astral-sh/ruff · warning

Expected at least one argument in outer function call

Error message

Expected at least one argument in outer function call

What it means

Raised by the C418/unnecessary-double-cast-or-process fix (e.g. `list(tuple(x))`, `set(list(x))`) when the outer call has no arguments at all. The fixer needs at least the inner argument to rebuild the simplified call; with zero arguments it bails.

Source

Thrown at crates/ruff_linter/src/rules/flake8_comprehensions/fixes.rs:568

    outer_call.args = match outer_call.args.split_first() {
        Some((first, rest)) => {
            let inner_call = match_call(&first.value)?;
            if let Some(arg) = inner_call
                .args
                .iter()
                .find(|argument| argument.keyword.is_none())
            {
                let mut arg = arg.clone();
                arg.comma.clone_from(&first.comma);
                arg.whitespace_after_arg = first.whitespace_after_arg.clone();
                iter::once(arg)
                    .chain(rest.iter().cloned())
                    .collect::<Vec<_>>()
            } else {
                rest.to_vec()
            }
        }
        None => bail!("Expected at least one argument in outer function call"),
    };

    Ok(Edit::range_replacement(
        tree.codegen_stylist(stylist),
        expr.range(),
    ))
}

/// (C416) Convert `[i for i in x]` to `list(x)`.
pub(crate) fn fix_unnecessary_comprehension(
    expr: &Expr,
    locator: &Locator,
    stylist: &Stylist,
) -> Result<Edit> {
    let module_text = locator.slice(expr);
    let mut tree = match_expression(module_text)?;

    match &tree {

View on GitHub (pinned to 26f38c119c)

Solutions

1. Re-run ruff so the fix operates on fresh diagnostics.
2. Rewrite manually: `list()` has no double-cast to remove; keep it as-is.
3. Suppress the fix and keep only the diagnostic if the call is intentionally empty.

Example fix

// before
list(tuple(items))
// after
list(items)

assert bool(cast_call.args), 'C418 fix needs at least one argument'

def has_args(expr):
    return isinstance(expr, ast.Call) and len(expr.args) >= 1

try:
    apply_autofix(diagnostic)
except Exception:
    keep_code_as_is()

When it happens

Trigger: `ruff check --fix` on a call like `list()` matched against the double-cast rule, or a stale diagnostic whose ranges no longer line up with the edited source.

Common situations: Applying cached/stale diagnostics to changed code; unusual constructs like `list(*args)` that parsed into an argument shape the fixer did not expect.

Related errors

• Expected Expression::Tuple | Expression::List
• Expected each argument to be a tuple of length two
• Expected one argument in outer function call
• Expected Expression::ListComp | Expression:SetComp | Express
• Expected two arguments

AI-assisted analysis of astral-sh/ruff@26f38c119c (2026-09-05). Data as JSON: /api/errors/b5929ff20a27fe4a. Report an issue: GitHub.