What is the purpose of python's inner classes?

Question

Python\'s inner/nested classes confuse me. Is there something that can\'t be accomplished without them? If so, what is that thing?

Answer 1

Is there something that can't be accomplished without them? If so, what is that thing?

There is something that cannot be easily done without: inheritance of related classes.

Here is a minimalist example with the related classes A and B:

class A(object):
    class B(object):
        def __init__(self, parent):
            self.parent = parent

    def make_B(self):
        return self.B(self)


class AA(A):  # Inheritance
    class B(A.B):  # Inheritance, same class name
        pass

This code leads to a quite reasonable and predictable behaviour:

>>> type(A().make_B())

>>> type(A().make_B().parent)

>>> type(AA().make_B())

>>> type(AA().make_B().parent)

If B were a top-level class, you could not write self.B() in the method make_B but would simply write B(), and thus lose the dynamic binding to the adequate classes.

Note that in this construction, you should never refer to class A in the body of class B. This is the motivation for introducing the parent attribute in class B.

Of course, this dynamic binding can be recreated without inner class at the cost of a tedious and error-prone instrumentation of the classes.