pydantic/monty · error · syn::Error

positional-only fields must come before positional-or-keywor

Error message

positional-only fields must come before positional-or-keyword, varargs, and keyword-only fields

What it means

This is a compile-time error from the `FromArgs` derive macro in monty-macros. It fires when a struct field marked `#[from_args(pos_only)]` is declared after a positional-or-keyword field, a `varargs` field, or a keyword-only field. Python signatures require positional-only parameters to come first, so the derived binder rejects the struct definition rather than producing a signature Python could never express.

Source

Thrown at crates/monty-macros/src/from_args.rs:564

    let mut varkwargs_idx = None;
    let mut seen_varargs = false;
    let mut seen_varkwargs = false;
    let mut seen_pos_or_kw = false;
    let mut seen_kw_only = false;
    let mut pos_counter: usize = 0;
    let mut slot_counter: usize = 0;
    for (idx, field) in fields.iter_mut().enumerate() {
        if seen_varkwargs {
            return Err(syn::Error::new(
                field.ident.span(),
                "no fields may appear after a `#[from_args(varkwargs)]` field",
            ));
        }

        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;
                }

View on GitHub (pinned to adc986b362)

Solutions

Move the `#[from_args(pos_only)]` field to the top of the struct, before all positional-or-keyword, varargs, and keyword-only fields
If the field does not need to be positional-only, remove the `#[from_args(pos_only)]` attribute so it becomes a normal positional-or-keyword field
If the field was intended to be keyword-only, change the attribute to `#[from_args(kw_only)]` and keep it after the positional fields

Example fix

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

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

Defensive patterns

Strategy: validation

Validate before calling

// Before compiling, check field order: all #[from_args(pos_only)] fields
// must be the first fields of the struct.
struct Fields<'a> { kinds: &'a [FieldKind] }
fn pos_only_first(kinds: &[&str]) -> bool {
    let after_non_pos_only = kinds.iter().position(|k| *k != "pos_only");
    match after_non_pos_only {
        Some(i) => !kinds[i..].contains(&"pos_only"),
        None => true,
    }
}

Prevention

Declare pos_only fields at the top of every arg struct
Mirror the target CPython function's parameter order when writing the struct
Run cargo check after each field reorder

When it happens

Trigger: Deriving `FromArgs` on a struct where a field annotated with `#[from_args(pos_only)]` appears in the struct body after any plain (positional-or-keyword) field, any `#[from_args(varargs)]` field, or any `#[from_args(kw_only)]` field.

Common situations: Reordering struct fields while converting a hand-written `into_parts()` parser to `FromArgs`, or adding a new pos_only parameter to the end of an existing struct instead of the top.

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/437fdf75d629633b. Report an issue: GitHub.