docs: def _get_read_preference(read_preference):
          """Converts read_preference from string to pymongo.ReadPreference value.
  
              Args:
                  read_preference: string containig the read_preference from the
                      config file
              Returns:
                  A value from the pymongo.ReadPreference enum
  
              Raises:
                  Exception: Invalid read preference"""
          read_preference = getattr(pymongo.ReadPreference, read_preference, None)
          if read_preference is None:
              raise ValueError('Invalid read preference: %s' % read_preference)
          return read_preference