Specifically, Ruff incorrectly identifies the scope a variable is chosen from when it's inside a comprehension inside a class inside a function and a variable with the same name exists inside the class scope.

Here is shortest code to reproduce this example, I named it scopetest.py:

def f():
    b = 'Function Scope'
    class C:
        b = 'Class Scope'
        d = [b for _ in range(1)]
    return C

print(f().d[0])  # Prints "Function Scope"

Running Ruff:

> ruff --version
ruff 0.0.267
> ruff scopetest.py
scopetest.py:2:5: F841 [*] Local variable `b` is assigned to but never used
Found 1 error.

If I run ruff with --fix it deletes the line that b is sourced from causing the code to now throw an exception.

The reason b is scoping from the function scope and not the class scope is that generators / comprehensions implicitly create an anonymous function and source their scope from locals, non-locals, globals, and builtins and skip the class scope as all other functions do.

I came across this testing different versions of code posted here where the Steering Council has decided to keep the behavior as-is for at least Python 3.12.