Python type hint for classes that support __getitem__

Question

I want to add type hints to a function that will accept any object with a __getitem__ method. For instance, in

def my_function(hasitems, locator):
    hasitems[locator]

I don't want to restrict hasitems to be a specific type like list or dict . As long as it supports __getitem__ , it's an appropriate argument to my_function . How can I annotate its type without being unnecessarily restrictive?

Edit: apparently PyCharm can deduce the appropriate hint in a number of common cases, but not in my actual use case. I can't post the code since it's for work, and I haven't been able to find a nonproprietary minimal example where PyCharm fails. In any case, the original question doesn't reference PyCharm and it is still a valid use case for type hints.

Answer 1

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]

Answer 2

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

Answer 3

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