reflex-dev/reflex · error · TypeError

Cannot pass a Var to a built-in function. Consider using .le

Error message

Cannot pass a Var to a built-in function. Consider using .length() for accessing the length of an iterable Var.

What it means

This TypeError is raised by reflex.compiler.compiler.into_component when rendering logic calls a Python built-in (typically len()) on a Var-backed value (ArrayCastedVar/ObjectCastedVar/StringCastedVar). Reflex Vars represent frontend state that only exists at runtime in JavaScript, so Python built-ins cannot operate on them during compilation. The error is a re-raise of the underlying 'has no len()' TypeError with a helpful message.

Source

Thrown at reflex/compiler/compiler.py:935

            raise
        key = e.args[0] if e.args else None
        if key is not None and isinstance(key, Var):
            raise TypeError(
                "Cannot access a primitive map with a Var. Consider calling rx.Var.create() on the map."
            ).with_traceback(e.__traceback__) from None
        raise
    except TypeError as e:
        if isinstance(e, ReflexError):
            _modify_exception(e)
            raise
        message = e.args[0] if e.args else None
        if message and isinstance(message, str):
            if message.endswith("has no len()") and (
                "ArrayCastedVar" in message
                or "ObjectCastedVar" in message
                or "StringCastedVar" in message
            ):
                raise TypeError(
                    "Cannot pass a Var to a built-in function. Consider using .length() for accessing the length of an iterable Var."
                ).with_traceback(e.__traceback__) from None
            if message.endswith((
                "indices must be integers or slices, not NumberCastedVar",
                "indices must be integers or slices, not BooleanCastedVar",
            )):
                raise TypeError(
                    "Cannot index into a primitive sequence with a Var. Consider calling rx.Var.create() on the sequence."
                ).with_traceback(e.__traceback__) from None
        if "CastedVar" in str(e):
            raise TypeError(
                "Cannot pass a Var to a built-in function. Consider moving the operation to the backend, using existing Var operations, or defining a custom Var operation."
            ).with_traceback(e.__traceback__) from None
        raise
    except ReflexError as e:
        _modify_exception(e)
        raise

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Replace len(var) with var.length(), e.g. rx.text(State.items.length())
2. For emptiness checks use var._bool() / boolean operations or var.length() > 0 via Var comparisons
3. Compute the length in an event handler on the backend and store it as a separate state field

Example fix

// before
rx.text(f"You have {len(State.items)} items")
// after
rx.text("You have ", State.items.length(), " items")

def safe_len(v):
    return v.length() if isinstance(v, rx.Var) or hasattr(v, "length") else len(v)

def is_var(v) -> bool:
    return isinstance(v, rx.Var) or type(v).__name__.endswith("CastedVar")

When it happens

Trigger: Calling len(State.some_list), len(State.some_string), or len(State.some_dict) inside a component render function, where the attribute is a Var. Any built-in function applied to an ArrayCastedVar, ObjectCastedVar, or StringCastedVar during component conversion triggers it.

Common situations: Rendering 'You have N items' with len(State.items), conditioning display on len(State.text) > 0, or migrating code from Python state objects to Reflex state vars without adjusting length checks.

Related errors

• Expected _js_expr to be a string, got value {self._js_expr!r
• Unsupported type {var_type} for guess_type.
• Cannot index into a primitive sequence with a Var. Consider
• Cannot pass a Var to a built-in function. Consider moving th
• The component `{comp_name}` only allows the components: {val

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/b0db5f4ea39ef336. Report an issue: GitHub.