"object" has no attribute "__iter__" false-positive

[tucked]

# sscce.py


def no_repro(bar: str = '4'):
    key_values = {
        'key': [1, 2, 3],
        'key2': [1, 2],
    }
    return all(
        value
        for key, values in key_values.items()
        for value in values
    )


def repro(bar: str = '4'):
    key_values = {
        'key': [1, 2, 3, bar],  # <--- bar is included
        'key2': [1, 2],
    }
    return all(
        value
        for key, values in key_values.items()
        for value in values
    )

• Note: no repro without 'key2': [1, 2],

mypy sscce.py

sscce.py:21: error: "object" has no attribute "__iter__"; maybe "__str__" or "__dir__"? (not iterable)
Found 1 error in 1 file (checked 1 source file)

• Python 3.6.8
• mypy 0.770 (mypy-0.770+dev.833fc2caec6773683d691a72058293ae87139210 too)

Potentially related: #4975

[msullivan]

This is frustrating but I don't think it's a bug.

We infer the type List[object] for the first list and List[int] for the second list. List[int] is not a subtype of List[object], so the best common subtype for them is unfortunately object.

[tucked]

Is the correct fix to use tuples instead?

[JukkaL]

The recommended fix is to add an annotation for key_values, but using tuples may also work.