Initial documentation for the `abc` module.

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+
+: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 abstract base classes
+(ABCs) in Python, as outlined in :pep:`3119`.
+
+Concrete base ABCs to derive from can be found in the :mod:`collections` module.
+
+
+The 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.  From now on,
+      ``issubclass(subclass, ABC)`` is true.
+
+
+   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 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.
+
+
+   To demonstrate these concepts, look at this example ABC definition::
+
+      class MyIterator:
+          pass
+
+      class Iterator(metaclass=ABCMeta):
+
+          @abstractmethod
+          def __next__(self):
+              raise StopIteration
+
+          def __iter__(self):
+              return self
+
+          @classmethod
+          def __subclasshook__(cls, C):
+              if cls is Iterator:
+                  if any("__next__" in B.__dict__ for B in C.__mro__):
+                      return True
+              return NotImplemented
+
+      Iterator.register(MyIterator)
+
+   The ABC ``Iterator`` defines the two standard iterator methods:
+   :meth:`__iter__` and :meth:`__next__`.  The :meth:`__iter__` method is given
+   a default implementation, while the :meth:`__next__` method is abstract.
+
+   .. XXX why is an implementation given then?
+
+   The :meth:`__subclasshook__` class method defined here says that any class
+   that has a :meth:`__next__` method in its :attr:`__dict__` (or in that of one
+   of its subclasses, accessed via the :attr:`__mro__`) is considered an
+   ``Iterator`` too.
+
+   Finally, the last line makes ``MyIterator`` a virtual subclass of
+   ``Iterator``, even though it does not define a :meth:`__next__` method.
+   (Of course, this doesn't make much sense in this context.)
+
+   .. XXX perhaps find better example
+
+
+It also provides the following decorators:
+
+.. function:: abstractmethod(function)
+
+   A decorator indicating abstract methods.
+
+   Requires that the metaclass is :class:`ABCMeta` or derived from it.  A class
+   that has a metaclass derived from :class:`ABCMeta` cannot be instantiated
+   unless all of its abstract methods are overridden.  The abstract methods can
+   be called using any of the the normal 'super' call mechanisms.
+
+   Usage::
+
+      class C(metaclass=ABCMeta):
+          @abstractmethod
+          def my_abstract_method(self, ...):
+              ...
+
+
+.. function:: abstractproperty(property)
+
+   A decorator indicating abstract properties.
+
+   Requires that the metaclass is :class:`ABCMeta` or derived from it.  A class
+   that has a metaclass derived from :class:`ABCMeta` cannot be instantiated
+   unless all of its abstract properties are overridden.  The abstract
+   properties can be called using any of the 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)
+