:mod:!abc --- Abstract Base Classes

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

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.abc submodule has some ABCs that can be used to test whether a class or instance provides a particular interface, for example, if it is :term:hashable or if it is a :term:mapping.

This module provides the metaclass :class:ABCMeta for defining ABCs and a helper class :class:ABC to alternatively define ABCs through inheritance:

.. class:: ABC

A helper class that has :class:ABCMeta as its metaclass. With this class, an abstract base class can be created by simply deriving from :class:!ABC avoiding sometimes confusing metaclass usage, for example::

 from abc import ABC

 class MyABC(ABC):
     pass

Note that the type of :class:!ABC is still :class:ABCMeta, therefore inheriting from :class:!ABC requires the usual precautions regarding metaclass usage, as multiple inheritance may lead to metaclass conflicts. One may also define an abstract base class by passing the metaclass keyword and using :class:!ABCMeta directly, for example::

 from abc import ABCMeta

 class MyABC(metaclass=ABCMeta):
     pass

.. versionadded:: 3.4

.. 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 ABC

     class MyABC(ABC):
         pass

     MyABC.register(tuple)

     assert issubclass(tuple, MyABC)
     assert isinstance((), MyABC)

  .. versionchanged:: 3.3
     Returns the registered subclass, to allow usage as a class decorator.

  .. versionchanged:: 3.4
     To detect calls to :meth:`!register`, you can use the
     :func:`get_cache_token` function.

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 :func:`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:`~type.__subclasscheck__` method of the ABC.)

  This method should return ``True``, ``False`` or :data:`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
  :data:`!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:
      def __getitem__(self, index):
          ...
      def __len__(self):
          ...
      def get_iterator(self):
          return iter(self)

  class MyIterable(ABC):

      @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:~object.__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:~object.__iter__ method in its :attr:~object.__dict__ (or in that of one of its base classes, accessed via the :attr:~type.__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:~object.__iter__ method (it uses the old-style iterable protocol, defined in terms of :meth:~object.__len__ and :meth:~object.__getitem__). Note that this will not make get_iterator available as a method of Foo, so it is provided separately.

The :mod:!abc module also provides the following decorator:

.. decorator:: abstractmethod

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. :func:!abstractmethod may be used to declare abstract methods for properties and descriptors.

Dynamically adding abstract methods to a class, or attempting to modify the abstraction status of a method or class once it is created, are only supported using the :func:update_abstractmethods function. The :func:!abstractmethod only affects subclasses derived using regular inheritance; "virtual subclasses" registered with the ABC's :meth:~ABCMeta.register method are not affected.

When :func:!abstractmethod is applied in combination with other method descriptors, it should be applied as the innermost decorator, as shown in the following usage examples::

  class C(ABC):
      @abstractmethod
      def my_abstract_method(self, arg1):
          ...
      @classmethod
      @abstractmethod
      def my_abstract_classmethod(cls, arg2):
          ...
      @staticmethod
      @abstractmethod
      def my_abstract_staticmethod(arg3):
          ...

      @property
      @abstractmethod
      def my_abstract_property(self):
          ...
      @my_abstract_property.setter
      @abstractmethod
      def my_abstract_property(self, val):
          ...

      @abstractmethod
      def _get_x(self):
          ...
      @abstractmethod
      def _set_x(self, val):
          ...
      x = property(_get_x, _set_x)

In order to correctly interoperate with the abstract base class machinery, the descriptor must identify itself as abstract using :attr:!__isabstractmethod__. In general, this attribute should be True if any of the methods used to compose the descriptor are abstract. For example, Python's built-in :class:property does the equivalent of::

  class Descriptor:
      ...
      @property
      def __isabstractmethod__(self):
          return any(getattr(f, '__isabstractmethod__', False) for
                     f in (self._fget, self._fset, self._fdel))

.. 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.

The :mod:!abc module also supports the following legacy decorators:

.. decorator:: abstractclassmethod

.. versionadded:: 3.2 .. deprecated:: 3.3 It is now possible to use :class:classmethod with :func:abstractmethod, making this decorator redundant.

A subclass of the built-in :func:classmethod, indicating an abstract classmethod. Otherwise it is similar to :func:abstractmethod.

This special case is deprecated, as the :func:classmethod decorator is now correctly identified as abstract when applied to an abstract method::

  class C(ABC):
      @classmethod
      @abstractmethod
      def my_abstract_classmethod(cls, arg):
          ...

.. decorator:: abstractstaticmethod

.. versionadded:: 3.2 .. deprecated:: 3.3 It is now possible to use :class:staticmethod with :func:abstractmethod, making this decorator redundant.

A subclass of the built-in :func:staticmethod, indicating an abstract staticmethod. Otherwise it is similar to :func:abstractmethod.

This special case is deprecated, as the :func:staticmethod decorator is now correctly identified as abstract when applied to an abstract method::

  class C(ABC):
      @staticmethod
      @abstractmethod
      def my_abstract_staticmethod(arg):
          ...

.. decorator:: abstractproperty

.. deprecated:: 3.3 It is now possible to use :class:property, :meth:property.getter, :meth:property.setter and :meth:property.deleter with :func:abstractmethod, making this decorator redundant.

A subclass of the built-in :func:property, indicating an abstract property.

This special case is deprecated, as the :func:property decorator is now correctly identified as abstract when applied to an abstract method::

  class C(ABC):
      @property
      @abstractmethod
      def my_abstract_property(self):
          ...

The above example defines a read-only property; you can also define a read-write abstract property by appropriately marking one or more of the underlying methods as abstract::

  class C(ABC):
      @property
      def x(self):
          ...

      @x.setter
      @abstractmethod
      def x(self, val):
          ...

If only some components are abstract, only those components need to be updated to create a concrete property in a subclass::

  class D(C):
      @C.x.setter
      def x(self, val):
          ...

The :mod:!abc module also provides the following functions:

.. function:: get_cache_token()

Returns the current abstract base class cache token.

The token is an opaque object (that supports equality testing) identifying the current version of the abstract base class cache for virtual subclasses. The token changes with every call to :meth:ABCMeta.register on any ABC.

.. versionadded:: 3.4

.. function:: update_abstractmethods(cls)

A function to recalculate an abstract class's abstraction status. This function should be called if a class's abstract methods have been implemented or changed after it was created. Usually, this function should be called from within a class decorator.

Returns cls, to allow usage as a class decorator.

If cls is not an instance of :class:ABCMeta, does nothing.

.. note::

  This function assumes that *cls*'s superclasses are already updated.
  It does not update any subclasses.

.. versionadded:: 3.10

.. rubric:: Footnotes

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