> something that is indexable by ints. > ...Dict[int, Any]...

If that is exactly what you want, then define a Protocol: from __future__ import annotations from typing import Protocol, TypeVar

    T = TypeVar("T")
    K = TypeVar("K")
    
    class GetItem(Protocol[K, T]):
        def __getitem__(self, key: K, /) -> T: ...
    
    
    def first(xs: GetItem[int, T]) -> T:
        return xs[0]

Then you can call "first" with a list or a tuple or a numpy array, but it will fail if you give it a dict. There is also collections.abc.Sequence, which is a type that has .__getitem__(int), .__getitem__(slice), .__len__ and is iterable. There are a couple of other useful ones in collections.abc as well, including Mapping (which you can use to do Mapping[int, t], which may be of interest to you), Reversible, Callable, Sized, and Iterable.