Failure to detect non-None Optional type in conditional expression

[jonafato]

• Are you reporting a bug, or opening a feature request?
  Bug

• Please insert below the code you are checking with mypy, or a mock-up repro if the source is private. We would appreciate if you try to simplify your case to a minimal repro.

from typing import Mapping, Optional

names_to_numbers: Mapping[str, Optional[int]] = {
    'one': 1,
    'two': 2,
    'foo': None,
}

squares = {
    1: 1,
    2: 4,
    3: 9,
}

squares[names_to_numbers['foo']] if names_to_numbers['foo'] else None

• What is the actual behavior/output?

bug.py:15: error: Invalid index type "Optional[int]" for "Dict[int, int]"; expected type "int"

This error is only thrown for conditional expressions of the form if container[index] else None. Changing it to any of the following works as expected:

squares[names_to_numbers['foo']] if names_to_numbers['foo'] is not None else None

squares[names_to_numbers['foo']] if isinstance(names_to_numbers['foo'], int) else None

foo = names_to_numbers['foo']
squares[foo] if foo else None

• What is the behavior/output you expect?
  No errors reported.

• What are the versions of mypy and Python you are using?
  mypy 0.701, reproducible on at least Python 2.7 and 3.7.

• Do you see the same issue after installing mypy from Git master?
  Yes

[ilevkivskyi]

I agree this behavior is inconsistent. Mypy already allows (on purpose) some code that may be technically unsafe (e.g. __getitem__() call is not guaranteed to return same return with same arguments every time) but are practically useful. We already allow this in other circumstances as you noticed. We should probably to update the branch in find_isinstance_check() (not the best name, I know) where we test for if isinstance(node, RefExpr) to accept any BindableExpression with a correct literal level.

Btw, the same same happens in normal if statement, not only in a ternary expression:

if names_to_numbers['foo']:  # error also disappears if I add 'is not None'
    squares[names_to_numbers['foo']]  # same error here

[JelleZijlstra]

@mbdevpl that's a separate issue with the ternary expression, similar to #4134.

[mbdevpl]

@JelleZijlstra OK, migrated there! Thanks!

[AlexWaygood]

This is no longer reproducible on mypy 0.930+