reflex-dev/reflex · error · TypeError

Expected Var return type of {fn.__name__} to have a generic

Error message

Expected Var return type of {fn.__name__} to have a generic type.

What it means

The operator-dispatch registration requires a fully generic return annotation Var[T]; a bare -> Var without a generic parameter cannot tell reflex which Python type the dispatcher handles.

Source

Thrown at packages/reflex-base/src/reflex_base/vars/base.py:3367

    Raises:
        TypeError: If the return type of the function is not a Var.
        TypeError: If the Var return type does not have a generic type.
        ValueError: If a function for the generic type is already registered.
    """
    types = get_type_hints(fn)
    return_type = types["return"]

    origin = get_origin(return_type)

    if origin is not Var:
        msg = f"Expected return type of {fn.__name__} to be a Var, got {origin}."
        raise TypeError(msg)

    generic_args = get_args(return_type)

    if not generic_args:
        msg = f"Expected Var return type of {fn.__name__} to have a generic type."
        raise TypeError(msg)

    generic_type = get_origin(generic_args[0]) or generic_args[0]

    if generic_type in dispatchers:
        msg = f"Function for {generic_type} already registered."
        raise ValueError(msg)

    dispatchers[generic_type] = fn

    return fn


def dispatch(
    field_name: str,
    var_data: VarData,
    result_var_type: GenericType,
    existing_var: Var | None = None,
) -> Var:

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Change the return annotation to a parameterized Var, e.g. -> Var[int] or -> Var[str]
2. Add from __future__ import annotations if typing forward references were the issue

Example fix

# before
def to_title(v: Var[str]) -> Var: ...
# after
def to_title(v: Var[str]) -> Var[str]: ...

from typing import get_args
assert get_args(fn.__annotations__['return']), 'Var return must be parameterized (Var[T])'

def returns_parameterized_var(fn) -> bool:
    from typing import get_origin, get_args
    from reflex.vars import Var
    rt = fn.__annotations__.get('return')
    return get_origin(rt) is Var and bool(get_args(rt))

When it happens

Trigger: Annotating the dispatch function's return as plain Var (no [T]) when registering it in the dispatcher table.

Common situations: Custom operator functions written quickly with a bare Var return type.

Related errors

• Expected generic type of {fn_first_arg_type} to be a type.
• `@rx.memo` on `{fn.__name__}` must return `rx.Component` or
• `{handler_name}` handler should have a parameter annotated a
• Could not compare types {args_types_without_vars[i]} and {ca
• MissingAnnotationError

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/f5d8fdd449b84b2c. Report an issue: GitHub.