Python - when is 'import' required?

Question

mod1.py

import mod2

class Universe:
    def __init__(self):
        pass
    def answer(self):
        return 42

u = Universe()
mod2.show_answer(u)

mod2.py

#import mod1 -- not necessary
def show_answer(thing):
    print thing.answer()

Coming from a C++ background I had the feeling it was necessary to import the module containing the Universe class definition before the show_answer function would work. I.e. everything had to be declared before it could be used.

Am I right in thinking this isn't necessary? This is duck typing, right? So if an import isn't required to see the methods of a class, I'd at least need it for the class definition itself and the top level functions of a module?

In one script I've written, I even went as far as writing a base class to declare an interface with a set of methods, and then deriving concrete classes to inherit that interface, but I think I get it now - that's just wrong in Python, and whether an object has a particular method is checked at runtime at the point where the call is made?

I realise Python is so much more dynamic than C++, it's taken me a while to see how little code you actually need to write!

I think I know the answer to this question, but I just wanted to get clarification and make sure I was on the right track.

UPDATE: Thanks for all the answers, I think I should clarify my question now:

Does mod2.show_answer() need an import (of any description) to know that thing has a method called answer(), or is that determined dynamically at runtime?

Answer 1

In this case you're right: show_answer() is given an object, of which it calls the method "answer". As long as the object given to show_answer() has such a method, it doesn't matter where the object comes from.

If, however, you wanted to create an instance of Universe inside mod2, you'd have to import mod1, because Universe is not in the mod2 namespace, even after mod2 has been imported by mod1.