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