nautechsystems/nautilus_trader · error

Python on_signal failed: {e}

Error message

Python on_signal failed: {e}

What it means

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}`.

Source

Thrown at crates/trading/src/python/strategy.rs:1121

    fn on_time_event(&mut self, event: &TimeEvent) -> anyhow::Result<()> {
        route_time_event(self, event);
        self.dispatch_on_time_event(event)
            .map_err(|e| anyhow::anyhow!("Python on_time_event failed: {e}"))
    }

    #[allow(unused_variables)]
    fn on_data(&mut self, data: &CustomData) -> anyhow::Result<()> {
        Python::attach(|py| {
            let py_data: Py<PyAny> = Py::new(py, data.clone())?.into_any();
            self.dispatch_on_data(py_data)
                .map_err(|e| anyhow::anyhow!("Python on_data failed: {e}"))
        })
    }

    fn on_signal(&mut self, signal: &Signal) -> anyhow::Result<()> {
        self.dispatch_on_signal(signal)
            .map_err(|e| anyhow::anyhow!("Python on_signal failed: {e}"))
    }

    fn on_queue_state(&mut self, event: &QueueStateChanged) -> anyhow::Result<()> {
        self.dispatch_on_queue_state(event)
            .map_err(|e| anyhow::anyhow!("Python on_queue_state failed: {e}"))
    }

    fn on_socket_state(&mut self, event: &SocketStateChanged) -> anyhow::Result<()> {
        self.dispatch_on_socket_state(event)
            .map_err(|e| anyhow::anyhow!("Python on_socket_state failed: {e}"))
    }

    fn on_instrument(&mut self, instrument: &InstrumentAny) -> anyhow::Result<()> {
        Python::attach(|py| {
            let py_instrument = instrument_any_to_pyobject(py, instrument.clone())
                .map_err(|e| anyhow::anyhow!("Failed to convert InstrumentAny to Python: {e}"))?;
            self.dispatch_on_instrument(py_instrument)
                .map_err(|e| anyhow::anyhow!("Python on_instrument failed: {e}"))

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Inspect `{e}` and fix the raising code in your Python `on_signal`.
  2. Validate/parse `signal.info` defensively (try/except json.JSONDecodeError, default handling).
  3. Dispatch by signal name with an explicit else-branch so unknown signals are logged, not crashed on.
  4. Decouple trading actions from signal handling: catch trading errors separately and log them.

Example fix

// before
def on_signal(self, signal):
    payload = json.loads(signal.info)  # JSONDecodeError on empty info
    self.order(signal.name, payload["side"], payload["qty"])

// after
def on_signal(self, signal):
    try:
        payload = json.loads(signal.info) if signal.info else {}
    except json.JSONDecodeError as e:
        self.log.error(f"bad signal info: {e}")
        return
    if "side" not in payload:
        return
    self.order(signal.name, payload["side"], payload.get("qty"))
Defensive patterns

Strategy: try-catch

Validate before calling

import json
def parse_signal_info(signal):
    if not signal.info:
        return {}
    try:
        return json.loads(signal.info)
    except json.JSONDecodeError:
        return {}

Type guard

def is_actionable(signal, known_names):
    return signal.name in known_names and bool(signal.info)

Try / catch

def on_signal(self, signal):
    try:
        self._act_on_signal(signal)
    except Exception as e:
        self.log.error(f"signal {signal.name} handling failed: {e}")

Prevention

When it happens

Trigger: 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.

Common situations: `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.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/b6235b11d2a65b7d. Report an issue: GitHub.