Input-Dependent Generics in Python

In PTB, we have the class CallbackContext, which is integral in the framework and we want to allow users to customize it by subclassing and passing the subclass to the Updater/Dispatcher. Now type hinting becomes a little bit more difficult, because we need to make sure that the callbacks have the annotation Callable[Update, CallbackContext] or Callable[Update, UsersCallbackContext] and of course we would like to make that dependent on the users input. (For the full details see ptb/#2068)

First, we think of Generics, i.e. we make Dispatcher a generic class and use the generic type T in the annotations like callback: Callable[Update, T]. This works fine, but it still requires the user to tell the type checker, what T is, i.e. with

dispatcher = Dispatcher(..., custom_type = UsersCallbackContext)

MyPy will ask the user to specify the dispatchers type as follows:

dispatcher: Dispatcher[UsersCallbackContext] = Dispatcher(..., custom_context = UsersCallbackContext)

That's unnecessary repetition. Also, we would like to have T fall back to CallbackContext, if the user doesn't provide a custom class.

After poking around a bit, I came across a solution in this thread. The idea is to overload the self argument in the __init__ method as follows:

@overload
def __init__(self: Dispatcher[CallbackContext], *args):  # not listing optional arguments!
    ...

@overload
def __init__(self: Dispatcher[T], ..., custom_context: Type[T]):
    ...

This works like a charm :)