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Key Concepts

Event Handlers

Introduction

Event handlers are called during an algorithm execution to pass information to your strategy. Most of the events are not needed for simple strategies, but can be helpful for debugging issues in more complex algorithms.

Data Events

Select Language: 
# Get data for a specific symbol to analyze its price movements or trading signals.
def on_data(self, slice: Slice) -> None:
    my_data = slice.get(self._symbol)

# Get split information to adjust holdings and calculations for the symbol.
def on_splits(self, splits: Splits) -> None:
    split = splits.get(self._symbol)
 
# Capture dividend events to account for income or adjust the strategy based on the dividend payout.
def on_dividends(self, dividends: Dividends) -> None:
    dividend = dividends.get(self._symbol)
   
# Track events when a security changes its ticker, allowing the algorithm to adapt to these changes.
def on_symbol_changed_events(self, symbol_changed_events: SymbolChangedEvents) -> None:
    symbol_changed_event = symbol_changed_events.get(self._symbol)

# Handle delisting events to manage the position or adjust the strategy when the symbol is delisted.
def on_delistings(self, delistings: Delistings) -> None:
    delisting = delistings.get(self._symbol)

The on_data method is the primary event handler for receiving financial data events to your algorithm. It is triggered sequentially at the point in time the data is available; in backtesting and live. For daily data, this means the event is processed at market close. In this method, all data for a given moment of time is grouped in a single event, including custom data types. This data is passed with the Slice object.

When fill-forward is enabled for your asset, the OnData event handler will be called regularly even if there was no new data. This is the default behavior.

In backtesting, if your algorithm takes a long time to process a slice, the following slice objects queue up and the next event triggers when your algorithm finishes processing the current slice. In live trading, if your algorithm takes longer to process the current slice than the time period that the slice spans, the next event triggers when your algorithm finishes processing the current slice, but the slice of the following event is the most recent slice. For example, say your algorithm consumes second resolution Crypto data but it takes your algorithm 3.5 seconds to process each slice. In backtesting, you'll get every slice. In live trading, if you deploy at 12:00:00 AM Coordinated Universal Time (UTC), you'll get the first slice at 12:00:01 AM UTC (spanning 12:00:00 AM UTC to 12:00:01 AM UTC) and you'll get the second slice at 12:00:04.5 AM UTC (roughly spanning 12:00:03 AM UTC to 12:00:04 AM UTC).

The on_splits, on_dividends, on_symbol_changed_events , and on_delistings event handlers provide data for their respective types in an isolated way. However, all of the data for these corporate actions is also available in the Slice in on_data.

To perform intensive computation before the market opens, use a Scheduled Event or the train method.

For more information on the Slice object and OnData event, see Handling Data.

Securities Changed Event

Select Language: 
# Log the universe changes.
def on_securities_changed(self, changes: SecurityChanges) -> None:
    for security in changes.added_securities:
        self.debug(f"{self.time}: Added {security.symbol}")

    for security in changes.removed_securities:
        self.debug(f"{self.time}: Removed {security.symbol}")
        
        if security.invested:
            self.liquidate(security.symbol, "Removed from Universe")

The on_securities_changed event notifies the algorithm when assets are added or removed from the universe. This can be due to changes in the Universe constituents, delisting, contract expiration, or an explicit call to the remove_security method.

The event is triggered immediately when the asset is removed from the universe; however the data feed for the asset may remain active if the algorithm has open orders.

For more information, see how to use Security Changed Events.

Order Events

Select Language: 
# Track filled orders and get the details of each fill.
def on_order_event(self, order_event: OrderEvent) -> None:
    order = self.transactions.get_order_by_id(order_event.order_id)
    if order_event.status == OrderStatus.FILLED:
        self.debug(f"{self.time}: {order.type}: {order_event}")

# Track Option assignment events to handle assigned Equity holdings. 
def on_assignment_order_event(self, assignment_event: OrderEvent) -> None:
    self.log(str(assignment_event))

The on_order_event method notifies the algorithm of new orders, and changes in the order status such as fill events and cancelations. For options assignment events there is a dedicated event handler on_assignment_order_event.

In backtesting order events are triggered synchronously after the main data events. In live trading, order events are asynchronously as they occur. To avoid infinite loops, we recommend not to place orders in the OnOrderEvent.

For more information, see how to use Order Events.

