base = [ ["1", "2", "3"], ["4", "", "5"], ["6", "7", "8"], ] def f(x): def concat(b): if not x: return b if not b: return f"[[{x}]]" return f"{x}-{b}" if x else b return apply_tree(base, concat) def apply_tree(tree, func): result = [] for e in tree: if isinstance(e, str): result.append(func(e)) else: result.append(apply_tree(e, func)) return result def print_tree_matrix(tree): for layer in tree: for row in zip(*layer): print("\t".join(sum(row, []))) print_tree_matrix(apply_tree(base, f))