code: def _NormalizedVolumeIdentifiers(
        self, volume_system, volume_identifiers, prefix='v'):
      
      normalized_volume_identifiers = []
      for volume_identifier in volume_identifiers:
        if isinstance(volume_identifier, int):
          volume_identifier = '{0:s}{1:d}'.format(prefix, volume_identifier)
  
        elif not volume_identifier.startswith(prefix):
          try:
            volume_identifier = int(volume_identifier, 10)
            volume_identifier = '{0:s}{1:d}'.format(prefix, volume_identifier)
          except (TypeError, ValueError):
            pass
  
        try:
          volume = volume_system.GetVolumeByIdentifier(volume_identifier)
        except KeyError:
          volume = None
  
        if not volume:
          raise errors.ScannerError(
              'Volume missing for identifier: {0:s}.'.format(volume_identifier))
  
        normalized_volume_identifiers.append(volume_identifier)
  
      return normalized_volume_identifiers
docs: Normalizes volume identifiers.
  
      Args:
        volume_system (VolumeSystem): volume system.
        volume_identifiers (list[int|str]): allowed volume identifiers, formatted
            as an integer or string with prefix.
        prefix (Optional[str]): volume identifier prefix.
  
      Returns:
        list[str]: volume identifiers with prefix.
  
      Raises:
        ScannerError: if the volume identifier is not supported or no volume
            could be found that corresponds with the identifier.