in operator does not check type of element

[Wilfred]

Given the following code:

from typing import List

def contains_foo(list_of_ints):
    # type: (List[int]) -> bool
    return 'foo' in list_of_ints

mypy --strict --py2 example.py does not complain that I'm looking for strings in a list of ints.

I don't think there are duplicate issues, but in is a tricky thing to search for. This comment in #4071 is possibly relevant, talking about narrowing.

[gvanrossum]

The problem is that quite often people use this behavior of in (and ==) in working code, depending on the fact that e.g. None or some other value is not in a given list (or not equal to a given value). So "fixing" this would cause a lot of pain for people who have painstakingly annotated legacy codebases. (Like my employer.)

[JukkaL]

This is a duplicate of #1749.