.. currentmodule:: attr
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 names for clarification.
What follows is the API explanation, if you'd like a more hands-on introduction, have a look at examples.
As of version 21.3.0, attrs consists of two to-level package names:
- The classic
attrthat powered the venerable attr.s and attr.ib - The modern
attrsthat only contains most modern APIs and relies on attrs.define and attrs.field to define your classes. Additionally it offers someattrAPIs 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.
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.
.. autodata:: attrs.NOTHING
.. autofunction:: attrs.define
.. function:: attrs.mutable(same_as_define) Alias for `attrs.define`. .. versionadded:: 20.1.0
.. function:: attrs.frozen(same_as_define) Behaves the same as `attrs.define` but sets *frozen=True* and *on_setattr=None*. .. versionadded:: 20.1.0
.. autofunction:: attrs.field
.. function:: define Old import path for `attrs.define`.
.. function:: mutable Old import path for `attrs.mutable`.
.. function:: frozen Old import path for `attrs.frozen`.
.. function:: field Old import path for `attrs.field`.
.. autoclass:: attrs.Attribute
:members: evolve
For example:
.. doctest::
>>> import attr
>>> @attr.s
... class C(object):
... 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)
.. autofunction:: attrs.make_class
This is handy if you want to programmatically create classes.
For example:
.. doctest::
>>> 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=[])
.. autoclass:: attrs.Factory
For example:
.. doctest::
>>> @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})
.. data:: attr.NOTHING Same as `attrs.NOTHING`.
.. autofunction:: 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)
.. note::
``attrs`` also comes with a serious business alias ``attr.attrs``.
For example:
.. doctest::
>>> 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.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)
.. autofunction:: attr.ib
.. 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:
.. doctest::
>>> @attr.s
... class C(object):
... 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
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!.. autoexception:: attrs.exceptions.PythonTooOldError
.. autoexception:: attrs.exceptions.FrozenError
.. autoexception:: attrs.exceptions.FrozenInstanceError
.. autoexception:: attrs.exceptions.FrozenAttributeError
.. autoexception:: attrs.exceptions.AttrsAttributeNotFoundError
.. autoexception:: attrs.exceptions.NotAnAttrsClassError
.. autoexception:: attrs.exceptions.DefaultAlreadySetError
.. autoexception:: attrs.exceptions.UnannotatedAttributeError
.. autoexception:: attrs.exceptions.NotCallableError
For example::
@attr.s(auto_attribs=True)
class C:
x: int
y = attr.ib() # <- ERROR!
attrs comes with a bunch of helper methods that make working with it easier:
.. autofunction:: attrs.cmp_using
.. function:: attr.cmp_using Same as `attrs.cmp_using`.
.. autofunction:: attrs.fields
For example:
.. doctest::
>>> @attr.s
... class C(object):
... 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
.. function:: attr.fields Same as `attrs.fields`.
.. autofunction:: attrs.fields_dict
For example:
.. doctest::
>>> @attr.s
... class C(object):
... 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
.. function:: attr.fields_dict Same as `attrs.fields_dict`.
.. autofunction:: attrs.has
For example:
.. doctest::
>>> @attr.s
... class C(object):
... pass
>>> attr.has(C)
True
>>> attr.has(object)
False
.. function:: attr.has Same as `attrs.has`.
.. autofunction:: attrs.resolve_types
For example:
.. doctest::
>>> 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'>
.. function:: attr.resolve_types Same as `attrs.resolve_types`.
.. autofunction:: attrs.asdict
For example:
.. doctest::
>>> @attrs.define
... class C:
... x: int
... y: int
>>> attrs.asdict(C(1, C(2, 3)))
{'x': 1, 'y': {'x': 2, 'y': 3}}
.. autofunction:: attr.asdict
.. autofunction:: attrs.astuple
For example:
.. doctest::
>>> @attrs.define
... class C:
... x = attr.field()
... y = attr.field()
>>> attrs.astuple(C(1,2))
(1, 2)
.. autofunction:: attr.astuple
attrs includes some handy helpers for filtering the attributes in attrs.asdict and attrs.astuple:
.. autofunction:: attrs.filters.include
.. autofunction:: attrs.filters.exclude
.. function:: attr.filters.include Same as `attrs.filters.include`.
.. function:: attr.filters.exclude Same as `attrs.filters.exclude`.
See :func:`attrs.asdict` for examples.
All objects from attrs.filters are also available from attr.filters.
.. autofunction:: attrs.evolve
For example:
.. doctest::
>>> @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.
.. function:: attr.evolve Same as `attrs.evolve`.
.. autofunction:: attrs.validate
For example:
.. doctest::
>>> @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'>).", ...)
.. function:: 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:
.. autofunction:: set_run_validators
.. autofunction:: get_run_validators
attrs comes with some common validators in the attrs.validators module.
All objects from attrs.converters are also available from attr.converters.
.. autofunction:: attrs.validators.lt
For example:
.. doctest::
>>> @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")
.. autofunction:: attrs.validators.le
For example:
.. doctest::
>>> @attrs.define
... class C(object):
... 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")
.. autofunction:: attrs.validators.ge
For example:
.. doctest::
>>> @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")
.. autofunction:: attrs.validators.gt
For example:
.. doctest::
>>> @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")
.. autofunction:: attrs.validators.max_len
For example:
.. doctest::
>>> @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")
.. autofunction:: attrs.validators.instance_of
For example:
.. doctest::
>>> @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)
.. autofunction:: attrs.validators.in_
For example:
.. doctest::
>>> 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')
>>> C(State.ON, 4)
Traceback (most recent call last):
...
ValueError: 'val' must be in [1, 2, 3] (got 4)
.. autofunction:: attrs.validators.provides
.. autofunction:: attrs.validators.and_
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])
.. autofunction:: attrs.validators.optional
For example:
.. doctest::
>>> @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)
.. autofunction:: attrs.validators.is_callable
For example:
.. doctest::
>>> @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'>).
.. autofunction:: attrs.validators.matches_re
For example:
.. doctest::
>>> @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')
.. autofunction:: attrs.validators.deep_iterable
For example:
.. doctest::
>>> @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')
.. autofunction:: attrs.validators.deep_mapping
For example:
.. doctest::
>>> @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:
.. autofunction:: attrs.validators.set_disabled
.. autofunction:: attrs.validators.get_disabled
.. autofunction:: attrs.validators.disabled
All objects from attrs.converters are also available from attr.converters.
.. autofunction:: attrs.converters.pipe
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])
.. autofunction:: attrs.converters.optional
For example:
.. doctest::
>>> @attr.s
... class C(object):
... x = attr.ib(converter=attr.converters.optional(int))
>>> C(None)
C(x=None)
>>> C(42)
C(x=42)
.. autofunction:: attrs.converters.default_if_none
For example:
.. doctest::
>>> @attr.s
... class C(object):
... x = attr.ib(
... converter=attr.converters.default_if_none("")
... )
>>> C(None)
C(x='')
.. autofunction:: attrs.converters.to_bool
For example:
.. doctest::
>>> @attr.s
... class C(object):
... 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
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.
.. autofunction:: attrs.setters.frozen
.. autofunction:: attrs.setters.validate
.. autofunction:: attrs.setters.convert
.. autofunction:: attrs.setters.pipe
.. autodata:: attrs.setters.NO_OP
For example, only ``x`` is frozen here:
.. doctest::
>>> @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.
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:
.. autoclass:: VersionInfo
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(object):
... 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.
.. autofunction:: assoc