28.8. "abc" — Abstract Base Classes
***********************************

New in version 2.6.

**Source code:** Lib/abc.py

======================================================================

This module provides the infrastructure for defining *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
"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, if it
is hashable or if it is 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

         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(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,
   "__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 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 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.
