code: def get_next_file_path(self, service, operation):
          
          base_name = '{0}.{1}'.format(service, operation)
          if self.prefix:
              base_name = '{0}.{1}'.format(self.prefix, base_name)
          LOG.debug('get_next_file_path: %s', base_name)
          next_file = None
          serializer_format = None
          index = self._index.setdefault(base_name, 1)
  
          while not next_file:
              file_name = os.path.join(
                  self._data_path, base_name + '_{0}'.format(index))
              next_file, serializer_format = self.find_file_format(file_name)
              if next_file:
                  self._index[base_name] += 1
              elif index != 1:
                  index = 1
                  self._index[base_name] = 1
              else:
                  raise IOError('response file ({0}.[{1}]) not found'.format(
                      file_name, "|".join(Format.ALLOWED)))
  
          return next_file, serializer_format
docs: Returns a tuple with the next file to read and the serializer
          format used