Need help converting 2 lines of code to backtrader

wykazox

Hi All,

Working on translating the famous Squeeze Momentum indicator from LazyBear (which is available in Pine on TradingView), to use it in Backtrader in Python.
More specifically, I have issues with the momentum indicator itself.
It uses the df.rolling.apply syntax from pandas and polyfit from numpy, but the syntax seems incompatible with Backtrader, so I need some help from the community.
I think I need help to help with the last 2 lines of code (fit_y and self.lines.Mom), basically.

The last 2 lines were transcribed from Pine to Python as:

fit_y = np.array(range(0,length_KC))
df['value'] = df['value'].rolling(window = length_KC).apply(lambda x: np.polyfit(fit_y, x, 1)[0] * (length_KC-1) + np.polyfit(fit_y, x, 1)[1], raw=True)

The code I have translated to Backtrader so far:

# Create Momentum indicator
class MomentumInd(bt.Indicator):
    lines = ('Mom',)
    params = (('period', 20),)
    plotinfo = dict(subplot=False)

    def _plotlabel(self):
        plabels = [self.p.period]
        return plabels

    def __init__(self):
    highest = bt.ind.Highest(self.data.high, period=self.p.period)
    lowest = bt.ind.Lowest(self.data.low, period=self.p.period)
    midline = (highest + lowest) / 2
    mavg = bt.ind.MovingAverageSimple(self.data.close, period=self.p.period)
    delta = self.data.close - ((midline + mavg) / 2)
    fit_y = np.array(range(0, self.p.period, 1))
    self.lines.Mom = delta.rolling(window=self.p.period).apply(lambda x: np.polyfit(fit_y, x, 1)[0] * (self.p.period - 1) + np.polyfit(fit_y, x, 1)[1], raw=True)

Unfortunately, I get the error:

AttributeError: 'LinesOperation' object has no attribute 'rolling'

I think it relates to the rolling window function in pandas, followed by the apply function, which seems incompatible with backtrader.
Any idea how to translate these last 2 lines in a way that backtrader can interpret the programme by any chance?
Thanks for your help!