old_contents: # Divide Function
  
  def div(x,y):
      print "DummyCal: ", x, " / ", y 
      if y == 0 :
          print "DummyCal: ZeroDivisionError, y cannot be 0"
          return None
      else:
          return x/y
  
  
new_contents: # Divide Function
  
  def div(x,y):
      print "DummyCal: ", x, " / ", y 
      if y == 0 :
          print "Error: denominator can not be 0!!!\n"
          return "0"
      else:
          return x/y
  
  
subject: Modify div.py file