:mod:`abc` --- Abstract Base Classes

.. module:: abc
   :synopsis: Abstract base classes according to PEP 3119.

.. moduleauthor:: Guido van Rossum

.. sectionauthor:: Georg Brandl

.. versionadded:: 2.6

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` 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:

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`). [1]

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:`~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.

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 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 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)

Footnotes