sudoku_solver.py

import re

class Solver(object):
    def solve_sudoku(self, board):
        if self._solve(board):
            return board
        return None
        
    def _solve(self, board):
        for r in range(9):
            for c in range(9):
                if board[r][c] == '.':
                    for d in '123456789':
                        if self._is_valid(board, r, c, d):
                            board[r][c] = d
                            if self._solve(board):
                                return True
                            board[r][c] = '.'
                    return False
        return True
                            
    def _is_valid(self, board, r, c, d):
        for row in range(9):
            if board[row][c] == d:
                return False
        for col in range(9):
            if board[r][col] == d:
                return False
        for row in range(r//3*3, r//3*3+3):
            for col in range(c//3*3, c//3*3+3):
                if board[row][col] == d:
                    return False
        return True

if __name__ == '__main__':
    board = []
    valid = re.compile(r'^[1-9.]{9}$')
    for i in range(9):
        while True:
            row = input('Row {0}: '.format(i+1))
            if valid.match(row):
                board.append(list(row))
                break
            print('Invalid input: row must only contain 1-9 and/or blanks represented by "."')
    solver = Solver()
    solution = solver.solve_sudoku(board)
    if solution:
        for l in solution:
            print(''.join(l))
    else:
        print('Unsolvable')