: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

.. versionadded:: 2.6

This module provides the infrastructure for defining an :term:`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` 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

	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(object):

          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.