An F841 error shows up only when enabling the preview mode

[suhaibmujahid]

Currently, F841 is not in preview. However, it will catch the following unused variable (i.e., secund) only when the preview mode is enabled.

Minimal code snippet

class Foo:
    def bar(self):
        return 1, 2

    def main(self):
        first, secund = self.bar()

        print(first)

Invoked commands

The following command shows no errors:

ruff ruff_F841.py --isolated

While enabling the preview shows an F841 error:

ruff ruff_F841.py --isolated --preview

ruff_F841.py:6:16: F841 Local variable `secund` is assigned to but never used
  |
5 |     def main(self):
6 |         first, secund = self.bar()
  |                ^^^^^^ F841
7 | 
8 |         print(first)
  |
  = help: Remove assignment to unused variable `secund`

Found 1 error.
No fixes available (1 hidden fix can be enabled with the `--unsafe-fixes` option).

The current Ruff settings

Just the defaults with no customization

The current Ruff version

ruff 0.2.1

[charliermarsh]

Hey sorry -- this is intended and it's covered in the rule documentation: https://docs.astral.sh/ruff/rules/unused-variable/. Right now, we don't lint against unused variables in tuples if at least one of them is used, but we're considering changing that behavior (hence why the revised logic is present in preview).

[charliermarsh]

(For completeness, there's also a separate issue here discussing whether / how to stabilize that change: #8884.)

[suhaibmujahid]

I appreciate the clarification.

BTW, this behaviour does not respect the explicit-preview-rules option. When having explicit-preview-rules = true, I was expecting to not have any preview behaviour unless the rule is explicitly selected.