Brokerage Events

Select Language: 
# Track brokerage messages.
def on_brokerage_message(self, message: BrokerageMessageEvent) -> None: 
    if message.type == BrokerageMessageType.RECONNECT:
        self.log(f"{self.time}: {message.type}: Message: {message.message}")

# Track brokerage disconnections and respond to connectivity interruptions.
def on_brokerage_disconnect(self) -> None:
    self.log(f"Brokerage disconnection detected at {self.time}")

 # Track brokerage reconnections to resume trading operations.
def on_brokerage_reconnect(self) -> None:
    self.log(f"Brokerage reconnected at {self.time}")

The on_brokerage_disconnect and on_brokerage_reconnect event handlers pass information to the algorithm about the brokerage connection status. This can be helpful for considering when to place a trade when a brokerage API is unreliable or under maintenance. The on_brokerage_message event handler provides information from the brokerages, including the disconnect and reconnect messages. Message content varies with each brokerage.

Brokerage events are triggered asynchronously in live trading, and are not created in backtesting.

Margin Call Events

Select Language: 
# Track when remaining margin is low.
def on_margin_call_warning(self) -> None:
    self.log("Margin call warning, 5% margin remaining")
    
# Review and adjust liquidation orders in response to a margin call.
def on_margin_call(self, requests): -> List[SubmitOrderRequest]: 
    # Modify order requests to choose what to liquidate.
    return requests

The on_margin_call_warning and on_margin_call event handlers provide information and control over how to reduce algorithm leverage when performance is poor.

The on_margin_call_warning method is called when there is less than 5% margin remaining to give you an opportunity to reduce leverage before an official margin call is performed by the brokerage. The on_margin_call event handler is passed a list of SubmitOrderRequest objects which will be executed on exiting the method.

Margin events are called before the data events are processed.

The following demonstration processes suggested orders and modifies the qualities to liquidate more than necessary and prevent a second margin call. For more information, see how to handle Margin Calls.

Warmup Finished Event

Select Language: 
# This event handler runs when the algorithm is done warming up.
def on_warmup_finished(self) -> None:
    pass # Done warming up.

The on_warmup_finished event handler runs once initialization and warm up is complete.

Command Events

Select Language: 
# An event handler for receiving external commands. 
def on_command(self, data):
    self.log(f'Got command at {self.time} with data: {data}')
    return True

The on_command event handler runs when you send external commands to your algorithm.

End Of Algorithm Events

Select Language: 
# The final step after algorithm execution. 
def on_end_of_algorithm(self) -> None:
    self.debug("Algorithm done")

The on_end_of_algorithm event handler is when the algorithm has finished executing as its final step. This event handler is helpful for performing final analysis, or saving algorithm state.

Event Flow

When you deploy an algorithm, LEAN first calls the initialize method. In live mode, the engine loads your holdings and open orders from your brokerage account to add data subscriptions and populate the securities, portfolio, and transactions objects. LEAN receives the data from the subscriptions, synchronizes the data to create a timeslice, and then performs the following steps. Your algorithm can spend up to 10 minutes on each timeslice unless you call the train method.

  1. If it's a backtest, check if there are Scheduled Events in the past that didn't fire because there was no data between the previous slice and the current slice. LEAN automatically creates a Scheduled Event to call the on_end_of_day method at the end of each day.

    2. In live mode, Scheduled Events occur in a separate thread from the algorithm manager, so they run at the correct time.

  2. Update the algorithm time.
  3. Update the current_slice.
  4. Pass the SymbolChangedEvents to the on_symbol_changed_events method.
  5. Cancel all open orders for securities that changed their ticker.
  6. Add a Security object to the securities collection for each new security in the universe.
  7. Update the Security objects with the latest data.
  8. Update the Cash objects in the CashBook with the latest data.
  9. Process fill models for non-market orders.
  10. Submit market on open orders to liquidate Equity Option contracts if the underlying Equity has a split warning.
  11. Process margin calls.
  12. If it's time to settle unsettled cash, perform settlement.
  13. Call the on_securities_changed method with the latest security changes.
  14. Apply dividends to the portfolio.
  15. For securities that have a split warning, update their portfolio holdings and adjust their open orders to account for the split.
  16. Update consolidators with the latest data.
  17. Pass the Dividends to the on_dividends method.
  18. Pass the Splits to the on_splits method.
  19. Pass the Delistings to the on_delistings method.
  20. Pass the Slice to the on_data method.
  21. Perform universe selection.
  22. Pass the Slice to the update method of each Alpha model.
  23. Pass the Insight objects from the Alpha model to the Portfolio Construction model.
  24. Pass the PortfolioTarget objects from the Portfolio Construction model to each Risk Management model.
  25. Pass the risk-adjusted PortfolioTarget objects from the Risk Management models to the Execution model.

When your algorithm stops executing, LEAN calls the on_end_of_algorithm method.

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