
``abc`` --- Abstract Base Classes
*********************************

This module provides the infrastructure for defining an *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
``numbers`` module regarding a type hierarchy for numbers based on
ABCs.)

The ``collections`` module has some concrete classes that derive from
ABCs; these can, of course, be further derived. In addition the
``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 abc.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
   ``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
   ``super()``). [1]

   Classes created with a metaclass of ``ABCMeta`` have the following
   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:

   __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 ``register()`` on every class
      you want to consider a subclass of the ABC.  (This class method
      is called from the ``__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.

   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,
   ``__iter__()``, as an abstract method.  The implementation given
   here can still be called from subclasses.  The ``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 ``__subclasshook__()`` class method defined here says that any
   class that has an ``__iter__()`` method in its ``__dict__`` (or in
   that of one of its base classes, accessed via the ``__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 ``__iter__()``
   method (it uses the old-style iterable protocol, defined in terms
   of ``__len__()`` and ``__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:

abc.abstractmethod(function)

   A decorator indicating abstract methods.

   Using this decorator requires that the class's metaclass is
   ``ABCMeta`` or is derived from it. A class that has a metaclass
   derived from ``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 ``abstractmethod()`` only affects
   subclasses derived using regular inheritance; "virtual subclasses"
   registered with the ABC's ``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 ``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.

abc.abstractproperty(fget[, fset[, fdel[, doc]]])

   A subclass of the built-in ``property()``, indicating an abstract
   property.

   Using this function requires that the class's metaclass is
   ``ABCMeta`` or is derived from it. A class that has a metaclass
   derived from ``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 ]-

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