reflex-dev/reflex · error · TypeError

`{definition.python_name}` is missing required prop `{param.

Error message

`{definition.python_name}` is missing required prop `{param.name}`.

What it means

A memoized function component invoked via `.call()` (or `functools.partial`) at runtime is missing a prop that has no default value. Reflex inspects the function's signature and requires every parameter without a default to be supplied.

Source

Thrown at packages/reflex-base/src/reflex_base/components/memo.py:1570

        )
        raise TypeError(msg)

    # Bind declared props before collecting any rest props.
    explicit_params = [
        param
        for param in definition.params
        if param.kind not in (MemoParamKind.REST, MemoParamKind.CHILDREN)
    ]
    explicit_values = {}
    remaining_props = kwargs.copy()
    for param in explicit_params:
        if param.name in remaining_props:
            explicit_values[param.name] = remaining_props.pop(param.name)
        elif param.default is not inspect.Parameter.empty:
            explicit_values[param.name] = param.default
        else:
            msg = f"`{definition.python_name}` is missing required prop `{param.name}`."
            raise TypeError(msg)

    # Reject unknown props unless a rest prop is declared.
    if remaining_props and rest_param is None:
        unexpected_prop = next(iter(remaining_props))
        msg = (
            f"`{definition.python_name}` does not accept prop `{unexpected_prop}`. "
            "Only declared props may be passed when no `rx.RestProp` is present."
        )
        raise TypeError(msg)

    # Return ordered explicit args when no packed props object is needed.
    if children_param is None and rest_param is None:
        return tuple(explicit_values[param.name] for param in explicit_params)

    # Build the props object passed to the imported FunctionVar.
    children_value: Any | None = None
    if children_param is not None:
        from reflex_components_core.base.fragment import Fragment

View on GitHub (pinned to 45b8ed5ab7)

Solutions

Add the missing keyword argument shown in the message to the `.call()` invocation
Give the parameter a default value in the memo function definition if the prop is optional
Check for typos in prop names at the call site

Example fix

// before
@rx.memo
def badge(label: str, tone: str) -> rx.Component: ...
badge.call(label="hi")

// after
badge.call(label="hi", tone="info")

Defensive patterns

Strategy: validation

Validate before calling

import inspect

def check_memo_call(fn, **props):
    sig = inspect.signature(fn)
    for name, p in sig.parameters.items():
        if p.default is inspect.Parameter.empty and name not in props:
            raise ValueError(f"missing required prop {name!r}")
    return props

badge.call(**check_memo_call(badge, label="hi"))

Prevention

Keep prop names in sync with the memo signature; rely on IDE/type checkers
Prefer giving optional params defaults so only truly required props raise
Write a tiny unit test per memo asserting required props are passed at each call site

When it happens

Trigger: Calling `my_memo.call()` without a keyword argument for a parameter declared without a default (e.g. `def item(x: int)` called as `item.call()`), or misspelling the prop name so the required one never gets bound.

Common situations: Typos in prop names (e.g. `colr=` vs `color=`), refactoring a memo signature to add a required parameter while old call sites are not updated, or conditionally building the props dict and forgetting a branch.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/6fa31551d6aad8e9. Report an issue: GitHub.