reflex-dev/reflex · error · ComputedVarSignatureError

ComputedVarSignatureError(fget.name, signature=str(sign)

Error message

ComputedVarSignatureError(fget.__name__, signature=str(sign))

What it means

ComputedVarSignatureError is raised when the getter function passed to rx.computed/rx.memo has a number of parameters other than exactly one (the state instance).

Source

Thrown at packages/reflex-base/src/reflex_base/vars/base.py:2989

        A ComputedVar instance.

    Raises:
        ValueError: If caching is disabled and an update interval is set.
        VarDependencyError: If user supplies dependencies without caching.
        ComputedVarSignatureError: If the getter function has more than one argument.
    """
    if cache is False and interval is not None:
        msg = "Cannot set update interval without caching."
        raise ValueError(msg)

    if cache is False and (deps is not None or auto_deps is False):
        msg = "Cannot track dependencies without caching."
        raise VarDependencyError(msg)

    if fget is not None:
        sign = inspect.signature(fget)
        if len(sign.parameters) != 1:
            raise ComputedVarSignatureError(fget.__name__, signature=str(sign))

        if inspect.iscoroutinefunction(fget):
            computed_var_cls = AsyncComputedVar
        else:
            computed_var_cls = ComputedVar
        return computed_var_cls(
            fget,
            initial_value=initial_value,
            cache=cache,
            deps=deps,
            auto_deps=auto_deps,
            interval=interval,
            backend=backend,
            **kwargs,
        )

    def wrapper(fget: Callable[[BASE_STATE], Any]) -> ComputedVar:
        if inspect.iscoroutinefunction(fget):

View on GitHub (pinned to 45b8ed5ab7)

Solutions

Make the getter take exactly one argument (self for state methods)
Pass the state explicitly when using rx.computed(fget) outside a class: rx.computed(lambda state: ...)
Close over extra values instead of taking them as parameters

Example fix

# before
rx.computed(lambda: State.count * 2)
# after
rx.computed(lambda state: state.count * 2)

Defensive patterns

Strategy: type-guard

Validate before calling

import inspect
assert len(inspect.signature(fget).parameters) == 1, 'computed var getter takes exactly one arg (state)'

Type guard

def is_valid_getter(f) -> bool:
    import inspect
    return callable(f) and len(inspect.signature(f).parameters) == 1

Prevention

Always define computed vars as methods on a State subclass taking only self
Lint new computed vars in CI with a signature check

When it happens

Trigger: Passing a zero-arg lambda or a multi-arg function: rx.computed(lambda: ...) or def f(self, x): ... used as a computed var getter.

Common situations: Using a plain function instead of a method on a State class, or converting a helper function with extra parameters into a computed var.

Related errors

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/ed236575b8392a0b. Report an issue: GitHub.