mirror of https://github.com/python/cpython
360 lines
12 KiB
ReStructuredText
360 lines
12 KiB
ReStructuredText
: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
|
|
|
|
**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:`~class.__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:`~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:`~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.
|