pydantic/monty · error · syn::Error

positional-or-keyword fields cannot appear after keyword-onl

Error message

positional-or-keyword fields cannot appear after keyword-only fields

What it means

A compile-time error from the `FromArgs` derive macro. It fires when a plain positional-or-keyword struct field is declared after a `#[from_args(kw_only)]` field. Since Python signatures place all positional-or-keyword parameters before keyword-only ones, the macro treats the struct field order as the parameter order and rejects the impossible arrangement. (Note: after a `varargs` field the macro silently promotes the field to keyword-only instead of erroring.)

Source

Thrown at crates/monty-macros/src/from_args.rs:576

            ));
        }

        match field.kind {
            FieldKind::PosOnly => {
                if seen_pos_or_kw || seen_kw_only || seen_varargs {
                    return Err(syn::Error::new(
                        field.ident.span(),
                        "positional-only fields must come before positional-or-keyword, varargs, and keyword-only fields",
                    ));
                }
            }
            FieldKind::PosOrKeyword => {
                if seen_varargs {
                    // Implicit kw_only after varargs.
                    field.kind = FieldKind::KwOnly;
                    seen_kw_only = true;
                } else if seen_kw_only {
                    return Err(syn::Error::new(
                        field.ident.span(),
                        "positional-or-keyword fields cannot appear after keyword-only fields",
                    ));
                } else {
                    seen_pos_or_kw = true;
                }
            }
            FieldKind::KwOnly => {
                seen_kw_only = true;
            }
            FieldKind::Varargs => {
                if seen_varargs {
                    return Err(syn::Error::new(
                        field.ident.span(),
                        "only one `#[from_args(varargs)]` field is allowed",
                    ));
                }
                if seen_kw_only {

View on GitHub (pinned to adc986b362)

Solutions

Reorder the fields so every positional-or-keyword field comes before all `#[from_args(kw_only)]` fields
Mark the offending field `#[from_args(kw_only)]` too, if it should be keyword-only
Remove `#[from_args(kw_only)]` from the earlier field if it should be positional

Example fix

// before
#[derive(FromArgs)]
#[from_args(name = "f"])
struct FArgs {
    #[from_args(kw_only)]
    flag: bool,
    x: i64,
}

// after
#[derive(FromArgs)]
#[from_args(name = "f"])
struct FArgs {
    x: i64,
    #[from_args(kw_only)]
    flag: bool,
}

Defensive patterns

Strategy: validation

Validate before calling

// No positional-or-keyword field may follow a kw_only field.
fn pos_or_kw_before_kw_only(kinds: &[&str]) -> bool {
    let first_kw_only = kinds.iter().position(|k| *k == "kw_only");
    match first_kw_only {
        Some(i) => !kinds[i..].iter().any(|k| *k == "pos_or_kw"),
        None => true,
    }
}

Prevention

Order struct fields exactly as the Python signature reads: positional, *args, keyword-only, **kwargs
Insert new parameters at the signature-correct position, not the end of the struct
Keep the crates/monty-macros/README.md style table open while writing arg structs

When it happens

Trigger: Deriving `FromArgs` on a struct where at least one field is marked `#[from_args(kw_only)]` and a later field has no kind attribute (positional-or-keyword).

Common situations: Inserting a new parameter in the middle of a struct without noticing a keyword-only field sits above the insertion point; refactoring field order alphabetically or by grouping rather than by signature position.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/2a9c5264730bf90d. Report an issue: GitHub.