:mod:`abc` --- Abstract Base Classes
====================================
.. module:: abc
:synopsis: Abstract base classes according to :pep:`3119`.
.. moduleauthor:: Guido van Rossum
.. sectionauthor:: Georg Brandl
.. much of the content adapted from docstrings
**Source code:** :source:`Lib/abc.py`
--------------
This module provides the infrastructure for defining :term:`abstract base
classes <abstract base class>` (ABCs) in Python, as outlined in :pep:`3119`;
see the PEP for why this was added to Python. (See also :pep:`3141` and the
:mod:`numbers` module regarding a type hierarchy for numbers based on ABCs.)
The :mod:`collections` module has some concrete classes that derive from
ABCs; these can, of course, be further derived. In addition, the
:mod:`collections.abc` submodule has some ABCs that can be used to test whether
a class or instance provides a particular interface, for example, if it is
hashable or if it is a mapping.
This module provides the metaclass :class:`ABCMeta` for defining ABCs and
a helper class :class:`ABC` to alternatively define ABCs through inheritance:
.. class:: ABC
A helper class that has :class:`ABCMeta` as its metaclass. With this class,
an abstract base class can be created by simply deriving from :class:`ABC`
avoiding sometimes confusing metaclass usage, for example::
from abc import ABC
class MyABC(ABC):
pass
Note that the type of :class:`ABC` is still :class:`ABCMeta`, therefore
inheriting from :class:`ABC` requires the usual precautions regarding
metaclass usage, as multiple inheritance may lead to metaclass conflicts.
One may also define an abstract base class by passing the metaclass
keyword and using :class:`ABCMeta` directly, for example::
from abc import ABCMeta
class MyABC(metaclass=ABCMeta):
pass
.. versionadded:: 3.4
.. class:: ABCMeta
Metaclass for defining Abstract Base Classes (ABCs).
Use this metaclass to create an ABC. An ABC can be subclassed directly, and
then acts as a mix-in class. You can also register unrelated concrete
classes (even built-in classes) and unrelated ABCs as "virtual subclasses" --
these and their descendants will be considered subclasses of the registering
ABC by the built-in :func:`issubclass` function, but the registering ABC
won't show up in their MRO (Method Resolution Order) nor will method
implementations defined by the registering ABC be callable (not even via
:func:`super`). [#]_
Classes created with a metaclass of :class:`ABCMeta` have the following method:
.. method:: register(subclass)
Register *subclass* as a "virtual subclass" of this ABC. For
example::
from abc import ABC
class MyABC(ABC):
pass
MyABC.register(tuple)
assert issubclass(tuple, MyABC)
assert isinstance((), MyABC)
.. versionchanged:: 3.3
Returns the registered subclass, to allow usage as a class decorator.
.. versionchanged:: 3.4
To detect calls to :meth:`register`, you can use the
:func:`get_cache_token` function.
You can also override this method in an abstract base class:
.. method:: __subclasshook__(subclass)
(Must be defined as a class method.)
Check whether *subclass* is considered a subclass of this ABC. This means
that you can customize the behavior of ``issubclass`` further without the
need to call :meth:`register` on every class you want to consider a
subclass of the ABC. (This class method is called from the
:meth:`__subclasscheck__` method of the ABC.)
This method should return ``True``, ``False`` or ``NotImplemented``. If
it returns ``True``, the *subclass* is considered a subclass of this ABC.
If it returns ``False``, the *subclass* is not considered a subclass of
this ABC, even if it would normally be one. If it returns
``NotImplemented``, the subclass check is continued with the usual
mechanism.
.. XXX explain the "usual mechanism"
For a demonstration of these concepts, look at this example ABC definition::
class Foo:
def __getitem__(self, index):
...
def __len__(self):
...
def get_iterator(self):
return iter(self)
class MyIterable(ABC):
@abstractmethod
def __iter__(self):
while False:
yield None
def get_iterator(self):
return self.__iter__()
@classmethod
def __subclasshook__(cls, C):
if cls is MyIterable:
if any("__iter__" in B.__dict__ for B in C.__mro__):
return True
return NotImplemented
MyIterable.register(Foo)
The ABC ``MyIterable`` defines the standard iterable method,
:meth:`~iterator.__iter__`, as an abstract method. The implementation given
here can still be called from subclasses. The :meth:`get_iterator` method
is also part of the ``MyIterable`` abstract base class, but it does not have
to be overridden in non-abstract derived classes.
The :meth:`__subclasshook__` class method defined here says that any class
that has an :meth:`~iterator.__iter__` method in its
:attr:`~object.__dict__` (or in that of one of its base classes, accessed
via the :attr:`~class.__mro__` list) is considered a ``MyIterable`` too.
Finally, the last line makes ``Foo`` a virtual subclass of ``MyIterable``,
even though it does not define an :meth:`~iterator.__iter__` method (it uses
the old-style iterable protocol, defined in terms of :meth:`__len__` and
:meth:`__getitem__`). Note that this will not make ``get_iterator``
available as a method of ``Foo``, so it is provided separately.
The :mod:`abc` module also provides the following decorator:
.. decorator:: abstractmethod
A decorator indicating abstract methods.
