nautechsystems/nautilus_trader · error

Python on_start failed: {e}

Error message

Python on_start failed: {e}

What it means

PyExecutionAlgorithm implements the DataActor lifecycle. Its on_start first runs the Rust-side ExecutionAlgorithm::on_start, then dispatches the Python `on_start` callback; a Python-side exception is re-wrapped with this message. It indicates the Python component failed to start.

Source

Thrown at crates/trading/src/python/algorithm.rs:547

        let _ = self
            .dispatch_position_event("on_position_changed", PositionEvent::PositionChanged(event));
    }

    fn on_position_closed(&mut self, event: PositionClosed) {
        let _ = self
            .dispatch_position_event("on_position_closed", PositionEvent::PositionClosed(event));
    }

    fn on_position_event(&mut self, event: PositionEvent) {
        let _ = self.dispatch_position_event("on_position_event", event);
    }
}

impl DataActor for PyExecutionAlgorithm {
    fn on_start(&mut self) -> anyhow::Result<()> {
        ExecutionAlgorithm::on_start(self)?;
        self.dispatch_no_args("on_start")
            .map_err(|e| anyhow::anyhow!("Python on_start failed: {e}"))
    }

    fn on_stop(&mut self) -> anyhow::Result<()> {
        ExecutionAlgorithm::on_stop(self)?;
        self.dispatch_no_args("on_stop")
            .map_err(|e| anyhow::anyhow!("Python on_stop failed: {e}"))
    }

    fn on_resume(&mut self) -> anyhow::Result<()> {
        ExecutionAlgorithm::on_resume(self)?;
        self.dispatch_no_args("on_resume")
            .map_err(|e| anyhow::anyhow!("Python on_resume failed: {e}"))
    }

    fn on_reset(&mut self) -> anyhow::Result<()> {
        ExecutionAlgorithm::on_reset(self)?;
        self.dispatch_no_args("on_reset")
            .map_err(|e| anyhow::anyhow!("Python on_reset failed: {e}"))

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Read the chained `{e}` traceback and fix the Python on_start callback.
  2. Ensure any external resources the callback needs (connections, config) are available before start.
  3. Call on_reset (or fresh construction) before restarting so stale state cannot break startup.
  4. Confirm the Python class defines a zero-argument on_start method.

Example fix

// before
def on_start(self, config): ...  # unexpected argument

// after
def on_start(self): ...
Defensive patterns

Strategy: try-catch

Validate before calling

# Python: preflight startup dependencies before the engine starts
assert self.ready, "state not initialized; call __init__/on_reset first"

Try / catch

match actor.on_start() {
    Ok(()) => {},
    Err(e) => {
        log::error!("python on_start failed: {e:#}");
        // route the actor to faulted state instead of continuing
    }
}

Prevention

When it happens

Trigger: Calling on_start when the Python on_start raises — e.g. failed initialization inside the user callback, resources unavailable at start, or invalid no-arg dispatch.

Common situations: Python on_start connecting to external services that are down; referencing attributes initialized in an overridden __init__ that was not called; state carried over from a previous run without reset.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/f3b55d5655dde59a. Report an issue: GitHub.