Python type hint for classes that support __getitem__

It sounds like you essentially want to define your own abstract base class (abc).

Following the documentation above, you can define a custom abc that only dictates the presence of __getitem__, but let's use a predefined one for an example. The Mapping abc consists of __getitem__ and a few other magic methods. You can use abcs in isinstance, but you can also directly use them as a type annotations:

def foo(bar: Mapping):
    pass

Or, using the extended type hinting support of ABCs do even fancies things, which you already saw in other answers:

def foo(bar: Mapping[Any, Any]):
    pass

This will work for dict and list, but not for any generic type:

from typing import Any, Mapping, Sequence, Union

def my_function(hasitems: Union[Mapping, Sequence], locator: Any) -> Any:
    return hasitems[locator]

If you're willing to install a not-quite-offical extension to typing, typing-extensions, you can use a Protocol, which should be an implementation of PEP-0544:

from typing_extensions import Protocol
from typing import Any

class GetItem(Protocol):
    def __getitem__(self: 'Getitem', key: Any) -> Any: pass

class BadGetItem:
    def __getitem__(self, a: int, b: int) -> Any: pass

def do_thing(arg: GetItem):
    pass

do_thing(dict())  # OK
do_thing(BadGetItem())  # Fails with explanation of correct signature
do_thing(1)  # Fails