Mypy is flagging an issue with my code that is really confusing to me. Here’s a snippet of code that illustrates the issue. Note that this is running in Python 3.12.3 and mypy 1.10.0

class A:
    pass


class B(A):
    pass


def func_of_a(var1: A) -> None:
    pass


def func_of_set_of_a(var1: set[A]) -> None:
    pass


a = A()
b = B()


b1: B = B()
func_of_a(b1)  # mypy does not flag this as being an issue (as expected)


set_of_a: set[A] = {a}
set_of_b: set[B] = {b}

func_of_set_of_a(set_of_a)   # mypy does not flag this as being an issue (as expected)

func_of_set_of_a(set_of_b)  # mypy flags this as an issue

mypy is flagging func_of_set_of_a(set_of_b) as being problematic with the following error:

error: Argument 1 to “func_of_set_of_a” has incompatible type “set[B]”; expected “set[A]” [arg-type]

I don’t understand why this is being flagged. func_of_a expects an object of type A and will also accept an object of any child class of A. Shouldn’t func_of_set_of_a behave similarly? func_of_set_of_a expects a set[A], but shouldn’t it also accept a set of objects of a child class of A? I know I can change the type hint to set[A] | set[B], but that would make my code unwieldy since in my actual use case, class A has many child classes.

What am I missing here?

I tried googling but could not find any info on this. Unless I’m missing it, the official Python documentation also does not address this.