About formal parameter is function, how to hint it?

[HouLingLXH]

For example:

def test(a: int, b:???):
print(b(a))

def add( x:int):
return x + 1

test(1, add)

About the ??? after b, how to hint the type of b is "(x:int) -> int" ？

[Zomatree]

You can use typing.Callable here, in this example you would do Callable[[int], int].

[srittau]

Please also see our discussion forum: https://github.com/python/typing/discussions

[HouLingLXH]

EventTest is the subclass of EventBase, how to fixed it?

[TeamSpen210]

Callable is somewhat unique in that the parameter types are contravariant, meaning that it accepts the specified class or its superclasses, not subclasses (which is covariant). It might seem odd, but if you think about it it makes sense. If your listener got given Callable[[object], None] that would work fine because that callable wouldn't use any EventBase functions. But if the callable was Callable[[EventTest], None], that would be invalid because it might use EventTest-specific functions which EventBase doesn't support.

The fix is to use generic types:

from typing import TypeVar
EventT = TypeVar('EventT', bound=EventBase)

def add_listener(self, event_class: type[EventT], want_do: Callable[[EventT], None]) -> None: ...

Then it can accept any subclasses, but the callable and event_class must match.