abc — Abstract Base Classes

This module provides the infrastructure for defining abstract base classes (ABCs) in Python, as outlined in PEP 3119; see the PEP for why this was added to Python. (See also, PEP 3141 regarding a type hierarchy for numbers based on ABCs.)

The collections module has some concrete classes that derive from ABCs; these can, of course, be further derived. In addition the collections module has some ABCs that can be used to test whether a class or instance provides a particular interface, for example, is it hashable or a mapping.

This module provides the following class:

class abc.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 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 super()). [1]

Classes created with a metaclass of ABCMeta have the following method:

abc.register(subclass)¶

Register subclass as a “virtual subclass” of this ABC. For example:

System Message: WARNING/2 (<string>, line 79)

Literal block expected; none found.

It also provides the following decorators:

abc.abstractmethod(function)¶

A decorator indicating abstract methods.

Using this decorator requires that the class’s metaclass is ABCMeta or is derived from it. A class that has a metaclass derived from ABCMeta cannot be instantiated unless all of its abstract methods and properties are overridden. The abstract methods can be called using any of the the normal ‘super’ call mechanisms.

Dynamically adding abstract methods to a class, or attempting to modify the abstraction status of a method or class once it is created, are not supported. The abstractmethod() only affects subclasses derived using regular inheritance; “virtual subclasses” registered with the ABC’s register() method are not affected.

Usage:

class C(metaclass=ABCMeta):
    @abstractmethod
    def my_abstract_method(self, ...):
        ...

Note

Unlike C++’s pure virtual functions, or Java abstract methods, these abstract methods may have an implementation. This implementation can be called via the 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.

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

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

Using this function requires that the class’s metaclass is ABCMeta or is derived from it. A class that has a metaclass derived from ABCMeta cannot be instantiated 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:

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)

Footnotes