pydantic/monty · error · syn::Error

`at_most_total` cannot be combined with `varargs` or `varkwa

Error message

`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

What it means

The `at_most_total` flag was set on a `#[derive(FromArgs)]` struct that also has a `varargs` or `varkwargs` field. The up-front total-count check only makes sense for signatures with a fixed maximum arity; with `*args`/`**kwargs` there is no upper bound to pre-check, so the combination is a compile-time error.

Source

Thrown at crates/monty-macros/src/from_args.rs:247

                         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");
            }
        }

        if self.vectorcall && !(self.at_most_total && self.style == Style::Clinic) {
            return err("`vectorcall` requires the default `clinic` style plus `at_most_total` \
                 — it models a `tp_vectorcall` fast path in front of a clinic parser");
        }

        if self.kwarg_error_name.is_some() && !matches!(self.style, Style::Def | Style::Clinic | Style::Unpack) {
            return err("`kwarg_error_name` is only meaningful with `style = def`, the default \
                 `clinic` style, or `style = unpack` (where it names the function in the \
                 `takes no keyword arguments` error) — the C families defer unknown-kwarg \
                 errors past binding");
        }

        if self.kwargs_not_supported_yet {

View on GitHub (pinned to adc986b362)

Solutions

Remove `at_most_total` from the struct.
If you need the pre-count check, drop the varargs/varkwargs field and give the struct a fixed set of fields.
If a cap on the number of arguments is truly needed, enforce it manually in the function body after collection.

Example fix

// before
#[derive(FromArgs)]
#[from_args(at_most_total)]
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 at_most_total_needs_fixed_max(has_varargs: bool, has_varkwargs: bool) -> bool {
    !has_varargs && !has_varkwargs
}

Prevention

A pre-count check needs an upper bound; any struct with *args/**kwargs cannot have one.
If a cap is needed on variadic input, enforce it in the function body.

When it happens

Trigger: Compiling `#[derive(FromArgs)] #[from_args(at_most_total)] struct Args { a: i64, #[from_args(varargs)] rest: Vec<Value> }` or with a `varkwargs` field. Checked in `Signature::validate` (crates/monty-macros/src/from_args.rs:246-250).

Common situations: Generalizing a fixed-arity C-family handler to accept `*args` while leaving the old `at_most_total` flag; mixing attributes from two structs with different shapes.

Related errors

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/c3ead5577bf3509f. Report an issue: GitHub.