Using singletons in Python (Django)

I was suggested to define a singleton in Python in the following way:

class Controller:
    pass
    # ...

controller = Controller() However, this way, controller cannot be replaced with a derived class.

What are other sensible ways to define singletons in Python Django?

Maybe, when defining a reusable app, I should not define the singleton instance right in the app, but define it in the program which uses it? What are advantages and disadvantages of doing this?