`module = None` optional import shows error locally but not from tox

[davidism]

In Flask, we have the following lines:

try:
    import dotenv
except ImportError:
    dotenv = None

Running mypy shows an error:

src/flask/cli.py:25: error: Incompatible types in assignment (expression has type "None", variable has type Module)

However, adding a # type: ignore comment, then running mypy through tox shows an error about the ignore:

src/flask/cli.py:25: error: unused 'type: ignore' comment

Running tox without the comment shows everything passing.

So we're left in the situation where we need to pass in CI, but if anyone runs mypy locally they see this error, fix it, then CI fails for them.

What's extra strange is that right below the failing code is another optional import, with the ignore comment, that passes both locally and through tox.

try:
    import ssl
except ImportError:
    ssl = None  # type: ignore

• Mypy version used: 0.812
• Mypy configuration options from setup.cfg: https://github.com/pallets/flask/blob/b5518e23f5b85ba6e069a8151ed42209947f3849/setup.cfg#L89-L105
• Python version used: 3.9.1

[hauntsaninja]

Duplicate of #8990 (comment) and #8823
ssl = cast(Any, None) should be a workaround, if you need an optional type inferred then you could alias and assign or use if TYPE_CHECKING to lie. Some more discussion in the linked issues.

[davidism]

Thanks. I want to make sure we're not doing anything incorrect with our configuration or testing though, it's not clear why it fails with mypy but not with tox -e typing. Same Python and mypy versions in both cases.