Don't mark re-exported symbols as unused (F401)

[layday]

Currently, an import in the form of:

from a import b as b

will trigger F401, but this idiom is in widespread use as a type checker-friendly alternative to __all__ which does not alter star import semantics. Type checkers which support flagging unused imports will ignore re-exported symbols, i.e. symbols where the as target is identical to the original name:

# pyright: strict

from os import environ  # Pyright reports this as unused
from os import environ as foo  # Likewise
from os import environb as environb   # No error

---

Thanks for ruff!

[charliermarsh]

This is easy to implement! My only question is whether it should be the default behavior, or designated under its own lint rule so as to be configurable (or via a configuration option).

Anyone have opinions on that? Maybe @andersk?

[charliermarsh]

I think this should be the default behavior since the re-export itself is effectively an explicit opt-in.

[andersk]

I think I agree. Although this was rejected upstream (PyCQA/pyflakes#474), it’s a pretty important pattern in the typed Python ecosystem. (Upstream is incorrect to expect that ecosystem to be entirely captured in PEPs—this reexport convention needs no special support from the Python language itself or its runtime implementations.) And I don’t see this significantly hurting anyone else.

[charliermarsh]

Going out in v0.0.117 (building now).

[layday]

whoops, I should've probably mentioned the same holds for import a as a, although not as common.

[andersk]

Also apparently import a.b.c as c. (I find that one surprising, but mypy accepts it; source.)

[charliermarsh]

Re-opening for those cases :)

[layday]

Also apparently import a.b.c as c. (I find that one surprising, but mypy accepts it; source.)

Interesting, Pyright rejects that.

[andersk]

Yeah, Pyright seems more correct. Opened python/mypy#14086.