sudoku

def solve_sudoku(file_name):
    ''' Solves sudoku given in a .txt file'''

    list_of_empties = None

    #TODO: Open up file and get a Python readable grid
    sudoku_grid = get_grid_from_file(file_name)

    #TODO: Get length of sudoku
    column_length = len(sudoku_grid)
    row_length = len(sudoku_grid[1])

    while len(list_of_empties) > 0 or list_of_empties == None:
    #Returns a list of all the empty blanks and the possible valid entries in each blank


        #TODO: Check all the blanks for valid entries
        for r in range(row_length):
            for c in range(column_length):
                #Finds and checks the value in the r-th row and c-th column if it is empty.
                if subgrid_values(sudoku_grid,r,c) == 'x':
                    list_of_valid_entries = []

                    #Checks all the possible valid entries for that particular blank of the sudoku
                    for i in range (1,10):
                        if valid_entry(sudoku_grid,i,r,c):
                            list_of_valid_entries.append(i)

                    #Stores the infomation (the number of possible valid entries to the blank, the list of valid entries, the row and column of the empty) in a list of all the empty blanks.
                    list_of_empties.append([len(list_of_valid_entries),list_of_valid_entries,r,c])

        #TODO: Solve starting with the blanks with the least number of valid entries.
        least_valid_entry = None

        for empty in list_of_empties:
            #Finds the empty blanks with the least number of valid entries
            if least_valid_entry == None:
                least_valid_entry = empty
             elif empty[0] < least_valid_entry[0]:
                least_valid_entry = empty

        #If there is only one valid entry for that empty blank, replace the empty blank with the values
        if least_valid_entry[0] == 1:
            sudoku_grid[least_valid_entry[3]][least_valid_entry[2]] = least_valid_entry[1][0]

        #Repeats the process till there is no more empty blanks left