astral-sh/ruff · error

Expected Expression::ListComp | Expression::SetComp

Error message

Expected Expression::ListComp | Expression::SetComp

What it means

An unreachable!/bail guard in fix_unnecessary_comprehension_in_call: the first argument of the call matched by the rule is neither a list comprehension nor a set comprehension, so there is nothing to unwrap. The rule trigger and the fixer disagree on the expression shape.

Source

Thrown at crates/ruff_linter/src/rules/flake8_comprehensions/fixes.rs:849

    let (whitespace_after, whitespace_before, elt, for_in, lpar, rpar) = match &call.args[0].value {
        Expression::ListComp(list_comp) => (
            &list_comp.lbracket.whitespace_after,
            &list_comp.rbracket.whitespace_before,
            &list_comp.elt,
            &list_comp.for_in,
            &list_comp.lpar,
            &list_comp.rpar,
        ),
        Expression::SetComp(set_comp) => (
            &set_comp.lbrace.whitespace_after,
            &set_comp.rbrace.whitespace_before,
            &set_comp.elt,
            &set_comp.for_in,
            &set_comp.lpar,
            &set_comp.rpar,
        ),
        _ => {
            bail!("Expected Expression::ListComp | Expression::SetComp");
        }
    };

    let mut new_empty_lines = vec![];

    if let ParenthesizableWhitespace::ParenthesizedWhitespace(ParenthesizedWhitespace {
        first_line,
        empty_lines,
        ..
    }) = &whitespace_after
    {
        // If there's a comment on the line after the opening bracket, we need
        // to preserve it. The way we do this is by adding a new empty line
        // with the same comment.
        //
        // Example:
        // ```python
        // any(

View on GitHub (pinned to 26f38c119c)

Solutions

1. Align the rule's detection with the fixer so only list/set comprehensions are fixed
2. Bail without a fix when the argument shape is unexpected

When it happens

Trigger: Thrown at crates/ruff_linter/src/rules/flake8_comprehensions/fixes.rs:849 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of astral-sh/ruff@26f38c119c (2026-09-05). Data as JSON: /api/errors/cab6f7eb6b399a3d. Report an issue: GitHub.