code: def average(old_avg, current_value, count):
      
      if old_avg is None:
          return current_value
      return (float(old_avg) * count + current_value) / (count + 1)
docs: Calculate the average. Count must start with 0
  
      >>> average(None, 3.23, 0)
      3.23
      >>> average(0, 1, 0)
      1.0
      >>> average(2.5, 5, 4)
      3.0