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

This module provides the infrastructure for defining :term:`abstract base
classes` (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` module 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 class:

.. 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)

   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:`__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:`__iter__` method in its :attr:`__dict__` (or in that of
   one of its base classes, accessed via the :attr:`__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:`__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.


It also provides the following decorators:

.. function:: abstractmethod(function)

   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 the normal 'super' call
   mechanisms.

   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.

   Usage::

      class C(metaclass=ABCMeta):
          @abstractmethod
          def my_abstract_method(self, ...):
              ...

   .. note::

      Unlike C++'s pure virtual functions, or 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.


.. function:: abstractproperty(fget[, fset[, fdel[, doc]]])

   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.

   Usage::

      class C(metaclass=ABCMeta):
          @abstractproperty
          def my_abstract_property(self):
              ...

   This defines a read-only property; you can also define a read-write abstract
   property using the 'long' form of property declaration::

      class C(metaclass=ABCMeta):
          def getx(self): ...
          def setx(self, value): ...
          x = abstractproperty(getx, setx)

.. rubric:: Footnotes

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