reflex-dev/reflex · error · TypeError

The _var_is_string argument is not supported for Var.

Error message

The _var_is_string argument is not supported for Var.

What it means

Var._replace rejects the legacy keyword _var_is_string. String formatting state now lives in VarData (formatting/transform hooks), so passing the old boolean raises TypeError. Companion guards reject _var_is_local and _var_full_name_needs_state_prefix.

Source

Thrown at packages/reflex-base/src/reflex_base/vars/base.py:791

        Args:
            _var_type: The new type of the Var.
            merge_var_data: VarData to merge into the existing VarData.
            **kwargs: Var fields to update.

        Returns:
            A new Var with the updated fields overwriting the corresponding fields in this Var.

        Raises:
            TypeError: If _var_is_local, _var_is_string, or _var_full_name_needs_state_prefix is not None.
        """
        if kwargs.get("_var_is_local", False) is not False:
            msg = "The _var_is_local argument is not supported for Var."
            raise TypeError(msg)

        if kwargs.get("_var_is_string", False) is not False:
            msg = "The _var_is_string argument is not supported for Var."
            raise TypeError(msg)

        if kwargs.get("_var_full_name_needs_state_prefix", False) is not False:
            msg = "The _var_full_name_needs_state_prefix argument is not supported for Var."
            raise TypeError(msg)
        value_with_replaced = dataclasses.replace(
            self,
            _var_type=_var_type or self._var_type,
            _var_data=VarData.merge(
                kwargs.get("_var_data", self._var_data), merge_var_data
            ),
            **kwargs,
        )

        if (js_expr := kwargs.get("_js_expr")) is not None:
            object.__setattr__(value_with_replaced, "_js_expr", js_expr)

        return value_with_replaced

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Drop the kwarg and express string-ness via the public API, e.g. Var.create(value, string=False) at creation time or VarData formatting hooks
2. Update to the current _replace signature
3. Upgrade pinned third-party reflex packages
4. Grep for _var_is_string across the project and migrate all uses

Example fix

# before
v = v._replace(_var_is_string=True)
# after
v = Var.create(v, string=True)  # set string-ness at creation

kwargs.pop('_var_is_string', None)
v = v._replace(**kwargs)

def is_supported_replace_kwarg(name: str) -> bool:
    return name not in {'_var_is_local', '_var_is_string', '_var_full_name_needs_state_prefix'}

When it happens

Trigger: Calling var._replace(_var_is_string=True); code or libraries written for an older reflex Var that accepted these flags; copy-pasted internal Reflex source after upgrading.

Common situations: Reflex version upgrades where Var was refactored (flags moved to VarData); stale component libraries; migration of large codebases using private Var APIs.

Related errors

• The _var_is_local argument is not supported for Var.
• The _var_full_name_needs_state_prefix argument is not suppor
• Expected _js_expr to be a string, got value {self._js_expr!r
• Unsupported type {var_type} for guess_type.
• Var of type {self._var_type} does not support item access.

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/200785be2fef9d08. Report an issue: GitHub.