code: def fetch(self, from_time, until_time=None):
          
          until_time = until_time or datetime.now()
          time_info, values = whisper.fetch(self.path,
                                            from_time.strftime('%s'),
                                            until_time.strftime('%s'))
          # build up a list of (timestamp, value)
          start_time, end_time, step = time_info
          current = start_time
          times = []
          while current <= end_time:
              times.append(current)
              current += step
          return zip(times, values)
docs: This method fetch data from the database according to the period
          given
  
          fetch(path, fromTime, untilTime=None)
  
          fromTime is an datetime
          untilTime is also an datetime, but defaults to now.
  
          Returns a tuple of (timeInfo, valueList)
          where timeInfo is itself a tuple of (fromTime, untilTime, step)
  
          Returns None if no data can be returned