[Python] Using a Python Protocol to type hint a specific implementation of a class that...

I have some code that looks approximately like this

from typing import Tuple

class Parent:
...

class Child1(Parent):
def method1(self): ...

class Child2(Parent):
def method2(self): ...

class Container:
def __init__(self, attr: Tuple[Parent, ...]):
self.attr = attr


and some legacy code that only accepts a Container argument when attr consists of elements of type Child_1, e.g.

def func(container_argument):
for e in container_argument.attr:
e.method_1()


I assumed that I could define a Protocol to structurally type-hint func, such that Containers that only contain instances of Child1 can be accepted as an argument. For example,

from typing import Protocol

class ContainerChild1Proto(Protocol):
attr: Tuple[Child1, ...]

def func(container_argument: ContainerChild1Proto):
for e in container_argument.attr:
e.method_1()

child1_only_container = Container((Child1(),))
mixed_container = Container((Child1(), Child2()))

func(child1_only_container) # hopefully passes
func(mixed_container) # hopefully fails



My hope was that a static type-checker would be able to recognise that child1_only_container matches the protocol, and that mixed_container doesn't, but pytype seems to just see that child1_only_container is of type Container which doesn't necessarily match ContainerChild1Proto and hence flags it as an error.

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