tkinter.Event is not generic at runtime

[Akuli]

In #4347 I made tkinter.Event generic, so that you could write

def event_handler(event: tkinter.Event[tkinter.Label]) -> None:
    label_stuff(event.widget)

instead of

def event_handler(event: tkinter.Event) -> None:
    assert isinstance(event.widget, tkinter.Label)
    label_stuff(event.widget)

This doesn't actually work because tkinter.Event is not generic at runtime, so you need to use a string instead (or on 3.8+, from __future__ import annotations):

def event_handler(event: "tkinter.Event[tkinter.Label]") -> None:
    label_stuff(event.widget)

The assert code still works though because plain tkinter.Event is equivalent to tkinter.Event[Any]. So the stubs currently allow writing event: tkinter.Event or event: "tkinter.Event[Foo]", but I didn't realize the need for using a string when I made the pull request.

What we can do about this:

• Do nothing and recommend using tkinter.Event or "tkinter.Event[Foo]".
• Create a pull request to cpython that makes tkinter.Event generic at runtime. This won't actually help because 3.8+ has from __future__ import annotations and tkinter.Event wouldn't become generic in older pythons.
• Revert make tkinter.Event generic and add missing type hints to bind methods #4347 and just require assert isinstance(event.widget, Foo).

Also, what else can go wrong with making something generic in stubs but not at runtime?

[srittau]

Stubs can be generic, even if the runtime is not. I didn't check, but I wouldn't be surprised if nearly all generic types outside of typing aren't generic at runtime. Quoting or using the future import are the best way to handle this.

[hauntsaninja]

I believe future annotations is 3.7+ :-)
This comes up often in mypy issues, going to link https://mypy.readthedocs.io/en/stable/common_issues.html#using-classes-that-are-generic-in-stubs-but-not-at-runtime here in case it helps people stumbling onto this issue

[gvanrossum]

Oh, we may need to revise that page. In the wake of PEP 585 we made every stdlib class that’s generic in typeshed also generic in the stdlib. Of course this only affects 3.9+.