reflex-dev/reflex · error · TypeError

`@rx.memo` does not support `*args` in `{fn_name}`.

Error message

`@rx.memo` does not support `*args` in `{fn_name}`.

What it means

`@rx.memo` inspects the wrapped function's signature and rejects `*args` (VAR_POSITIONAL) parameters. Memoized component functions must have a fully explicit signature so Reflex can statically map each parameter to a typed prop at compile time; variadic positional arguments cannot be classified as props.

Source

Thrown at packages/reflex-base/src/reflex_base/components/memo.py:1359

        )

    return tuple(params)


def _check_parameter_kind(parameter: inspect.Parameter, fn_name: str) -> None:
    """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.

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Remove `*args` and declare each prop explicitly with `rx.Var[...]` annotations, e.g. `def item(label: rx.Var[str], count: rx.Var[int])`.
2. If you need a variable set of props, declare a single `rx.RestProp` parameter instead of `*args`.
3. Keep variadic-argument helpers as plain functions outside `@rx.memo` and pass their result in as an explicit prop.

Example fix

// before
@rx.memo
def item(*args):
    return rx.box(*args)

# after
@rx.memo
def item(a: rx.Var[str], b: rx.Var[str]):
    return rx.box(a, b)

import inspect

def memo_signature_ok(fn) -> bool:
    for p in inspect.signature(fn).parameters.values():
        if p.kind is inspect.Parameter.VAR_POSITIONAL:
            return False
    return True

assert memo_signature_ok(item), "remove *args before applying @rx.memo"

When it happens

Trigger: Decorating a function with `@rx.memo` where the signature contains `*args`, e.g. `@rx.memo def item(*args): ...`. The error is raised eagerly at decoration time by `_analyze_params` -> `_check_parameter_kind`.

Common situations: Copying a plain Python helper that used `*args` into a memoized component; refactoring a render function to accept flexible arguments; converting a wrapper/decorator-style function to `@rx.memo`.

Related errors

• `@rx.memo` does not support `**kwargs` in `{fn_name}`.
• `@rx.memo` does not support positional-only parameters in `{
• 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/70d1b28adcd64419. Report an issue: GitHub.