pydantic/monty · error · syn::Error

unknown style `{other}`; expected `def`, `clinic`, `c`, `c_n

Error message

unknown style `{other}`; expected `def`, `clinic`, `c`, `c_named`, or `unpack`

What it means

A compile-time error from the `FromArgs` derive macro's struct-attribute parser. The `#[from_args(style = "...")]` attribute only accepts one of `def`, `clinic`, `c`, `c_named`, or `unpack`; any other string is rejected with this message naming the invalid value.

Source

Thrown at crates/monty-macros/src/from_args.rs:790

        }
        attr.parse_nested_meta(|meta| {
            if meta.path.is_ident("name") {
                let value: LitStr = meta.value()?.parse()?;
                name = Some(value.value());
                Ok(())
            } else if meta.path.is_ident("style") {
                if style.is_some() {
                    return Err(meta.error("duplicate `style` attribute"));
                }
                let value: Ident = meta.value()?.parse()?;
                style = Some(match value.to_string().as_str() {
                    "def" => Style::Def,
                    "clinic" => Style::Clinic,
                    "c" => Style::C,
                    "c_named" => Style::CNamed,
                    "unpack" => Style::Unpack,
                    other => {
                        return Err(syn::Error::new(
                            value.span(),
                            format!("unknown style `{other}`; expected `def`, `clinic`, `c`, `c_named`, or `unpack`"),
                        ));
                    }
                });
                Ok(())
            } else if meta.path.is_ident("at_most_total") {
                at_most_total = true;
                Ok(())
            } else if meta.path.is_ident("vectorcall") {
                vectorcall = true;
                Ok(())
            } else if meta.path.is_ident("kwarg_error_name") {
                let value: LitStr = meta.value()?.parse()?;
                kwarg_error_name = Some(value.value());
                Ok(())
            } else if meta.path.is_ident("bad_arg") {
                if bad_arg.is_some() {

View on GitHub (pinned to adc986b362)

Solutions

1. Use one of the exact style names: `def`, `clinic`, `c`, `c_named`, or `unpack`
2. Match the style to the CPython parser family of the target function (see crates/monty-macros/README.md for the family table)
3. If no style is needed, remove the attribute entirely and let the default apply

Example fix

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

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

// Validate the style string before writing the attribute.
const STYLES: [&str; 5] = ["def", "clinic", "c", "c_named", "unpack"];
fn is_valid_style(s: &str) -> bool { STYLES.contains(&s) }

When it happens

Trigger: Writing `#[from_args(style = "method")]`, `#[from_args(style = "C")]` (wrong case), a typo like `clinci`, or any other unrecognized style string on a struct deriving `FromArgs`.

Common situations: Guessing style names from CPython terminology, copying a style from a different macro family, or capitalization drift (`c` vs `C`).

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• positional-only fields must come before positional-or-keywor
• positional-or-keyword fields cannot appear after keyword-onl
• only one `#[from_args(varargs)]` field is allowed
• `varargs` cannot appear after keyword-only fields — Python h
• only one `#[from_args(varkwargs)]` field is allowed

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/8ac85d840c3c41bb. Report an issue: GitHub.