leetcode Sudoku Solver python

#the define of Sudoku is on this link : http://sudoku.com.au/TheRules.aspx

Write a program to solve a Sudoku puzzle by filling the empty cells.

Empty cells are indicated by the character '.'.

You may assume that there will be only one unique solution.

A sudoku puzzle...

...and its solution numbers marked in red.

class Solution(object):
    def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        def isValid(x,y):
            tmp=board[x][y]
            board[x][y]='D'
            for i in range(9):
                if board[i][y] == tmp:
                    return False
            for i in range(9):
                if board[x][i] == tmp:
                    return False
            for i in range(3):
                for j in range(3):
                    if board[(x/3)*3+i][(y/3)*3+j] == tmp:
                        return False
            board[x][y]=tmp
            return True
        def dfs(board):
            for i in range(9):
                for j in range(9):
                    if board[i][j] == '.':
                        for k in '':
                            board[i][j] = k
                            if isValid(i,j) and dfs(board):
                                return True
                            board[i][j] = '.'
                        return False
            return True
        dfs(board)