pola-rs/polars · error

expected an argument

Error message

expected an argument

What it means

Compile-time panic in the polars_expr proc macro (create_expression_function): a function argument in the decorated expression function is not a simple typed identifier pattern (syn::Pat::Ident), e.g. a tuple or destructuring pattern was used where a plain `name: Type` argument was expected.

Source

Thrown at pyo3-polars/pyo3-polars-derive/src/lib.rs:107

            // Set latest error, but leave return value in empty state.
            pyo3_polars::derive::_update_last_error(err);
        },
    })
}

fn create_expression_function(ast: syn::ItemFn) -> proc_macro2::TokenStream {
    // count how often the user define a kwargs argument.
    let args = ast
        .sig
        .inputs
        .iter()
        .skip(1)
        .map(|fn_arg| {
            if let FnArg::Typed(pat) = fn_arg {
                if let syn::Pat::Ident(pat) = pat.pat.as_ref() {
                    pat.ident.to_string()
                } else {
                    panic!("expected an argument")
                }
            } else {
                panic!("expected a type argument")
            }
        })
        .collect::<Vec<_>>();

    let fn_name = &ast.sig.ident;

    // Get the tokenstream of the call logic.
    let quote_call = match args.len() {
        0 => quote_call_no_kwargs(&ast, fn_name),
        1 => match args[0].as_str() {
            "kwargs" => quote_call_kwargs(&ast, fn_name),
            "context" => quote_call_context(&ast, fn_name),
            a => panic!("didn't expect argument {a}"),
        },
        2 => match (args[0].as_str(), args[1].as_str()) {

View on GitHub (pinned to 9b5d73fd00)

Solutions

1. Provide the required argument to the attribute, e.g. `#[polars_expr(output_type=...)]` with its argument.

When it happens

Trigger: Thrown at pyo3-polars/pyo3-polars-derive/src/lib.rs:107 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@9b5d73fd00 (2026-08-19). Data as JSON: /api/errors/dd19cd1ff0d1d5e5. Report an issue: GitHub.