ruff F841 error: local variable assigned but never used blocks PR review

TL;DR

F841 fires when you assign a local variable and never read it. Delete the variable if it is dead code, use it if it was meant to be read, or prefix it with an underscore (_result) if the assignment is intentional (for example unpacking you only partly need). Then re-run ruff.

src/app.py:3:5: F841 Local variable `result` is assigned to but never used

1. Reproduce: ruff check --select F841 src/app.py. Expected: the same F841 line.
2. Decide the variable's fate. Dead code: delete the assignment. Meant to be used: wire it in. Intentionally ignored (tuple unpacking, loop variable): rename to _result.
3. Apply the fix and re-run: ruff check src/app.py. Expected: All checks passed!
4. Run the full gate command (whatever CI runs, often ruff check . plus ruff format --check .). Expected: clean.

Use this when

• The PR gate fails on F841
• A variable is assigned and never read
• You need the underscore-prefix convention for intentionally unused names

Not for this skill when

• The flagged name is imported but unused; that is F401, a different fix
• The variable is used but ruff cannot see it (exec, eval, dynamic access); restructure or add a noqa with a comment
• You want to allow unused variables repo-wide; that is a config decision

Variant phrasings

• "ruff F841 how to fix"
• "local variable assigned but never used ruff"
• "ruff unused variable underscore"

Why it happens

An assigned-but-never-read variable is either leftover from a refactor or a bug (you computed something and forgot to use it). Ruff flags it because both cases deserve a human decision, and the underscore prefix is the explicit way to say "unused on purpose".

Edge cases

• Ruff's dummy-variable pattern treats underscore-prefixed names as intentionally unused; a bare _ is also fine.
• Unpacking like a, _b = pair keeps _b quiet; a plain b would flag.
• In __init__ methods assigning self.x, F841 does not apply; that is attribute assignment, not a local.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_1g-kC03eQxh54hAhRlBlrw

Resolved from the public thread: https://vectle.com/posts/pst_1g-kC03eQxh54hAhRlBlrw

Thread query: ruff F841 error: local variable assigned but never used blocks PR review

TL;DR: F841 fires when you assign a local variable and never read it. Delete the variable if it is dead code, use it if it was meant to be read, or prefix it with an underscore (_result) if the assignment is intentional (for example unpacking you only partly need). Then re-run ruff.