{"record":{"id":"b293683ee5cee91e","repo":"reflex-dev/reflex","slug":"could-not-compare-types-args-types-without-vars-i","errorCode":null,"errorMessage":"Could not compare types {args_types_without_vars[i]} and {callback_param_type} for argument {arg}{callback_name_context}{key_context}.","messagePattern":"Could not compare types (.+?) and (.+?) for argument (.+?)(.+?)(.+?)\\.","errorType":"exception","errorClass":"TypeError","httpStatus":null,"severity":"error","filePath":"packages/reflex-base/src/reflex_base/event/__init__.py","lineNumber":1999,"sourceCode":"        # check that args of event handler are matching the spec if type hints are provided\n        for i, arg in enumerate(callback_params_names[: len(args_types_without_vars)]):\n            if arg not in callback_param_name_to_type:\n                continue\n\n            type_match_found.setdefault(arg, False)\n            callback_param_type = callback_param_name_to_type[arg]\n\n            try:\n                compare_result = typehint_issubclass(\n                    args_types_without_vars[i], callback_param_type\n                ) or _is_on_submit_mapping_event_arg_compatible_with_typed_dict(\n                    args_types_without_vars[i], callback_param_type, key\n                )\n            except TypeError as te:\n                callback_name_context = f\" of {callback_name}\" if callback_name else \"\"\n                key_context = f\" for {key}\" if key else \"\"\n                msg = f\"Could not compare types {args_types_without_vars[i]} and {callback_param_type} for argument {arg}{callback_name_context}{key_context}.\"\n                raise TypeError(msg) from te\n\n            if compare_result:\n                type_match_found[arg] = True\n                continue\n            type_match_found[arg] = False\n            as_annotated_in = (\n                f\" as annotated in {callback_name}\" if callback_name else \"\"\n            )\n            delayed_exceptions.append(\n                EventHandlerArgTypeMismatchError(\n                    f\"Event handler {key} expects {args_types_without_vars[i]} for argument {arg} but got {callback_param_type}{as_annotated_in} instead.\"\n                )\n            )\n\n        if all(type_match_found.values()):\n            delayed_exceptions.clear()\n            if event_spec_index:\n                args = get_args(provided_event_types[0])","sourceCodeStart":1981,"sourceCodeEnd":2017,"githubUrl":"https://github.com/reflex-dev/reflex/blob/45b8ed5ab735f8a56bbb09a42384f030eb0208e7/packages/reflex-base/src/reflex_base/event/__init__.py#L1981-L2017","documentation":"When binding a callback/event handler to a component prop, Reflex compares the handler's parameter types with the arg types the component provides; if Python's issubclass check between the two types itself raises TypeError (e.g. unrelated generic alias combinations or unparameterized generics), the comparison error is re-raised with both types and the argument name.","triggerScenarios":"Passing a handler with parameter annotations like dict (bare generic) or complex typing constructs to props such as on_change/on_select where the component supplies a parameterized generic type, causing issubclass(A, B) TypeError.","commonSituations":"Upgrading Reflex versions where component callback arg types became parameterized generics while user handlers kept bare annotations; using custom Generic classes in handler signatures.","solutions":["Annotate the handler parameter with the exact parameterized type the callback provides (e.g. list[str] instead of list, dict[str, Any] instead of dict)","Simplify/avoid exotic typing constructs in handler signatures used as component callbacks","Update Reflex — newer releases widen compatible type pairs"],"exampleFix":"# before\nclass State(rx.State):\n    @rx.event\n    def on_change(self, value: list): ...\n# after\nfrom typing import Any\nclass State(rx.State):\n    @rx.event\n    def on_change(self, value: list[str]): ...","handlingStrategy":"try-catch","validationCode":"# pre-check that issubclass works for your annotation pair\nimport typing\ntry:\n    typing.get_type_hints(fn)\nexcept Exception:\n    raise  # fix annotations before binding as callback","typeGuard":"def safe_param_annotation(t) -> TypeGuard[type]: return isinstance(t, type) or typing.get_origin(t) is not None","tryCatchPattern":"try:\n    rx.select(options, on_change=State.pick)\nexcept TypeError as e:\n    if 'Could not compare types' in str(e):\n        # re-annotate handler params with parameterized generics and retry\n        ...","preventionTips":["Always use fully parameterized generics (list[str], dict[str, Any]) in handler signatures","Keep Reflex updated; pin versions in CI to catch type-compat regressions"],"tags":["reflex","type-checking","generics","event-handler"],"backgroundTag":"type-annotation-mismatch","analyzedSha":"45b8ed5ab735f8a56bbb09a42384f030eb0208e7","analyzedAt":"2026-08-28T19:25:27.644Z","schemaVersion":2},"datasetVersion":"2026-08-28T21:17:43.275Z"}