nautechsystems/nautilus_trader · error

Python on_trade failed: {e}

Error message

Python on_trade failed: {e}

What it means

Wraps any failure raised when the Rust core dispatches a TradeTick into the Python actor's `on_trade` handler. The Python `on_trade` implementation raised (or dispatch into Python failed), and Rust re-raises with the 'Python on_trade failed' prefix.

Source

Thrown at crates/common/src/python/actor.rs:1108

    }

    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)
            .map_err(|e| anyhow::anyhow!("Python on_trade failed: {e}"))
    }

    fn on_bar(&mut self, bar: &Bar) -> anyhow::Result<()> {
        self.dispatch_on_bar(*bar)
            .map_err(|e| anyhow::anyhow!("Python on_bar failed: {e}"))
    }

    fn on_book_deltas(&mut self, deltas: &OrderBookDeltas) -> anyhow::Result<()> {
        self.dispatch_on_book_deltas(deltas.clone())
            .map_err(|e| anyhow::anyhow!("Python on_book_deltas failed: {e}"))
    }

    fn on_book_depth(&mut self, depth: &OrderBookDepth10) -> anyhow::Result<()> {
        self.dispatch_on_book_depth(depth)
            .map_err(|e| anyhow::anyhow!("Python on_book_depth failed: {e}"))
    }

    fn on_book(&mut self, order_book: &OrderBook) -> anyhow::Result<()> {

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Inspect the wrapped `{e}` traceback and fix the exception inside `on_trade`
  2. Verify the override signature `on_trade(self, tick)` taking a TradeTick
  3. Initialize all handler state in on_start before subscribing to trade ticks
  4. Add defensive checks for None/empty aggressor side or price fields

Example fix

// before
def on_trade(self, tick):
    self.vwap.update(tick.price, None)  # TypeError on None size
// after
def on_trade(self, tick):
    if tick.size is None or tick.size.as_double() <= 0:
        return
    self.vwap.update(tick.price, tick.size)
Defensive patterns

Strategy: try-catch

Validate before calling

def _valid_trade(tick) -> bool:
    return tick.price is not None and tick.size is not None and tick.size > 0

Try / catch

try:
    self.dispatch_on_trade(tick)
except Exception as e:
    self.log.error(f'Python on_trade failed: {e}', exc_info=True)

Prevention

When it happens

Trigger: A trade tick arrives after subscribe_trade_ticks and the Python `on_trade(self, tick)` override raises an exception.

Common situations: Trade aggregation logic with uninitialized state, wrong handler signature, or exceptions from order submission triggered on every trade in fast markets.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/f7879e9614d9dc1d. Report an issue: GitHub.