Question

Suppose that I have the following python base class:

class BaseClass(object):
    def a():
        """This method uses method b(), defined in the inheriting class"""

And also a class that inherites BaseClass:

class UsedByUser(BaseClass):
    def b():
        """b() is defined here, yet is used by the base class"""

My user would only create instances of class UsedByUser. Typical use would be:

if __name__ == '__main__':
    # initialize the class used by the user        
    usedByUser = UsedByUser()

    # invoke method a()
    usedByUser.a()

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My questions is, is the above use problematic? is this a valid approach, or must I also define method b() in BaseClass and then override it in UsedByUser?

Answer 1

I would define the b method in the BaseClass too:

class BaseClass(object):
    def b(self):
        raise NotImplementedError('b must be implemented by a subclass')

Remember: explicit is better than implicit and given that the method a needs the method b anyways, better raise a meaningful exception rather than a general AttributeError.

It is worth to point out that this is absolutely NOT needed from a syntactic point of view, but it adds clarity to the code and enforces the subclass to provide an implementation.