https://qiita.com/sekishoku/items/ffb5d4c35970de3219f0#comment-51d891aa67c28702620c

suudoku.py

board = """
| 45|  6| 3 |
|   | 49|126|
|   |   |   |
+---+---+---+
| 12|   |  4|
|9  | 62|   |
| 87|   |  9|
+---+---+---+
|   |   |   |
|   | 95|687|
| 76|  3| 9 |
""".splitlines()

SPACE = 0
board = [[int(number) if number.isdigit() else SPACE
          for number in row.replace('|', '')]
         for row in board[1:4]+board[5:8]+board[9:12]]


def solve(board, row=0, col=0):
    while board[row][col] != SPACE:
        if col < 8:
            col += 1
        elif row < 8:
            col = 0
            row += 1
        else:
            return True

    for number in candidate(board, row, col):
        board[row][col] = number
        if solve(board, row, col):
            return True

    board[row][col] = SPACE
    return False


def candidate(board, row, col):
    if board[row][col] != SPACE:
        return []

    row_numbers = board[row]
    col_numbers = [board[r][col] for r in range(9)]
    box_row = row - row % 3
    box_col = col - col % 3
    box_numbers = [board[r][c]
                   for r in range(box_row, box_row + 3)
                   for c in range(box_col, box_col + 3)]
    conflict_numbers = row_numbers + col_numbers + box_numbers
    return set(range(1, 10)) - set(conflict_numbers)


def main():
    if solve(board):
        for row in board:
            print(*row)
    else:
        print("No answer")


if __name__ == '__main__':
    main()