Initialization

In Python, instance intialization 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(object):
    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:

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

    @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 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 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
>>> @attr.s
... class C(object):
...    _x = attr.ib()
>>> inspect.signature(C.__init__)
<Signature (self, x) -> 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:

>>> @attr.s
... class C(object):
...    _1 = attr.ib()
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:

>>> import attr
>>> @attr.s
... class C(object):
...     a = attr.ib(default=42)
...     b = attr.ib(default=attr.Factory(list))
...     c = attr.ib(factory=list)  # syntactic sugar for above
...     d = attr.ib()
...     @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. You also cannot use type annotations to elide the attr.ib() call for d as explained in Type Annotations.

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

>>> @attr.s
... class C(object):
...     x = attr.ib(default=[])
>>> i = C()
>>> j = C()
>>> i.x.append(42)
>>> j.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 into __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.

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

>>> @attr.s
... class C(object):
...     x = attr.ib()
...     @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 you can’t elide the call to attr.ib().

Callables

If you want to re-use your validators, you should have a look at the validator argument to attr.ib().

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.

Since the validators runs 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'!")
>>> @attr.s
... class C(object):
...     x = attr.ib(validator=[attr.validators.instance_of(int),
...                            x_smaller_than_y])
...     y = attr.ib()
>>> 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 attr.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 attr.validate() on any instance to verify that it’s still valid:

>>> i = C(4, 5)
>>> i.x = 5  # works, no magic here
>>> attr.validate(i)
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:

>>> @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, 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:

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

>>> attr.set_run_validators(False)
>>> C("128")
C(x='128')
>>> attr.set_run_validators(True)
>>> 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.

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

Post-Init Hook

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 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)

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

    1. default factory

    2. converter

  2. all validators

  3. __attrs_post_init__

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.