Difference between abstract class and interface in Python

Question

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

Answer 1

What you'll see sometimes is the following:

class Abstract1( object ):
    """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( object ):
    pass # lots of stuff - but missing something

class Mixin1( object ):
    def something( self ):
        pass # one implementation

class Mixin2( object ):
    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.