CODEWITHSUNDEEP
pythonlistrecursionbacktracking
atl15
atl15

Reputation: 3

Python list changing for no reason even with copy method

def return_solved_board(board):
    solution = board.copy()

Backtracking recursion loop starts

def solve(board): 
    for y in range(9):
        for x in range(9):
            if solution[y][x] == 0:
                for number in range(1,10):
                    if check_rules_at_point(x, y, number, solution):
                        solution[y][x] = number
                        solve(solution)
    #Where backtracking comes into play, if the solve method fails on the next cell than it resets current one.
                        solution[y][x] = 0
    #Breaks out of recursion to try a differant number in cell prior
                return
    #Stores a solved copy of the solution into global variable
    global returned_information
    returned_information = solution.copy()
    print(returned_information) #1st print statement

Checks if the board can be solved or not

if not check_for_valid_board(board):
    return "This board is not possible to solve."
else:
    solve(board)
    global returned_information
    print(returned_information) #2nd print statement
    return returned_information

print(return_solved_board(valid_sudoku_board1))

I am trying to write a program to solve sudoku puzzles. The puzzle is stored in a list of lists. When I run this program the first print statement returns the correct solved list of lists. However, the second return statement returns the original unsolved puzzle. Both of their id's are the same however I can't figure out why returned_information in the second print statement changes back as returned_information is only called once.

Upvotes: 0

Views: 165

Answers (1)

AKX
AKX

Reputation: 169032

You're probably bumping into dicts'/lists' copy() being a shallow copy; use copy.deepcopy(...) instead to do a deep copy.

Upvotes: 1

Related Questions