reflex-dev/reflex · error · TypeError

`@rx.memo` does not support positional-only parameters in `{

Error message

`@rx.memo` does not support positional-only parameters in `{fn_name}`.

What it means

`@rx.memo` rejects positional-only parameters (those before a `/` in the signature). Memo props are always passed by keyword at the call site, and positional-only names cannot participate in that binding convention, so the signature is rejected at decoration time.

Source

Thrown at packages/reflex-base/src/reflex_base/components/memo.py:1365

    """Reject Python parameter kinds (``*args`` / ``**kwargs`` / positional-only)
    that memo does not support.

    Args:
        parameter: The parameter to check.
        fn_name: The function name for error messages.

    Raises:
        TypeError: If the parameter uses an unsupported kind.
    """
    if parameter.kind is inspect.Parameter.VAR_POSITIONAL:
        msg = f"`@rx.memo` does not support `*args` in `{fn_name}`."
        raise TypeError(msg)
    if parameter.kind is inspect.Parameter.VAR_KEYWORD:
        msg = f"`@rx.memo` does not support `**kwargs` in `{fn_name}`."
        raise TypeError(msg)
    if parameter.kind is inspect.Parameter.POSITIONAL_ONLY:
        msg = f"`@rx.memo` does not support positional-only parameters in `{fn_name}`."
        raise TypeError(msg)


def _classify_parameter(
    annotation: Any, param_name: str, fn_name: str
) -> tuple[MemoParamKind, Any]:
    """Walk ``_CLASSIFICATION_ORDER`` and return the first matching kind.

    Args:
        annotation: The parameter annotation.
        param_name: The parameter name (some kinds care, e.g. ``children``).
        fn_name: The function name for error messages.

    Returns:
        The matched ``(kind, kind_data)``.

    Raises:
        TypeError: If no kind matches.
    """

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Remove the `/` marker so all parameters are normal positional-or-keyword parameters.
2. Annotate each parameter as `rx.Var[...]` or `rx.RestProp` and pass props by keyword when calling the memo.

Example fix

# before
@rx.memo
def item(value, /):
    return rx.text(value)

# after
@rx.memo
def item(value: rx.Var[str]):
    return rx.text(value)

import inspect

def memo_signature_ok(fn) -> bool:
    return all(
        p.kind is not inspect.Parameter.POSITIONAL_ONLY
        for p in inspect.signature(fn).parameters.values()
    )

When it happens

Trigger: Decorating a function with `@rx.memo` that uses the `/` positional-only marker, e.g. `@rx.memo def item(x, /, y: rx.Var[str]): ...`, or a lambda/function defined via `inspect` or C extensions with implicit positional-only args.

Common situations: Copying modern Python API signatures that use `/`; using `functools.partial` or wrappers that surface parameters as positional-only; mixing conventions when porting typed APIs into memo components.

Related errors

• `@rx.memo` does not support `*args` in `{fn_name}`.
• `@rx.memo` does not support `**kwargs` in `{fn_name}`.
• All parameters of `{fn_name}` must be annotated as `rx.Var[.
• Component-returning `@rx.memo` `{fn.__name__}` must return a
• `{definition.python_name}` only accepts children positionall

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/1cad98527a6b38c9. Report an issue: GitHub.