code: def mark_time(times, msg=None):
      
      tt = time.clock()
      times.append(tt)
      if (msg is not None) and (len(times) > 1):
          print msg, times[-1] - times[-2]
docs: Time measurement utility.
  
      Measures times of execution between subsequent calls using
      time.clock(). The time is printed if the msg argument is not None.
  
      Examples
      --------
  
      >>> times = []
      >>> mark_time(times)
      ... do something
      >>> mark_time(times, 'elapsed')
      elapsed 0.1
      ... do something else
      >>> mark_time(times, 'elapsed again')
      elapsed again 0.05
      >>> times
      [0.10000000000000001, 0.050000000000000003]