The problem of using [-1] with the first bar when backtesting

LUS8806

I find there is something wrong in the QuickStart example:

 if self.dataclose[0] < self.dataclose[-1]:

Actually, when the bar of the first day comes (2000-01-03), self.dataclose[-1] actually refers the bar of the last date (2000-12-28). This is unrealistic.

I am not sure if this is actually a problem. Or there is some misunderstanding?

LUS8806

@LUS8806

so it should add some logic before this line, the following should be works:

  def next(self):
        self.log('Close, {:.2f}'.format(self.dataclose[0]))
        # added lines
        if len(self) < 3:
            return

        if self.dataclose[0] < self.dataclose[-1]:
            # current close less than previous close

            if self.dataclose[-1] < self.dataclose[-2]:
                # previous close less than the previous close
                      ....

run-out

@LUS8806 said in The problem of using [-1] with the first bar when backtesting:

Actually, when the bar of the first day comes (2000-01-03), self.dataclose[-1] actually refers the bar of the last date (2000-12-28). This is unrealistic.

How are you concluding that the previous bar, which would have been the last trading day in 1999, is now the last day in 2000? You should know that all of the data in the data file (1995-2014) is available to backtrader. So if backtrader needs to access information from the last trading day of 1999 it can. Hope that helps.

LUS8806

@run-out

I've set the start date as the first date in the data file. Please see the code:

import backtrader as bt
import datetime


class TestStrategy(bt.Strategy):
    def __init__(self):
        self.dataclose = self.datas[0].close

        self.order = None

    def next(self):
        self.log('Close, {:.2f}'.format(self.dataclose[0]))
        print({"current_date": self.datas[0].datetime.date(0),
               "current_close": self.dataclose[0],
               "pre_date": self.datas[0].datetime.date(-1),
               "pre_close": self.dataclose[-1]})


if __name__ == '__main__':
    cerebro = bt.Cerebro()
    cerebro.addstrategy(TestStrategy)
    data_path = '../backtrader/datas/orcl-1995-2014.txt'
    data = bt.feeds.YahooFinanceCSVData(
        # 数据文件路径
        dataname=data_path,
        # 开始日期
        fromdate=datetime.datetime(1995, 1, 3),
        # 结束日期
        todate=datetime.datetime(1995, 1, 10),
        reverse=False
    )
    cerebro.adddata(data)
    cerebro.broker.setcash(100000.0)

    print('Starting Portfolio Value: %.2f' % cerebro.broker.getvalue())
    cerebro.run()
    print('Final Portfolio Value: %.2f' % cerebro.broker.getvalue())

the output is:

Starting Portfolio Value: 100000.00
{'current_date': datetime.date(1995, 1, 3), 'current_close': 1.88, 'pre_date': datetime.date(1995, 1, 10), 'pre_close': 1.94}
{'current_date': datetime.date(1995, 1, 4), 'current_close': 1.9, 'pre_date': datetime.date(1995, 1, 3), 'pre_close': 1.88}
{'current_date': datetime.date(1995, 1, 5), 'current_close': 1.86, 'pre_date': datetime.date(1995, 1, 4), 'pre_close': 1.9}
{'current_date': datetime.date(1995, 1, 6), 'current_close': 1.88, 'pre_date': datetime.date(1995, 1, 5), 'pre_close': 1.86}
{'current_date': datetime.date(1995, 1, 9), 'current_close': 1.94, 'pre_date': datetime.date(1995, 1, 6), 'pre_close': 1.88}
{'current_date': datetime.date(1995, 1, 10), 'current_close': 1.94, 'pre_date': datetime.date(1995, 1, 9), 'pre_close': 1.94}
Final Portfolio Value: 100000.00

Have you see this line, when i get the pre_close in the first bar, the system returns the data of last bar which is 1995-01-10

{'current_date': datetime.date(1995, 1, 3), 'current_close': 1.88, 'pre_date': datetime.date(1995, 1, 10), 'pre_close': 1.94}

run-out

Since you are looking back to before the data starts, you need to let Backtrader know that is the case by adding in a minimum period before it starts the backtest. Insert after next()

        if len(self.dataclose) <= 1:
            return

LUS8806

@run-out
Thanks a lot. I'll try.