nautechsystems/nautilus_trader · error

Python on_book failed: {e}

Error message

Python on_book failed: {e}

What it means

This error wraps any Python exception raised inside the actor's `on_book` callback when an `OrderBook` snapshot is delivered to the Python subclass. The Rust `PyDataActor` calls `call_method1(py, "on_book", ...)` and converts the resulting `PyErr` into an anyhow error with this message. The original Python traceback (including line number and exception type) is appended after the `{e}` placeholder, so the actual bug is in user Python code, not the Rust core.

Source

Thrown at crates/common/src/python/actor.rs:1128

    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<()> {
        self.dispatch_on_book(order_book)
            .map_err(|e| anyhow::anyhow!("Python on_book failed: {e}"))
    }

    fn on_mark_price(&mut self, mark_price: &MarkPriceUpdate) -> anyhow::Result<()> {
        self.dispatch_on_mark_price(*mark_price)
            .map_err(|e| anyhow::anyhow!("Python on_mark_price failed: {e}"))
    }

    fn on_index_price(&mut self, index_price: &IndexPriceUpdate) -> anyhow::Result<()> {
        self.dispatch_on_index_price(*index_price)
            .map_err(|e| anyhow::anyhow!("Python on_index_price failed: {e}"))
    }

    fn on_funding_rate(&mut self, funding_rate: &FundingRateUpdate) -> anyhow::Result<()> {
        self.dispatch_on_funding_rate(*funding_rate)
            .map_err(|e| anyhow::anyhow!("Python on_funding_rate failed: {e}"))
    }

    fn on_instrument_status(&mut self, data: &InstrumentStatus) -> anyhow::Result<()> {

View on GitHub (pinned to 18893faf8b)

Solutions

  1. Read the full traceback appended after 'Python on_book failed:' — it names the exact line in your on_book override that raised.
  2. Add a defensive check inside on_book for missing data (e.g. book.best_bid_price() returning None/NaN) before use.
  3. Call super().on_book(book) at the start of your override if you changed the method signature or forgot it.
  4. Reproduce locally with the same data by running the actor in a sandbox/backtest before live deployment.

Example fix

// before (Rust actor.rs propagates the wrapped exception)
self.dispatch_on_book(order_book)
    .map_err(|e| anyhow::anyhow!("Python on_book failed: {e}"))

// after (user Python hardening)
# before
def on_book(self, book):
    px = book.best_bid_price()
    qty = book.best_bid_qty()
    self.order(px, qty)

# after
def on_book(self, book):
    px = book.best_bid_price()
    qty = book.best_bid_qty()
    if px is None or qty is None or qty <= 0:
        return
    self.order(px, qty)
Defensive patterns

Strategy: try-catch

Validate before calling

if book.best_bid_price() is None or book.best_ask_price() is None:
    return  # skip empty/uninitialized book

Type guard

def is_valid_book(book):
    bid, ask = book.best_bid_price(), book.best_ask_price()
    return bid is not None and ask is not None and bid != 0 and ask != 0

Try / catch

def on_book(self, book):
    try:
        self._process_book(book)
    except Exception as e:
        self.log.error(f"on_book handler failed: {e}", exc_info=True)

Prevention

When it happens

Trigger: Any exception raised by the user's `on_book(self, book)` override, or during `into_py_any` conversion of the OrderBook to a Python object. E.g. accessing `book.instrument_id` and calling methods on None, dividing by a zero book.best_bid_price(), indexing into an empty bids list, or a typo'd helper method invoked inside on_book.

Common situations: Live trading with a new data handler added to an existing actor; upgrading nautilus_trader where OrderBook Python bindings changed shape (e.g. attribute renamed), so attribute access in the handler now raises AttributeError; handling the first snapshot before book initialization assumptions hold.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/ef4bbaebf8a01130. Report an issue: GitHub.