def METHOD_NAME(cls, reader):
    array_names = set()
    n_ghost_layers = reader.GetUpdateGhostLevel()
    n_pieces = reader.GetNumberOfPieces()
    for piece_index in range(n_pieces):
        reader.UpdatePiece(piece_index, n_pieces, n_ghost_layers)
        piece = reader.GetOutput()
        cell = piece.GetCellData()
        for i in range(cell.GetNumberOfArrays()):
            array_names.add(cell.GetArrayName(i))
    return array_names