28.8.abc — Abstract Base Classes¶

Source code:Lib/abc.py

This module provides the infrastructure for definingabstract baseclasses (ABCs) in Python, as outlined inPEP 3119; see the PEP for why thiswas added to Python. (See alsoPEP 3141 and thenumbers moduleregarding a type hierarchy for numbers based on ABCs.)

Thecollections module has some concrete classes that derive fromABCs; these can, of course, be further derived. In addition, thecollections module has some ABCs that can be used to test whethera class or instance provides a particular interface, for example, if it ishashable or if it is a mapping.

This module provides the following class:

classabc.ABCMeta¶

Metaclass for defining Abstract Base Classes (ABCs).

Use this metaclass to create an ABC. An ABC can be subclassed directly, andthen acts as a mix-in class. You can also register unrelated concreteclasses (even built-in classes) and unrelated ABCs as “virtual subclasses” –these and their descendants will be considered subclasses of the registeringABC by the built-inissubclass() function, but the registering ABCwon’t show up in their MRO (Method Resolution Order) nor will methodimplementations defined by the registering ABC be callable (not even viasuper()).1

Classes created with a metaclass ofABCMeta have the following method:

register(subclass)¶

Registersubclass as a “virtual subclass” of this ABC. Forexample:

fromabcimportABCMetaclassMyABC:__metaclass__=ABCMetaMyABC.register(tuple)assertissubclass(tuple,MyABC)assertisinstance((),MyABC)

You can also override this method in an abstract base class:

__subclasshook__(subclass)¶

(Must be defined as a class method.)

Check whethersubclass is considered a subclass of this ABC. This meansthat you can customize the behavior ofissubclass further without theneed to callregister() on every class you want to consider asubclass of the ABC. (This class method is called from the__subclasscheck__() method of the ABC.)

This method should returnTrue,False orNotImplemented. Ifit returnsTrue, thesubclass is considered a subclass of this ABC.If it returnsFalse, thesubclass is not considered a subclass ofthis ABC, even if it would normally be one. If it returnsNotImplemented, the subclass check is continued with the usualmechanism.

For a demonstration of these concepts, look at this example ABC definition:

classFoo(object):def__getitem__(self,index):...def__len__(self):...defget_iterator(self):returniter(self)classMyIterable:__metaclass__=ABCMeta@abstractmethoddef__iter__(self):whileFalse:yieldNonedefget_iterator(self):returnself.__iter__()@classmethoddef__subclasshook__(cls,C):ifclsisMyIterable:ifany("__iter__"inB.__dict__forBinC.__mro__):returnTruereturnNotImplementedMyIterable.register(Foo)

The ABCMyIterable defines the standard iterable method,__iter__(), as an abstract method. The implementation givenhere can still be called from subclasses. Theget_iterator() methodis also part of theMyIterable abstract base class, but it does not haveto be overridden in non-abstract derived classes.

The__subclasshook__() class method defined here says that any classthat has an__iter__() method in its__dict__ (or in that of one of its base classes, accessedvia the__mro__ list) is considered aMyIterable too.

Finally, the last line makesFoo a virtual subclass ofMyIterable,even though it does not define an__iter__() method (it usesthe old-style iterable protocol, defined in terms of__len__() and__getitem__()). Note that this will not makeget_iteratoravailable as a method ofFoo, 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 isABCMeta oris derived from it.A class that has a metaclass derived fromABCMetacannot be instantiated unless all of its abstract methods andproperties are overridden.The abstract methods can be called using any of the normal ‘super’ callmechanisms.

Dynamically adding abstract methods to a class, or attempting to modify theabstraction status of a method or class once it is created, are notsupported. Theabstractmethod() only affects subclasses derived usingregular inheritance; “virtual subclasses” registered with the ABC’sregister() method are not affected.

Usage:

classC:__metaclass__=ABCMeta@abstractmethoddefmy_abstract_method(self,...):...

Note

Unlike Java abstract methods, these abstractmethods may have an implementation. This implementation can becalled via thesuper() mechanism from the class thatoverrides it. This could be useful as an end-point for asuper-call in a framework that uses cooperativemultiple-inheritance.

abc.abstractproperty([fget[,fset[,fdel[,doc]]]])¶

A subclass of the built-inproperty(), indicating an abstract property.

Using this function requires that the class’s metaclass isABCMeta oris derived from it.A class that has a metaclass derived fromABCMeta cannot beinstantiated 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:

classC:__metaclass__=ABCMeta@abstractpropertydefmy_abstract_property(self):...

This defines a read-only property; you can also define a read-write abstractproperty using the ‘long’ form of property declaration:

classC:__metaclass__=ABCMetadefgetx(self):...defsetx(self,value):...x=abstractproperty(getx,setx)

Footnotes

1

C++ programmers should note that Python’s virtual base classconcept is not the same as C++’s.