API Reference

attrs works by decorating a class using attr.s() and then optionally defining attributes on the class using attr.ib().

Note

When this documentation speaks about “attrs attributes” it means those attributes that are defined using attr.ib() in the class body.

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

Core

attr.s(these=None, repr_ns=None, repr=True, cmp=True, hash=None, init=True, slots=False, frozen=False, str=False)

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.

  • 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.
  • 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.
  • cmp (bool) – Create __eq__, __ne__, __lt__, __le__, __gt__, and __ge__ methods that compare the class as if it were a tuple of its attrs attributes. But the attributes are only compared, if the type of both classes is identical!
  • hash (bool or None) –

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

    1. If both are True, attrs will generate a __hash__ for you.
    2. If cmp is True and frozen is False, __hash__ will be set to None, marking it unhashable (which it is).
    3. If cmp is False, __hash__ will be left untouched meaning the __hash__ method of the superclass will be used (if superclass 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 the Python documentation 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_post_init__ method exists on the class, it will be called after the class is fully initialized.
  • slots (bool) – Create a slots-style class that’s more memory-efficient. See Slots for further ramifications.
  • frozen (bool) –

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

    Please note:

    1. This is achieved by installing a custom __setattr__ method on your class so you can’t implement an own one.
    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).
  • 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 Factory also works as expected.

    Attributes annotated as typing.ClassVar are ignored.

New in version 16.0.0: slots

New in version 16.1.0: frozen

New in version 16.3.0: str, and 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

Note

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

For example:

>>> import attr
>>> @attr.s
... class C(object):
...     _private = attr.ib()
>>> C(private=42)
C(_private=42)
>>> class D(object):
...     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.ib(default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, convert=None, metadata={}, type=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 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 attr.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.

  • 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 (bool) – Include this attribute in the generated __repr__ method.
  • cmp (bool) – Include this attribute in the generated comparison methods (__eq__ et al).
  • hash (bool or None) – Include this attribute in the generated __hash__ method. If None (default), mirror cmp’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.
  • convert (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.

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

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

New in version 17.3.0: type

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(object):
...     x = attr.ib()
...     y = attr.ib()
...     @x.validator
...     def name_can_be_anything(self, attribute, value):
...         if value < 0:
...             raise ValueError("x must be positive")
...     @y.default
...     def name_does_not_matter(self):
...         return self.x + 1
>>> C(1)
C(x=1, y=2)
>>> C(-1)
Traceback (most recent call last):
    ...
ValueError: x must be positive
class attr.Attribute(name, default, validator, repr, cmp, hash, init, convert=None, metadata=None, type=None)

Read-only representation of an attribute.

Attribute name:The name of the attribute.

Plus all arguments of attr.ib().

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.

Warning

You should never instantiate this class yourself!

>>> import attr
>>> @attr.s
... class C(object):
...     x = attr.ib()
>>> attr.fields(C).x
Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, convert=None, metadata=mappingproxy({}), type=None)
attr.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.
  • 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

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 attr.Factory(factory, takes_self=False)

Stores a factory callable.

If passed as the default value to attr.ib(), 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(object):
...     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})
exception attr.exceptions.FrozenInstanceError

A frozen/immutable instance has been attempted to be modified.

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

New in version 16.1.0.

exception attr.exceptions.AttrsAttributeNotFoundError

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

New in version 16.2.0.

exception attr.exceptions.NotAnAttrsClassError

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

New in version 16.2.0.

exception attr.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 attr.exceptions.UnannotatedAttributeError

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

New in version 17.3.0.

