Placing Stop Loss on self.buy()

[ghost]

Sorry for being daft but Im not tracking this in the documentation.

How do I set a trailing 80% stop loss when I initiate a trade? See the bold italics line below.

Really appreciate it.

class SmaCross(Strategy):
    # Define the two MA lags as *class variables*
    # for later optimization
    n1 = 10
    n2 = 40
    
    def init(self):
        # Precompute the two moving averages
        self.sma1 = self.I(SMA, self.data.Close, self.n1)
        self.sma2 = self.I(SMA, self.data.Close, self.n2)
    
    def next(self):
        # If sma1 crosses above sma2, close any existing
        # short trades, and buy the asset
        if crossover(self.sma1, self.sma2):
            self.position.close()
            **_self.buy(sl=self.price*0.8)_**

        # Else, if sma1 crosses below sma2, close any existing
        # long trades, and sell the asset if you are long_short
        elif crossover(self.sma2, self.sma1):
            self.position.close()
 #           self.sell()

[kernc]

Your sl= above is initialized correctly, but it's not trailing the price moves upwards.

One of the examples features TrailingStrategy that does roughly that. To use it, you need to transform your 20 % into ATR units. Simplified:

tr = data.High - data.Low
one_atr_in_pct = (tr / data.Close).mean()

def to_atr(pct):
    assert 0 < pct < 1, pct
    return pct / one_atr_in_pct

# >>> to_atr(.20)
# 12.214876329014

[ghost]

Thank you.