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Will this fix #12570 ?
Also F401 causes infinite loop, but without __all__

Will this fix #12570 ?

Doesn't look like it (not as the PR currently stands, anyway). I can still repro that infinite loop even with this PR branch.

I'm still looking into it, but it looks like #12570 is a distinct bug from the one this PR is fixing. So we'll fix it, but not in this PR 👍

What's the reason that you decided not to add sub imports (by adding a) to __ALL__? Is it because that's not what sub imports are commonly used for?

Submodule imports have... odd semantics in Python. Here's what happens if I import a regular module: the email module is loaded and assigned to an email symbol in the global namespace:

>>> globals().keys()
dict_keys(['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__annotations__', '__builtins__'])
>>> import email
>>> globals().keys()
dict_keys(['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__annotations__', '__builtins__', 'email'])
<module 'email' from '/Users/alexw/.pyenv/versions/3.12.4/lib/python3.12/email/__init__.py'>

That's not what happens with a submodule import, however:

Python 3.12.4 (main, Jun 12 2024, 09:54:41) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import email.utils
>>> globals().keys()
dict_keys(['__name__', '__doc__', '__package__', '__loader__', '__spec__', '__annotations__', '__builtins__', 'email'])
>>> email
<module 'email' from '/Users/alexw/.pyenv/versions/3.12.4/lib/python3.12/email/__init__.py'>
>>> email.utils
<module 'email.utils' from '/Users/alexw/.pyenv/versions/3.12.4/lib/python3.12/email/utils.py'>

In the REPL snippet above, the import email.utils statement does the following things:

1. Loads the email module
2. Assigns the email module to an email symbol in the global namespace
3. Loads the email.utils submodule
4. Assigns the email.utils submodule as an email attribute on the email module that's now in the global namespace.

__all__ is a list of symbols in the global namespace that are publicly exported from a given module. If __all__ is validly defined, it should only contain strings which are valid identifiers, and all the identifiers listed in __all__ should refer to symbols that exist in the global namespace. Therefore, we shouldn't add submodule.a or a to __all__: the first isn't a valid identifier, and neither of them refers to a symbol that exists in the global namespace as a result of the statement import submodule.a.

Possibly we could autofix this by adding submodule to __all__, however... possibly that would be better than just removing the import if we know that the import submodule.a statement is a first-party import. @carljm, do you have any opinion here on what the better autofix would be?

I might be missing some context here, but if we autofix this:

import mod
__all__ = ["FOO"]
FOO = 42

to this:

import mod
__all__ = ["FOO", "mod"]
FOO = 42

Then I think we should also autofix this:

import mod.sub
__all__ = ["FOO"]
FOO = 42

to this:

import mod.sub
__all__ = ["FOO", "mod"]
FOO = 42

It doesn't make sense to me that we would remove import mod.sub, but if its import mod we would add "mod" to __all__. Both import mod and import mod.sub define the symbol mod, and we should handle them consistently; either remove the import as autofix in both cases, or add to __all__ in both cases.

I don't think you're missing something! I think I just spent too long tracking down the source of the bug, and didn't think hard enough about what the correct autofix should be. I'll make that change.

I found another bug! If you have import submodule.a in an __init__.py file that doesn't have an __all__ definition, our autofix currently produces invalid syntax:

__init__.py:1:8: F401 [*] `submodule.a` imported but unused; consider removing, adding to `__all__`, or using a redundant alias
  |
1 | import submodule.a
  |        ^^^^^^^^^^^ F401
  |
  = help: Use an explicit re-export: `submodule.a as submodule.a`

ℹ Safe fix
1   |-import submodule.a
  1 |+import submodule.a as submodule.a

I pushed some updates. They required a bit of refactoring as the code for this rule is quite complex.

To summarise, if you have an unused first-party submodule import (e.g. import submodule.a) in an __init__.py file:

• Without a preexisting __all__ definition:
  • We used to produce invalid syntax in our autofix: import submodule.a as submodule.a.
  • We now offer no autofix
• With a preexisting __all__ definition:
  • We used to enter an infinite loop when attempting to provide an autofix
  • We now don't enter an infinite loop, and add "submodule" to __all__ in our autofix

The error message has also changed slightly if you have an unused import in an __init__.py file that isn't a first-party import. This was a side effect of the refactoring, but I think it's for the better. For import sys in an __init__.py file:

• The error message used to be:

  sys imported but unused; consider removing, adding to __all__, or using a redundant alias

• It is now:

  sys imported but unused

I think that's an improvement. If it's not a first-party import, there's very little chance that the user wants it to be reexported, either by adding it to __all__ or by using a redundant alias, so the more concise error message feels like an improvement to me.