Persistent state and trailing stop

sdk.state is the SDK mechanism for non-trivial strategies. It is a dictionary that the engine keeps alive between script calls.

Common patterns for managing state bar-by-bar, from flags to trailing stops.

def on_bar_strategy(sdk, params):
    # Pattern 1: Initialization
    if not isinstance(sdk.state, dict): sdk.state = {}
    
    # Pattern 2: Trail Stop Logic
    if sdk.position > 0:
        hw = sdk.state.get("high_water")
        sdk.state["high_water"] = max(hw, close) if hw else close
        new_stop = sdk.state["high_water"] * (1 - trail_pct)
        sdk.update_exits(stop_loss=new_stop)
    
    # Pattern 3: Cooldowns
    now = sdk.candles[-1]["time"]
    if now - sdk.state.get("last_entry", 0) < cooldown_ms:
        return

Visual flowchart of how data persists across engine calls and restarts.

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Principle

Always initialize keys before using them, at least for non-numeric objects. Missing numeric keys return 0.0 automatically, which simplifies counters but can hide bugs for lists or dicts.

Size limit

sdk.state lives in sandbox memory (64MB limit). Avoid memory leaks by capping list sizes.