Ruff Cannot Recognise a Variable Usage inside typing.cast

[barmanroys]

Summary

Play ground link.

Context

A test method within a unittest class.

Code

from typing import cast, Tuple
...

def test_run(self):
    """Use the sample initial states and action choices for testing."""
    initial_input: int = 1 # Ruff warns this variable is defined but not used 
    actions: Tuple[int, ...] = (199, 213, 111, 333) # Ruff warns this variable is defined but not used 
    result = cast(
        typ=Tuple[Any],
        val=self.interactor.run(llm_input=initial_input, action_sequence=actions),
    )
    self.assertTupleEqual(tuple1=result, tuple2=(1, 0, True, {}))
    logging.info(msg="Multi-round interactor passed testing.")

Problem

Ruff Cannot recognise the usage of a variable inside typing.cast. Even though it is being used.

System Details

• ruff 0.11.5
• uv 0.5.13
• Operating system: Linux mint 22

Version

0.11.5

[sharkdp]

Here's a smaller example that shows the same problem. It seems to depend on the fact that keyword arguments are used for typing.cast:

from typing import cast

def f():
    x = 1
    print(cast(typ=int, val=x))

[barmanroys]

@sharkdp thanks for reducing my example to a minimal variant.

So, as typing.cast accepts keyword arguments, should this behaviour be fixed on the ruff-end of things (perdon the pun)?

Also, weird enough, the bug/warning from ruff is absent when the cast is being called at the top level of the module. So the following code works okay.

from typing import cast
x = 1
print(cast(typ=int, val=x))

[sharkdp]

So, as typing.cast accepts keyword arguments, should this behaviour be fixed on the ruff-end of things

Yes, I think so.

Also, weird enough, the bug/warning from ruff is absent when the cast is being called at the top level of the module

F841 only applies to unused variables in function scopes. If you define an variable in module-global scope, it could be imported from another module. So there's no way to tell if it is used or not (in a single-file analysis).

[ntBre]

I think we're only visiting the positional args for cast (and the other typing imports in this block?):

ruff/crates/ruff_linter/src/checkers/ast/mod.rs

Lines 1541 to 1553 in d4695fe

	Some(typing::Callable::Cast) => {
	let mut args = arguments.args.iter();
	if let Some(arg) = args.next() {
	self.visit_type_definition(arg);
	if !self.source_type.is_stub() && self.enabled(Rule::RuntimeCastValue) {
	flake8_type_checking::rules::runtime_cast_value(self, arg);
	}
	}
	for arg in args {
	self.visit_expr(arg);
	}
	}

[dhruvmanila]

I think we're only visiting the positional args for cast (and the other typing imports in this block?):

Yeah, this seems like a clear issue. We should probably use find_argument_value or find_argument which finds an argument via both position and name.