valid sudoku

def checkduplicate(self,list):
        counter = [0 for i in range(9)]
        for x in list:
            if x != '.':
                counter[int(x)-1] += 1
                if counter[int(x)-1] > 1:
                    return False
        return True
    
    def isValidSudoku(self, board):
        for xrow in board: #convert into each row
            if self.checkduplicate(xrow) == False:
                return False
        len_col,len_row = len(board[0]),len(board)
        for xcolumn in [[board[i][j] for i in range(len_row)] for j in range(len_col)]: #convert into each column
            if self.checkduplicate(xcolumn) == False:
                return False
        for c in range(0,len_col,3):
            for r in range(0,len_row,3):
                cube_list = [item for row in [board[r][c:c+3] for r in range(r,r+3)] for item in row] #convert into each 3x3 submatrix
                if self.checkduplicate(cube_list) == False:
                    return False
        return True