Fix for false positive on F401 breaks valid code

[RUrlus]

• ruff v0.0.269
• CPython 3.10.9

Issue

ruff flags a false positive F401 for imports that are used in __all__ that is guarded by a conditional statement.
Running ruff with --fix on will result in broken imports.

MRE

import random


def some_dependency_check():
    return random.uniform(0.0, 1.0) > 0.49999

if some_dependency_check():
    import math

    __all__ = ["math"]
else:
    __all__ = []

-> ruff mre.py
mre.py:9:12: F401 [*] `math` imported but unused
Found 1 error.
[*] 1 potentially fixable with the --fix option.

[charliermarsh]

So, can and should avoid flagging this as unused...

I do feel obligated to mention that you should try to avoid using conditionally-defined __all__ assignments :) They will cause issues with all sorts of tooling, since many conditions are undecidable at "compile" time, e.g., Pyright treats x as undefined if you use a wildcard import on this file:

x = 1

if x > 0:
    __all__ = ["x"]
else:
    __all__ = []

[JonathanPlasse]

We should maybe merge all __all__s.

[RUrlus]

@charliermarsh Thanks for the quick response!

In the example you gave, x is an ast.Constant and thus pyright could/ should conclude that the else branch is unreachable at import time .

I do feel obligated to mention that you should try to avoid using conditionally-defined all assignments

I agree, but I don't know how to avoid it in the pattern where I encountered this. I want to support different backends/types for dispatching without making all of them dependencies:

from foo.backends import pandas, spark

@singledispatch
def strip_punctuation(data):
    raise NotImplementedError(f"no backend with support for type {type(data)} was found")

if pandas._supported:
    strip_punctuation.register(pandas.strip_punctuation)

if spark._supported:
    strip_punctuation.register(spark.strip_punctuation)

Where the imports are guarded, foo.backends.pandas.__init__.py:

_supported = can_import("pandas")
if _supported:
    from foo.backends.pandas.string import strip_punctuation
    
    __all__ = ["strip_punctuation"]
else:
    __all__ = []

[charliermarsh]

@JonathanPlasse - Yeah I think that's probably what we need to do.

[JonathanPlasse]

I can work on it if you want.

[MichaReiser]

@JonathanPlasse - Yeah I think that's probably what we need to do.

I'm not sure if merging gives us the desired result in all situations:

• When testing for public docstrings -> Merging them is what we want
• When testing for invalid imports -> Merging is unsound because it then wont flag imports that may not exist at runtime. A better approach for this use case is to explicitly determine the "conditional" exports and potentially warn about them (but not flag any that are in both sets)
• others?

[charliermarsh]

Yeah that's true. Another option could be to iterate over all Export bindings (even those that are overridden) when we do these checks.

[charliermarsh]

I'm looking into this.