Best base type which support `in` operator?

Question

I have a function:

def f(a):
    return 100 in a

I want to annotate the argument a, for example

from typing import List
def f(a: List[int]):
    return 100 in a

But types which support in operator is not only list but also set or tuple (or maybe dict or keyview or something).

My question is:

What type is the best for a, which support in operator?
As in, which is the simplest type which provides this particular functionality?

It seems that typing.Container or typing.Collection is ok, but I have no idea about which type is the best.

Answer 1

I would choose Union[Container, Iterable]. The python documentation for membership testing states that it first tries __contains__ then it tries __iter__ to determine membership. It also states that it tries __getitem__ failing both but only for old style iteration. To be honest I'm not sure what that means but I think we're safe here to ignore it.

Your arguments should be as generic as possible. If you chose Collection that would prohibit generators since they don't implement len for example, yet generators support the in keyword.

https://docs.python.org/3/library/collections.abc.html#collections.abc.Container