cpython/Doc/library/abc.rst

: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, is it
hashable or a mapping.


This module provides the following classes:

.. 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 ABCMeta

        class MyABC(metaclass=ABCMeta):
            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(metaclass=ABCMeta):

          @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.


.. 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.

   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.

   .. versionadded:: 3.4


The :mod:`abc` module also provides the following decorators:

.. 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 not
   supported.  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(metaclass=ABCMeta):
          @abstractmethod
          def my_abstract_method(self, ...):
              ...
          @classmethod
          @abstractmethod
          def my_abstract_classmethod(cls, ...):
              ...
          @staticmethod
          @abstractmethod
          def my_abstract_staticmethod(...):
              ...

          @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 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.


.. decorator:: abstractclassmethod

   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(metaclass=ABCMeta):
          @classmethod
          @abstractmethod
          def my_abstract_classmethod(cls, ...):
              ...

   .. versionadded:: 3.2
   .. deprecated:: 3.3
       It is now possible to use :class:`classmethod` with
       :func:`abstractmethod`, making this decorator redundant.


.. decorator:: abstractstaticmethod

   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(metaclass=ABCMeta):
          @staticmethod
          @abstractmethod
          def my_abstract_staticmethod(...):
              ...

   .. versionadded:: 3.2
   .. deprecated:: 3.3
       It is now possible to use :class:`staticmethod` with
       :func:`abstractmethod`, making this decorator redundant.


.. decorator:: abstractproperty(fget=None, fset=None, fdel=None, doc=None)

   A subclass of the built-in :func:`property`, indicating an abstract
   property.

   Using this function 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 properties can be called using
   any of the normal 'super' call mechanisms.

   This special case is deprecated, as the :func:`property` decorator
   is now correctly identified as abstract when applied to an abstract
   method::

      class C(metaclass=ABCMeta):
          @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(metaclass=ABCMeta):
          @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):
              ...


   .. 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.


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


.. rubric:: Footnotes

.. [#] C++ programmers should note that Python's virtual base class
   concept is not the same as C++'s.