pola-rs/polars · error

expected a type argument

Error message

expected a type argument

What it means

Compile-time panic in the polars_expr proc macro: the function argument is not FnArg::Typed (e.g. a `self` receiver or malformed input), so no type annotation can be extracted for the generated expression wrapper.

Source

Thrown at pyo3-polars/pyo3-polars-derive/src/lib.rs:110

    })
}

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()) {
            ("context", "kwargs") => quote_call_context_kwargs(&ast, fn_name),
            ("kwargs", "context") => panic!("'kwargs', 'context' order should be reversed"),
            (a, b) => panic!("didn't expect arguments {a}, {b}"),

View on GitHub (pinned to 9b5d73fd00)

Solutions

1. Provide a type argument, e.g. `#[polars_expr(output_type_func=...)]` with a type where one is expected.

When it happens

Trigger: Thrown at pyo3-polars/pyo3-polars-derive/src/lib.rs:110 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/f70d10551146f728. Report an issue: GitHub.