nautechsystems/nautilus_trader · error

Python on_socket_state failed: {e}

Error message

Python on_socket_state failed: {e}

What it means

Wraps any failure raised when the Rust core dispatches a `SocketStateChanged` event into the Python actor's `on_socket_state` handler. The error indicates the Python handler raised (or dispatch into Python failed); Rust re-raises with the 'Python on_socket_state failed' prefix.

Source

Thrown at crates/common/src/python/actor.rs:1089

            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}"))
        })
    }

    fn on_quote(&mut self, quote: &QuoteTick) -> anyhow::Result<()> {
        self.dispatch_on_quote(*quote)
            .map_err(|e| anyhow::anyhow!("Python on_quote failed: {e}"))
    }

    fn on_trade(&mut self, tick: &TradeTick) -> anyhow::Result<()> {
        self.dispatch_on_trade(*tick)

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Read the chained `{e}` message for the underlying Python traceback and fix the handler
  2. Verify the override signature `on_socket_state(self, event)` with a SocketStateChanged argument
  3. Handle all socket-state enum cases (Connected, Disconnected, etc.) without assuming prior state
  4. Wrap risky reconnect logic in try/except and log instead of raising

Example fix

// before
def on_socket_state(self, event):
    self.client.close()  # AttributeError when never connected
// after
def on_socket_state(self, event):
    if self.client is not None:
        self.client.close()
Defensive patterns

Strategy: try-catch

Validate before calling

def _check_on_socket_state(self, event):
    try:
        self.on_socket_state(event)
    except Exception as e:
        self.log.error(f'on_socket_state failed: {e!r}')

Try / catch

try:
    self.dispatch_on_socket_state(event)
except Exception as e:
    self.log.error(f'Python on_socket_state failed: {e}', exc_info=True)

Prevention

When it happens

Trigger: A socket state change event is delivered to a Python actor overriding `on_socket_state`, and the Python method raises an exception or cannot be dispatched (e.g. not callable).

Common situations: Connection/reconnect logic in `on_socket_state` that touches sockets or clients not yet created, wrong handler arity, or exceptions raised while reacting to Disconnected/Connected transitions.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/6e1991c2410706cf. Report an issue: GitHub.