pythoninterfaceabstract-class

Difference between abstract class and interface in Python


What is the difference between abstract class and interface in Python?


Solution

  • What you'll see sometimes is the following:

    class Abstract1:
        """Some description that tells you it's abstract,
        often listing the methods you're expected to supply."""
    
        def aMethod(self):
            raise NotImplementedError("Should have implemented this")
    

    Because Python doesn't have (and doesn't need) a formal Interface contract, the Java-style distinction between abstraction and interface doesn't exist. If someone goes through the effort to define a formal interface, it will also be an abstract class. The only differences would be in the stated intent in the docstring.

    And the difference between abstract and interface is a hairsplitting thing when you have duck typing.

    Java uses interfaces because it doesn't have multiple inheritance.

    Because Python has multiple inheritance, you may also see something like this

    class SomeAbstraction:
        pass  # lots of stuff - but missing something
    
    class Mixin1:
        def something(self):
            pass  # one implementation
    
    class Mixin2:
        def something(self):
            pass  # another
    
    class Concrete1(SomeAbstraction, Mixin1):
        pass
    
    class Concrete2(SomeAbstraction, Mixin2):
        pass
    

    This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint.