attr
attrs
works by decorating a class using attr.define or attr.s and then optionally defining attributes on the class using attr.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.
Note
attrs
20.1.0 added a bunch of nicer APIs (sometimes referred to as next generation -- or NG -- APIs) that were intended to become the main way of defining classes in the future. As of 21.1.0, they are not provisional anymore and are the recommended way to use attrs
! The next step will be adding an importable attrs
namespace. The documentation will be updated successively.
Please have a look at next-gen
!
attr.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)
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.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
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 _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
attr.Attribute
For example:
>>> 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)
attr.make_class
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=[])
attr.Factory
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})
attr.exceptions.PythonTooOldError
attr.exceptions.FrozenError
attr.exceptions.FrozenInstanceError
attr.exceptions.FrozenAttributeError
attr.exceptions.AttrsAttributeNotFoundError
attr.exceptions.NotAnAttrsClassError
attr.exceptions.DefaultAlreadySetError
attr.exceptions.UnannotatedAttributeError
attr.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:
attr.cmp_using
attr.fields
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, 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)) >>> attr.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) >>> attr.fields(C).y is attr.fields(C)[1] True
attr.fields_dict
For example:
>>> @attr.s ... class C(object): ... x = attr.ib() ... y = attr.ib() >>> attr.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) >>> attr.fields_dict(C)['y'] is attr.fields(C).y True
attr.has
For example:
>>> @attr.s ... class C(object): ... pass >>> attr.has(C) True >>> attr.has(object) False
attr.resolve_types
For example:
>>> import typing >>> @attr.s(auto_attribs=True) ... class A: ... a: typing.List['A'] ... b: 'B' ... >>> @attr.s(auto_attribs=True) ... class B: ... a: A ... >>> attr.fields(A).a.type typing.List[ForwardRef('A')] >>> attr.fields(A).b.type 'B' >>> attr.resolve_types(A, globals(), locals()) <class 'A'> >>> attr.fields(A).a.type typing.List[A] >>> attr.fields(A).b.type <class 'B'>
attr.asdict
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
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 the attributes in attr.asdict and `attr.astuple`:
attr.filters.include
attr.filters.exclude
See asdict
for examples.
attr.evolve
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 withevolve
. - the usual
__init__
validators will validate the new values.
validate
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 <class 'int'> (got '1' that is a <class 'str'>).", ...)
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:
set_run_validators
get_run_validators
attrs
comes with some common validators in the attrs.validators
module:
attr.validators.lt
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.lt(42)) >>> C(41) C(x=41) >>> C(42) Traceback (most recent call last): ... ValueError: ("'x' must be < 42: 42")
attr.validators.le
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.le(42)) >>> C(42) C(x=42) >>> C(43) Traceback (most recent call last): ... ValueError: ("'x' must be <= 42: 43")
attr.validators.ge
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.ge(42)) >>> C(42) C(x=42) >>> C(41) Traceback (most recent call last): ... ValueError: ("'x' must be => 42: 41")
attr.validators.gt
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.gt(42)) >>> C(43) C(x=43) >>> C(42) Traceback (most recent call last): ... ValueError: ("'x' must be > 42: 42")
attr.validators.max_len
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.max_len(4)) >>> C("spam") C(x='spam') >>> C("bacon") Traceback (most recent call last): ... ValueError: ("Length of 'x' must be <= 4: 5")
attr.validators.instance_of
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, 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)
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
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
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, kw_only=False), <type 'int'>, '42') >>> C(None) C(x=None)
attr.validators.is_callable
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.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'>).
attr.validators.matches_re
For example:
>>> @attr.s ... class User(object): ... email = attr.ib(validator=attr.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')
attr.validators.deep_iterable
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.deep_iterable( ... member_validator=attr.validators.instance_of(int), ... iterable_validator=attr.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')
attr.validators.deep_mapping
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(validator=attr.validators.deep_mapping( ... key_validator=attr.validators.instance_of(str), ... value_validator=attr.validators.instance_of(int), ... mapping_validator=attr.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:
attr.validators.set_disabled
attr.validators.get_disabled
attr.validators.disabled
attr.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])
attr.converters.optional
For example:
>>> @attr.s ... class C(object): ... x = attr.ib(converter=attr.converters.optional(int)) >>> C(None) C(x=None) >>> C(42) C(x=42)
attr.converters.default_if_none
For example:
>>> @attr.s ... class C(object): ... x = attr.ib( ... converter=attr.converters.default_if_none("") ... ) >>> C(None) C(x='')
attr.converters.to_bool
For example:
>>> @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 attr.s's and attr.ib's on_setattr
arguments.
attr.setters.frozen
attr.setters.validate
attr.setters.convert
attr.setters.pipe
attr.setters.NO_OP
For example, only x
is frozen here:
>>> @attr.s(on_setattr=attr.setters.frozen) ... class C(object): ... x = attr.ib() ... y = attr.ib(on_setattr=attr.setters.NO_OP) >>> c = C(1, 2) >>> c.y = 3 >>> c.y 3 >>> c.x = 4 Traceback (most recent call last): ... attr.exceptions.FrozenAttributeError: ()
N.B. Please use attr.s's frozen argument to freeze whole classes; it is more efficient.
These are Python 3.6 and later-only, and keyword-only APIs that call attr.s with different default values.
The most notable differences are:
- automatically detect whether or not auto_attribs should be True
- slots=True (see
slotted classes
for potentially surprising behaviors) - auto_exc=True
- auto_detect=True
- eq=True, but order=False
- Converters and validators are run when you set an attribute (on_setattr=[attr.setters.convert, attr.setters.validate]).
- Some options that aren't relevant to Python 3 have been dropped.
Please note that these are defaults and you're free to override them, just like before.
Since the Python ecosystem has settled on the term field
for defining attributes, we have also added attr.field as a substitute for attr.ib.
21.3.0 Converters are also run on_setattr
.
Note
attr.s and attr.ib (and their serious business cousins) aren't going anywhere. The new APIs build on top of them.
attr.define
mutable(same_as_define)
Alias for attr.define.
20.1.0
frozen(same_as_define)
Behaves the same as attr.define but sets frozen=True and on_setattr=None.
20.1.0
attr.field
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`:
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.
assoc