code: def log(self, txt, dt=None):
      if self.p.printout:
          dt = dt or self.data.datetime[0]
          dt = bt.num2date(dt)
          print("%s, %s" % (dt.isoformat(), txt))
  
file_path: backtrader/contrib/samples/pair-trading/pair-trading.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import argparse
  import datetime
  import backtrader
  import backtrader.feeds
  import backtrader.indicators
label_desc: Use image node_id 1 for calling the PairTradingStrategy obj's underlying member method code with example usage: obj.log(txt, dt) without return types