For example:

@attr.s(auto_attribs=True)
class C:
    x: int
    y = attr.ib()

Influencing Initialization

Generally speaking, it’s best to keep logic out of your __init__. The moment you need a finer control over how your class is instantiated, 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 after your class is initialized. And for that attrs offers the __attrs_post_init__ hook that is automatically detected and run after attrs is done initializing your instance:

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     y = attr.ib(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:

>>> @attr.s(frozen=True)
... class FrozenBroken(object):
...     x = attr.ib()
...     y = attr.ib(init=False)
...     def __attrs_post_init__(self):
...         self.y = self.x + 1
>>> FrozenBroken(1)
Traceback (most recent call last):
   ...
attr.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__():

>>> @attr.s(frozen=True)
... class Frozen(object):
...     x = attr.ib()
...     y = attr.ib(init=False)
...     def __attrs_post_init__(self):
...         object.__setattr__(self, "y", self.x + 1)
>>> Frozen(1)
Frozen(x=1, y=2)

Helpers

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

attr.fields(cls)

Returns 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:
Return type:

tuple (with name accessors) of attr.Attribute

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

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     y = attr.ib()
>>> attr.fields(C)
(Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, convert=None, metadata=mappingproxy({}), type=None), Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, convert=None, metadata=mappingproxy({}), type=None))
>>> attr.fields(C)[1]
Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=None, init=True, convert=None, metadata=mappingproxy({}), type=None)
>>> attr.fields(C).y is attr.fields(C)[1]
True
attr.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(object):
...     pass
>>> attr.has(C)
True
>>> attr.has(object)
False
attr.asdict(inst, recurse=True, filter=None, dict_factory=<class ‘dict’>, retain_collection_types=False)

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 attr.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.
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

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     y = attr.ib()
>>> attr.asdict(C(1, C(2, 3)))
{'x': 1, 'y': {'x': 2, 'y': 3}}
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 attr.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.

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     y = attr.ib()
>>> attr.astuple(C(1,2))
(1, 2)

attrs includes some handy helpers for filtering:

attr.filters.include(*what)

Whitelist what.

Parameters:what (list of type or attr.Attributes) – What to whitelist.
Return type:callable
attr.filters.exclude(*what)

Blacklist what.

Parameters:what (list of classes or attr.Attributes.) – What to blacklist.
Return type:callable

See Converting to Collections Types for examples.

attr.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:

New in version 17.1.0.

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     y = attr.ib()
>>> i1 = C(1, 2)
>>> i1
C(x=1, y=2)
>>> i2 = attr.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.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:

>>> @attr.s
... class C(object):
...     x = attr.ib(validator=attr.validators.instance_of(int))
>>> i = C(1)
>>> i.x = "1"
>>> attr.validate(i)
Traceback (most recent call last):
   ...
TypeError: ("'x' must be <type 'int'> (got '1' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, validator=<instance_of validator for type <type 'int'>>, repr=True, cmp=True, hash=None, init=True, type=None), <type 'int'>, '1')

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.

attr.get_run_validators()

Return whether or not validators are run.

Validators

attrs comes with some common validators in the attrs.validators module:

attr.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 attr.Attribute), the expected type, and the value it got.

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib(validator=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), <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), <type 'int'>, None)
attr.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 attr.Attribute), the expected options, and the value it got.

New in version 17.1.0.

For example:

>>> import enum
>>> class State(enum.Enum):
...     ON = "on"
...     OFF = "off"
>>> @attr.s
... class C(object):
...     state = attr.ib(validator=attr.validators.in_(State))
...     val = attr.ib(validator=attr.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')
>>> C(State.ON, 4)
Traceback (most recent call last):
   ...
ValueError: 'val' must be in [1, 2, 3] (got 4)
attr.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 attr.Attribute), the expected interface, and the value it got.
attr.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 attr.ib()’s validator argument.

Thus the following two statements are equivalent:

x = attr.ib(validator=attr.validators.and_(v1, v2, v3))
x = attr.ib(validator=[v1, v2, v3])
attr.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:

>>> @attr.s
... class C(object):
...     x = attr.ib(validator=attr.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), <type 'int'>, '42')
>>> C(None)
C(x=None)

Converters

attr.converters.optional(converter)

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

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

New in version 17.1.0.

For example:

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

Deprecated APIs

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:

Deprecated since version 17.1.0: Use evolve() instead.