2
:mod:`abc` --- Abstract Base Classes
3
====================================
6
:synopsis: Abstract base classes according to PEP 3119.
7
.. moduleauthor:: Guido van Rossum
8
.. sectionauthor:: Georg Brandl
9
.. much of the content adapted from docstrings
11
This module provides the infrastructure for defining an :term:`abstract base
12
class` (ABCs) in Python, as outlined in :pep:`3119`; see the PEP for why this
13
was added to Python. (See also :pep:`3141` and the :mod:`numbers` module
14
regarding a type hierarchy for numbers based on ABCs.)
16
The :mod:`collections` module has some concrete classes that derive from
17
ABCs; these can, of course, be further derived. In addition the
18
:mod:`collections` module has some ABCs that can be used to test whether
19
a class or instance provides a particular interface, for example, is it
20
hashable or a mapping.
23
This module provides the following class:
27
Metaclass for defining Abstract Base Classes (ABCs).
29
Use this metaclass to create an ABC. An ABC can be subclassed directly, and
30
then acts as a mix-in class. You can also register unrelated concrete
31
classes (even built-in classes) and unrelated ABCs as "virtual subclasses" --
32
these and their descendants will be considered subclasses of the registering
33
ABC by the built-in :func:`issubclass` function, but the registering ABC
34
won't show up in their MRO (Method Resolution Order) nor will method
35
implementations defined by the registering ABC be callable (not even via
38
Classes created with a metaclass of :class:`ABCMeta` have the following method:
40
.. method:: register(subclass)
42
Register *subclass* as a "virtual subclass" of this ABC. For
45
from abc import ABCMeta
47
class MyABC(metaclass=ABCMeta):
52
assert issubclass(tuple, MyABC)
53
assert isinstance((), MyABC)
55
You can also override this method in an abstract base class:
57
.. method:: __subclasshook__(subclass)
59
(Must be defined as a class method.)
61
Check whether *subclass* is considered a subclass of this ABC. This means
62
that you can customize the behavior of ``issubclass`` further without the
63
need to call :meth:`register` on every class you want to consider a
64
subclass of the ABC. (This class method is called from the
65
:meth:`__subclasscheck__` method of the ABC.)
67
This method should return ``True``, ``False`` or ``NotImplemented``. If
68
it returns ``True``, the *subclass* is considered a subclass of this ABC.
69
If it returns ``False``, the *subclass* is not considered a subclass of
70
this ABC, even if it would normally be one. If it returns
71
``NotImplemented``, the subclass check is continued with the usual
74
.. XXX explain the "usual mechanism"
77
For a demonstration of these concepts, look at this example ABC definition::
80
def __getitem__(self, index):
84
def get_iterator(self):
87
class MyIterable(metaclass=ABCMeta):
94
def get_iterator(self):
95
return self.__iter__()
98
def __subclasshook__(cls, C):
100
if any("__iter__" in B.__dict__ for B in C.__mro__):
102
return NotImplemented
104
MyIterable.register(Foo)
106
The ABC ``MyIterable`` defines the standard iterable method,
107
:meth:`__iter__`, as an abstract method. The implementation given here can
108
still be called from subclasses. The :meth:`get_iterator` method is also
109
part of the ``MyIterable`` abstract base class, but it does not have to be
110
overridden in non-abstract derived classes.
112
The :meth:`__subclasshook__` class method defined here says that any class
113
that has an :meth:`__iter__` method in its :attr:`__dict__` (or in that of
114
one of its base classes, accessed via the :attr:`__mro__` list) is
115
considered a ``MyIterable`` too.
117
Finally, the last line makes ``Foo`` a virtual subclass of ``MyIterable``,
118
even though it does not define an :meth:`__iter__` method (it uses the
119
old-style iterable protocol, defined in terms of :meth:`__len__` and
120
:meth:`__getitem__`). Note that this will not make ``get_iterator``
121
available as a method of ``Foo``, so it is provided separately.
124
It also provides the following decorators:
126
.. function:: abstractmethod(function)
128
A decorator indicating abstract methods.
130
Using this decorator requires that the class's metaclass is :class:`ABCMeta` or
132
A class that has a metaclass derived from :class:`ABCMeta`
133
cannot be instantiated unless all of its abstract methods and
134
properties are overridden.
135
The abstract methods can be called using any of the the normal 'super' call
138
Dynamically adding abstract methods to a class, or attempting to modify the
139
abstraction status of a method or class once it is created, are not
140
supported. The :func:`abstractmethod` only affects subclasses derived using
141
regular inheritance; "virtual subclasses" registered with the ABC's
142
:meth:`register` method are not affected.
146
class C(metaclass=ABCMeta):
148
def my_abstract_method(self, ...):
153
Unlike Java abstract methods, these abstract
154
methods may have an implementation. This implementation can be
155
called via the :func:`super` mechanism from the class that
156
overrides it. This could be useful as an end-point for a
157
super-call in a framework that uses cooperative
158
multiple-inheritance.
161
.. function:: abstractproperty(fget[, fset[, fdel[, doc]]])
163
A subclass of the built-in :func:`property`, indicating an abstract property.
165
Using this function requires that the class's metaclass is :class:`ABCMeta` or
167
A class that has a metaclass derived from :class:`ABCMeta` cannot be
168
instantiated unless all of its abstract methods and properties are overridden.
169
The abstract properties can be called using any of the normal
170
'super' call mechanisms.
174
class C(metaclass=ABCMeta):
176
def my_abstract_property(self):
179
This defines a read-only property; you can also define a read-write abstract
180
property using the 'long' form of property declaration::
182
class C(metaclass=ABCMeta):
184
def setx(self, value): ...
185
x = abstractproperty(getx, setx)
187
.. rubric:: Footnotes
189
.. [#] C++ programmers should note that Python's virtual base class
190
concept is not the same as C++'s.