Using this decorator requires that the class's metaclass is :class:`ABCMeta`
or is derived from it. A class that has a metaclass derived from
:class:`ABCMeta` cannot be instantiated unless all of its abstract methods
and properties are overridden. The abstract methods can be called using any
of the normal 'super' call mechanisms. :func:`abstractmethod` may be used
to declare abstract methods for properties and descriptors.
Dynamically adding abstract methods to a class, or attempting to modify the
abstraction status of a method or class once it is created, are only
supported using the :func:`update_abstractmethods` function. The
:func:`abstractmethod` only affects subclasses derived using regular
inheritance; "virtual subclasses" registered with the ABC's :meth:`register`
method are not affected.
When :func:`abstractmethod` is applied in combination with other method
descriptors, it should be applied as the innermost decorator, as shown in
the following usage examples::
class C(ABC):
@abstractmethod
def my_abstract_method(self, arg1):
...
@classmethod
@abstractmethod
def my_abstract_classmethod(cls, arg2):
...
@staticmethod
@abstractmethod
def my_abstract_staticmethod(arg3):
...
@property
@abstractmethod
def my_abstract_property(self):
...
@my_abstract_property.setter
@abstractmethod
def my_abstract_property(self, val):
...
@abstractmethod
def _get_x(self):
...
@abstractmethod
def _set_x(self, val):
...
x = property(_get_x, _set_x)
In order to correctly interoperate with the abstract base class machinery,
the descriptor must identify itself as abstract using
:attr:`__isabstractmethod__`. In general, this attribute should be ``True``
if any of the methods used to compose the descriptor are abstract. For
example, Python's built-in :class:`property` does the equivalent of::
class Descriptor:
...
@property
def __isabstractmethod__(self):
return any(getattr(f, '__isabstractmethod__', False) for
f in (self._fget, self._fset, self._fdel))
.. note::
Unlike Java abstract methods, these abstract
methods may have an implementation. This implementation can be
called via the :func:`super` mechanism from the class that
overrides it. This could be useful as an end-point for a
super-call in a framework that uses cooperative
multiple-inheritance.
The :mod:`abc` module also supports the following legacy decorators:
.. decorator:: abstractclassmethod
.. versionadded:: 3.2
.. deprecated:: 3.3
It is now possible to use :class:`classmethod` with
:func:`abstractmethod`, making this decorator redundant.
A subclass of the built-in :func:`classmethod`, indicating an abstract
classmethod. Otherwise it is similar to :func:`abstractmethod`.
This special case is deprecated, as the :func:`classmethod` decorator
is now correctly identified as abstract when applied to an abstract
method::
class C(ABC):
@classmethod
@abstractmethod
def my_abstract_classmethod(cls, arg):
...
.. decorator:: abstractstaticmethod
.. versionadded:: 3.2
.. deprecated:: 3.3
It is now possible to use :class:`staticmethod` with
:func:`abstractmethod`, making this decorator redundant.
A subclass of the built-in :func:`staticmethod`, indicating an abstract
staticmethod. Otherwise it is similar to :func:`abstractmethod`.
This special case is deprecated, as the :func:`staticmethod` decorator
is now correctly identified as abstract when applied to an abstract
method::
class C(ABC):
@staticmethod
@abstractmethod
def my_abstract_staticmethod(arg):
...
.. decorator:: abstractproperty
.. deprecated:: 3.3
It is now possible to use :class:`property`, :meth:`property.getter`,
:meth:`property.setter` and :meth:`property.deleter` with
:func:`abstractmethod`, making this decorator redundant.
A subclass of the built-in :func:`property`, indicating an abstract
property.
This special case is deprecated, as the :func:`property` decorator
is now correctly identified as abstract when applied to an abstract
method::
class C(ABC):
@property
@abstractmethod
def my_abstract_property(self):
...
The above example defines a read-only property; you can also define a
read-write abstract property by appropriately marking one or more of the
underlying methods as abstract::
class C(ABC):
@property
def x(self):
...
@x.setter
@abstractmethod
def x(self, val):
...
If only some components are abstract, only those components need to be
updated to create a concrete property in a subclass::
class D(C):
@C.x.setter
def x(self, val):
...
The :mod:`abc` module also provides the following functions:
.. function:: get_cache_token()
Returns the current abstract base class cache token.
The token is an opaque object (that supports equality testing) identifying
the current version of the abstract base class cache for virtual subclasses.
The token changes with every call to :meth:`ABCMeta.register` on any ABC.
.. versionadded:: 3.4
.. function:: update_abstractmethods(cls)
A function to recalculate an abstract class's abstraction status. This
function should be called if a class's abstract methods have been
implemented or changed after it was created. Usually, this function should
be called from within a class decorator.
Returns *cls*, to allow usage as a class decorator.
If *cls* is not an instance of :class:`ABCMeta`, does nothing.
.. note::
This function assumes that *cls*'s superclasses are already updated.
It does not update any subclasses.
.. versionadded:: 3.10
.. rubric:: Footnotes
.. [#] C++ programmers should note that Python's virtual base class
concept is not the same as C++'s.
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