sudoku_print.py

def border_line():
    return ("=" * 7).join("+" * 4)


def get_fmt(i):
    return "{}" if i % 3 else "+ {}"


def sudoku_line(i, line):
    s = "" if i % 3 else border_line() + "\n"
    return (
        s
        + " ".join(
            get_fmt(i).format(x if x != "0" else "_") for i, x in enumerate(line)
        )
        + " +"
    )


def sudoku_board(board):
    return (
        "\n".join(sudoku_line(i, line) for i, line in enumerate(board.splitlines()))
        + "\n"
        + border_line()
    )


board = """123456789\n234567890\n345678901\n456789012
567890123\n678901234\n789012345\n890123456\n901234567"""

print(sudoku_board(board))