Python's super(), abstract base classes, and NotImplementedError

Question

Abstract base classes can still be handy in Python. In writing an abstract base class where I want every subclass to have, say, a spam() method, I want to write something like this:

class Abstract(object):
    def spam(self):
        raise NotImplementedError

The challenge comes in also wanting to use super(), and to do it properly by including it in the entire chain of subclasses. In this case, it seems I have to wrap every super call like the following:

class Useful(Abstract):
    def spam(self):
        try:
            super(Useful, self).spam()
        except NotImplementedError, e:
            pass
        print("It's okay.")

That's okay for a simple subclass, but when writing a class that has many methods, the try-except thing gets a bit cumbersome, and a bit ugly. Is there a more elegant way of subclassing from abstract base classes? Am I just Doing It Wrong?

Answer 1

You can do this cleanly in python 2.6+ with the abc module:

import abc
class B(object):
    __metaclass__ = abc.ABCMeta
    @abc.abstractmethod
    def foo(self):
        print 'In B'

class C(B):
    def foo(self):
        super(C, self).foo()
        print 'In C'

C().foo()

The output will be

In B
In C