Which Exception for notifying that subclass should implement a method?

In Python 2.6 and better, you can use the abc module to make Base an "actually" abstract base class:

import abc

class Base:
    __metaclass__ = abc.ABCMeta
    @abc.abstractmethod
    def g(self):
        pass
    def f(self): # &c

this guarantees that Base cannot be instantiated -- and neither can any subclass which fails to override g -- while meeting @Aaron's target of allowing subclasses to use super in their g implementations. Overall, a much better solution than what we used to have in Python 2.5 and earlier!

Side note: having Base inherit from object would be redundant, because the metaclass needs to be set explicitly anyway.

Make a method that does nothing, but still has a docstring explaining the interface. Getting a NameError is confusing, and raising NotImplementedError (or any other exception, for that matter) will break proper usage of super.