{"record":{"id":"b6235b11d2a65b7d","repo":"nautechsystems/nautilus_trader","slug":"python-on-signal-failed-e-b6235b","errorCode":null,"errorMessage":"Python on_signal failed: {e}","messagePattern":"Python on_signal failed: (.+?)","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"crates/trading/src/python/strategy.rs","lineNumber":1121,"sourceCode":"\n    fn on_time_event(&mut self, event: &TimeEvent) -> anyhow::Result<()> {\n        route_time_event(self, event);\n        self.dispatch_on_time_event(event)\n            .map_err(|e| anyhow::anyhow!(\"Python on_time_event failed: {e}\"))\n    }\n\n    #[allow(unused_variables)]\n    fn on_data(&mut self, data: &CustomData) -> anyhow::Result<()> {\n        Python::attach(|py| {\n            let py_data: Py<PyAny> = Py::new(py, data.clone())?.into_any();\n            self.dispatch_on_data(py_data)\n                .map_err(|e| anyhow::anyhow!(\"Python on_data failed: {e}\"))\n        })\n    }\n\n    fn on_signal(&mut self, signal: &Signal) -> anyhow::Result<()> {\n        self.dispatch_on_signal(signal)\n            .map_err(|e| anyhow::anyhow!(\"Python on_signal failed: {e}\"))\n    }\n\n    fn on_queue_state(&mut self, event: &QueueStateChanged) -> anyhow::Result<()> {\n        self.dispatch_on_queue_state(event)\n            .map_err(|e| anyhow::anyhow!(\"Python on_queue_state failed: {e}\"))\n    }\n\n    fn on_socket_state(&mut self, event: &SocketStateChanged) -> anyhow::Result<()> {\n        self.dispatch_on_socket_state(event)\n            .map_err(|e| anyhow::anyhow!(\"Python on_socket_state failed: {e}\"))\n    }\n\n    fn on_instrument(&mut self, instrument: &InstrumentAny) -> anyhow::Result<()> {\n        Python::attach(|py| {\n            let py_instrument = instrument_any_to_pyobject(py, instrument.clone())\n                .map_err(|e| anyhow::anyhow!(\"Failed to convert InstrumentAny to Python: {e}\"))?;\n            self.dispatch_on_instrument(py_instrument)\n                .map_err(|e| anyhow::anyhow!(\"Python on_instrument failed: {e}\"))","sourceCodeStart":1103,"sourceCodeEnd":1139,"githubUrl":"https://github.com/nautechsystems/nautilus_trader/blob/18893faf8b356be3320add8de2f861b0b647cf06/crates/trading/src/python/strategy.rs#L1103-L1139","documentation":"The strategy's Python-level `on_signal` callback raised an exception. `dispatch_on_signal(signal)` forwards the received `Signal` to the Python method; any exception escaping the handler is wrapped at strategy.rs:1121 as `Python on_signal failed: {e}`. The real cause is the Python exception text in `{e}`.","triggerScenarios":"An external signal (published by another strategy or via the signal API, e.g. `Signal` with a particular name) arrives and `on_signal` raises — e.g. parsing `signal.info` JSON that is not valid JSON, or acting on a signal name the handler doesn't expect.","commonSituations":"`json.loads(signal.info)` on non-JSON or empty info strings; strategy code that trades immediately in `on_signal` and hits an unhandled market/venue error; signal producers changing the info schema; missing handler branches for newly added signal names.","solutions":["Inspect `{e}` and fix the raising code in your Python `on_signal`.","Validate/parse `signal.info` defensively (try/except json.JSONDecodeError, default handling).","Dispatch by signal name with an explicit else-branch so unknown signals are logged, not crashed on.","Decouple trading actions from signal handling: catch trading errors separately and log them."],"exampleFix":"// before\ndef on_signal(self, signal):\n    payload = json.loads(signal.info)  # JSONDecodeError on empty info\n    self.order(signal.name, payload[\"side\"], payload[\"qty\"])\n\n// after\ndef on_signal(self, signal):\n    try:\n        payload = json.loads(signal.info) if signal.info else {}\n    except json.JSONDecodeError as e:\n        self.log.error(f\"bad signal info: {e}\")\n        return\n    if \"side\" not in payload:\n        return\n    self.order(signal.name, payload[\"side\"], payload.get(\"qty\"))","handlingStrategy":"try-catch","validationCode":"import json\ndef parse_signal_info(signal):\n    if not signal.info:\n        return {}\n    try:\n        return json.loads(signal.info)\n    except json.JSONDecodeError:\n        return {}","typeGuard":"def is_actionable(signal, known_names):\n    return signal.name in known_names and bool(signal.info)","tryCatchPattern":"def on_signal(self, signal):\n    try:\n        self._act_on_signal(signal)\n    except Exception as e:\n        self.log.error(f\"signal {signal.name} handling failed: {e}\")","preventionTips":["Always try/except JSON parsing of signal.info.","Dispatch by signal name with a logged default branch for unknown names.","Keep trading calls out of the parse path; validate payload first.","Version the signal info schema shared between producer and consumer."],"tags":["python","strategy","signal","callback"],"backgroundTag":"python-callback-failed","analyzedSha":"18893faf8b356be3320add8de2f861b0b647cf06","analyzedAt":"2026-09-08T20:49:34.690Z","contentChangedAt":"2026-09-08T20:49:34.690Z","schemaVersion":2},"datasetVersion":"2026-09-14T05:17:10.506Z"}