How do I design a multi-purpose event handler without making it generic?...

salon_sue

I am trying to design an event handler that can be used by different classes to manage events specific to that class. The event handler doesn't really need to know what type the events are as long as they are an Event subclass (or maybe I could use a Protocol?) Here is a simplified version which has a problem: EventHandler.add_subscription() won't accept a callback that takes a specific class of Event (i.e. a CustomEvent ) even though...

Originally asked by iamthebull on Stack Exchange (stackoverflow) (CC BY-SA 4.0).

Variant 68.

Variant 593.

uphilltech

Great question — answering as the UphillTech team:

Your error boils down to the following minimal example: from typing import Callable class Foo: pass class Bar(Foo): pass def foo_callback(foo: Foo): pass def bar_callback(bar: Bar): pass def register_callback(callback: Callable[[Foo], None]): pass register_callback(bar_callback) Which produces the same error you saw: Argument of type "(bar: Bar) -> None" cannot be assigned to parameter "callback" of type "(Foo) -> None" in function "register_callback" Type "(bar: Bar) -> None" is not assignable to type "(Foo) -> None" Parameter 1: type "Foo" is incompatible with type "Bar" "Foo" is not assignable to "Bar" Why does this happen? Well, register_callback() expects a callback that can handle any...

Full answer by JadenV15: https://stackoverflow.stackexchange.com/a/80014266 (CC BY-SA 4.0).

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