Implement F402

[harupy]

#170

> flake8 resources/test/fixtures/F402.py
resources/test/fixtures/F402.py:5:5: F402 import 'os' from line 1 shadowed by loop variable
resources/test/fixtures/F402.py:8:5: F402 import 'path' from line 2 shadowed by loop variable

> cargo run -- --no-cache resources/test/fixtures/F402.py
./resources/test/fixtures/F402.py:5:5: F402 import 'os' from line 1 shadowed by loop variable
./resources/test/fixtures/F402.py:8:5: F402 import 'path' from line 2 shadowed by loop variable

[charliermarsh]

Flake8 has this check for self.differentForks(node, existing.source), which I think enables them to avoid flagging errors in cases like this:

if False:
    import typing
else:
    for typing in range(3):
        pass

(ruff raises F402 here, but Flake8 doesn't.)

It might be okay to have false positives for this rule in particular, but it seems like something we'll have to solve to support RedefinedWhileUnused and some others. differentForks requires that we store the parent stack for every binding, and then implement the differentForks logic.

Do you think that's worth doing here, as part of this PR? Or would you rather merge w/ these limitations?

(Either way, can we add an example using the if/else to F402.py, even if it has a TODO saying that we shouldn't be flagging that line?)

[charliermarsh]

I guess pylint doesn't really do this, so maybe we should just not worry about it for now.

For example, pylint has a rule (W0621) around using a variable name that shadows a variable in the outer scope, and it does get triggered here:

if True:
    typing = 1
else:
    def f(typing):
        pass