializing attributes that are set to init=False. This allows for clean initializer parameter lists while being able to initialize attributes to default values. #32

• attr.asdict() can now produce arbitrary mappings instead of Python dicts when provided with a dict_factory argument. #40

• Multiple performance improvements.

---

15.2.0 (2015-12-08)

Changes:

• Added a convert argument to attr.ib, which allows specifying a function to run on arguments. This allows for simple type conversions, e.g. with attr.ib(convert=int). #26

• Speed up object creation when attribute validators are used. #28

---

15.1.0 (2015-08-20)

Changes:

• Added attr.validators.optional() that wraps other validators allowing attributes to be None. #16

• Multi-level inheritance now works. #24

• __repr__() now works with non-redecorated subclasses. #20

---

15.0.0 (2015-04-15)

Changes:

Initial release.

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