API

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 Examples.

Core

attr.s(these=None, repr_ns=None, repr=True, cmp=True, hash=True, 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 (e.g. if you want to use properties).

    If these is not None, the class body is ignored.

  • 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 represantation 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) – Create a __hash__ method that returns the hash() of a tuple of all attrs attribute values.
  • init (bool) – Create a __init__ method that initialiazes 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.

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__.

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=True, init=True, convert=None, metadata={})

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 datatypes 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.

  • validator (callable) –

    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.

    They can be globally disabled and re-enabled using get_run_validators().

  • 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) – Include this attribute in the generated __hash__ method.
  • 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.

Note

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

class attr.Attribute(name, default, validator, repr, cmp, hash, init, convert=None, metadata=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()
>>> C.x
Attribute(name='x', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({}))
attr.make_class(name, attrs, **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.
  • attributes_arguments – Passed unmodified to attr.s().
Returns:

A new class with attrs.
Return type:

type

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)

Stores a factory callable.

If passed as the default value to attr.ib(), the factory is used to generate a new value.

For example:

>>> @attr.s
... class C(object):
...     x = attr.ib(default=attr.Factory(list))
>>> C()
C(x=[])
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.

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 accesors) 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=True, init=True, convert=None, metadata=mappingproxy({})), Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({})))
>>> attr.fields(C)[1]
Attribute(name='y', default=NOTHING, validator=None, repr=True, cmp=True, hash=True, init=True, convert=None, metadata=mappingproxy({}))
>>> 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 deteremines 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)))
{'y': {'y': 3, 'x': 2}, 'x': 1}
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.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:

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.assoc(i1, y=3)
>>> i2
C(x=1, y=3)
>>> i1 == i2
False
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=True, init=True), <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 perfomed 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.

The TypeError is raised 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 '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=True, init=True), <type 'int'>, None)
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.

The TypeError is raised with a human readable error message, the attribute (of type attr.Attribute), the expected interface, and the value it got.

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 – A validator that is used for non-None values.

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 'int'>, '42')
>>> C(None)
C(x=None)

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.