docs: def fetch( call_hash ):
      """
      Fetches CallDescriptor from the local database given a hash key representing the call. If it doesn't exist returns None.
  
      :param str hash: The sha1 hexdigest to look the CallDescriptor up by.
  
      :rtype: CallDescriptor corresponding to the hash passed or None if it wasn't found.
      """
      res = select_expected_value(call_hash)
      if not res:
          return None
  
      last_packet_number = -1
      expected_value = ""
      for packet in res:
          call_hash, expected_value_fragment, packet_num = packet
          expected_value += expected_value_fragment
          if packet_num <= last_packet_number:
              raise Exception("Received expected_value data out of order!")
  
          last_packet_number = packet_num
  
      return ExpectedValue( call_hash      = call_hash,
                            expected_value = pickle.loads(str(expected_value)) )