{"record":{"id":"dc6abb12478f5629","repo":"reflex-dev/reflex","slug":"the-function-passed-to-operation-name-should-tak","errorCode":null,"errorMessage":"The function passed to {operation_name} should take at most one argument.","messagePattern":"The function passed to (.+?) should take at most one argument\\.","errorType":"exception","errorClass":"VarTypeError","httpStatus":null,"severity":"error","filePath":"packages/reflex-base/src/reflex_base/vars/sequence.py","lineNumber":456,"sourceCode":"    ) -> tuple[tuple[str, ...], Var]:\n        \"\"\"Call fn with a placeholder element to get function arg names and return expression.\n\n        Args:\n            fn: The function to trace, taking at most one argument.\n            operation_name: The name of the calling operation, for error messages.\n\n        Returns:\n            A tuple of the function's argument names and its return expression.\n\n        Raises:\n            VarTypeError: If the function takes more than one argument.\n        \"\"\"\n        if not callable(fn):\n            raise_unsupported_operand_types(operation_name, (type(self), type(fn)))\n        num_args = len(inspect.signature(fn).parameters)\n        if num_args > 1:\n            msg = f\"The function passed to {operation_name} should take at most one argument.\"\n            raise VarTypeError(msg)\n        if num_args == 0:\n            return (), Var.create(fn())\n        element = self._element_placeholder()\n        return (element._js_expr,), Var.create(fn(element))\n\n    def map(self, fn: Any):\n        \"\"\"Apply a function to each element of the array.\n\n        Args:\n            fn: The function to apply, taking at most one argument.\n\n        Returns:\n            The array after applying the function.\n        \"\"\"\n        from .function import ArgsFunctionOperation\n\n        args, return_expr = self._trace_element_fn(fn, \"map\")\n        return map_array_operation(","sourceCodeStart":438,"sourceCodeEnd":474,"githubUrl":"https://github.com/reflex-dev/reflex/blob/45b8ed5ab735f8a56bbb09a42384f030eb0208e7/packages/reflex-base/src/reflex_base/vars/sequence.py#L438-L474","documentation":"For ArrayVar operations map/filter/flat_map, Reflex traces the passed Python function to generate the equivalent JS callback. The callback may take zero or one argument (the element); more than one parameter cannot be mapped to JS, so VarTypeError is raised.","triggerScenarios":"Passing a lambda/function with 2+ positional params to .map()/.filter()/.flat_map(), e.g. `items.map(lambda item, i: ...)` or a function with extra defaulted args counted by inspect.signature (e.g. keyword-only or *args in some forms).","commonSituations":"Trying to get the index alongside the element; reusing a helper that takes additional config arguments; copying JS .map((el, i) => ...) patterns into Python.","solutions":["Rewrite the callback to take exactly one argument (the element); if the index is needed, use a different approach (e.g. enumerate in a computed var).","Close over extra configuration values instead of declaring them as parameters."],"exampleFix":"# before\nState.items.map(lambda item, idx: f\"{idx}: {item}\")\n\n# after\nclass State(rx.State):\n    @rx.var\n    def items_indexed(self) -> list[str]:\n        return [f\"{i}: {x}\" for i, x in enumerate(self.items)]","handlingStrategy":"validation","validationCode":"import inspect\n\ndef takes_at_most_one_arg(fn) -> bool:\n    return len(inspect.signature(fn).parameters) <= 1","typeGuard":"import inspect\n\ndef unary_callback(fn) -> bool:\n    return callable(fn) and len(inspect.signature(fn).parameters) <= 1","tryCatchPattern":null,"preventionTips":["Write map/filter/flat_map callbacks with exactly one parameter.","Capture extra config via closures, not parameters.","Avoid *args in traced callbacks."],"tags":["reflex","arrayvar","map","callback-signature"],"backgroundTag":"var-callback-signature-mismatch","analyzedSha":"45b8ed5ab735f8a56bbb09a42384f030eb0208e7","analyzedAt":"2026-08-28T19:25:27.644Z","schemaVersion":2},"datasetVersion":"2026-08-28T21:17:43.275Z"}