code: def buy(self, quantity, price, commission=0.0):
      assert quantity > 0, "Invalid quantity"
      self.update(quantity, price, commission)
  
imports: import math
  from pyalgotrade import stratanalyzer
  from pyalgotrade import observer
  from pyalgotrade import dataseries
label_desc: Use image node_id 12 for calling the PositionTracker obj's underlying member method code with example usage: obj.buy(quantity, price, commission) without return types