pydantic/monty · error · syn::Error

`style = unpack` cannot be combined with `varargs` or `varkw

Error message

`style = unpack` cannot be combined with `varargs` or `varkwargs` — it models a fixed positional min..max range

What it means

A `#[derive(FromArgs)]` struct with `style = unpack` also declares a `varargs` (`*args`) or `varkwargs` (`**kwargs`) field. `unpack` models `PyArg_UnpackTuple`, a fixed positional min..max range; variadic collection is outside that model, so the macro rejects the combination at compile time.

Source

Thrown at crates/monty-macros/src/from_args.rs:232

        match self.style {
            Style::Def => {
                if self.bad_arg.is_some() {
                    return err("`bad_arg`/`bad_arg_named` cannot be combined with `style = def` \
                         — CPython `def` binding never type-checks while binding; declare \
                         fields as raw `Value` and coerce in the function body");
                }
                if self.varargs_idx.is_some() {
                    return err("`style = def` cannot be combined with `varargs` — a `*args` \
                         signature can never raise too-many-positional, so the style has no effect");
                }
            }
            Style::Unpack => {
                if self.fields.iter().any(|f| matches!(f.kind, FieldKind::PosOrKeyword)) {
                    return err("`style = unpack` models a positional-only `PyArg_UnpackTuple` \
                         signature — every positional field must be `pos_only`");
                }
                if self.varargs_idx.is_some() || self.varkwargs_idx.is_some() {
                    return err("`style = unpack` cannot be combined with `varargs` or `varkwargs` \
                         — it models a fixed positional min..max range");
                }
            }
            Style::Clinic | Style::C | Style::CNamed => {}
        }

        if self.at_most_total {
            if matches!(self.style, Style::Def | Style::Unpack) {
                return err(
                    "`at_most_total` cannot be combined with `style = def` or `style = unpack` \
                     — the total pre-count models `PyArg_ParseTupleAndKeywords`-family C parsers",
                );
            }
            if self.varargs_idx.is_some() || self.varkwargs_idx.is_some() {
                return err("`at_most_total` cannot be combined with `varargs` or `varkwargs` \
                     — the up-front total-count check is only meaningful for \
                     signatures with a fixed maximum");
            }

View on GitHub (pinned to adc986b362)

Solutions

Remove `style = "unpack"` and let the default `clinic` style parse; it supports varargs/varkwargs fields.
If the signature really is fixed-width, drop the varargs/varkwargs field and enumerate the positional fields explicitly with `pos_only`.

Example fix

// before
#[derive(FromArgs)]
#[from_args(style = "unpack")]
struct Args { a: Value, #[from_args(varargs)] rest: Vec<Value> }

// after
#[derive(FromArgs)]
struct Args { a: Value, #[from_args(varargs)] rest: Vec<Value> }

Defensive patterns

Strategy: validation

Validate before calling

fn unpack_is_fixed_arity(has_varargs: bool, has_varkwargs: bool) -> bool {
    !has_varargs && !has_varkwargs
}

Prevention

Treat style = "unpack" as strictly fixed min..max positional; variadic fields require another style.
Check the README style table when adding varargs/varkwargs fields to an existing struct.

When it happens

Trigger: Compiling `#[derive(FromArgs)] #[from_args(style = "unpack")] struct Args { a: i64, #[from_args(varargs)] rest: Vec<Value> }` (or a `varkwargs` field). Checked in `Signature::validate` (crates/monty-macros/src/from_args.rs:231-234).

Common situations: Adding a catch-all `*args` field to an existing unpack-style struct while generalizing a builtin; confusing `unpack` (fixed tuple unpack) with general C-style variadic parsing.

Related errors

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/3c616a4581e9b160. Report an issue: GitHub.