Python sudoku backtracking

Question

I was trying out the backtracking algorithm with an easy example (sudoku). I first tried another approach where more possibilities are canceled, but after I got the same error I switched to an easier solution.

1. look for the first unsolved spot
2. fill in every number between 1 and 9 and backtrack the new field if it is valid

When I run it and output the non-valid fields I can see that when the algorithm goes out of a recursion call the spot that was in that recursion call is still a 9 (so the algorithm couldn't find anything for that spot)

e.g. the first two lines look something like this (it's trying to solve an empty field):

[1, 2, 3, 4, 6, 9, 9, 9, 9]

[9, 9, 9, 9, 9, 9, 9, 0, 0]

I thought it was a reference error and inserted [e for e in field] in the backtracking call so that the old field doesn't get altered although that didn't seem to help.

Here is my code:

    for i in range(9):
        a = [field[i][j] for j in range(9) if field[i][j] != 0]
        if len(a) != len(set(a)):
            return False

    for i in range(9):
        a = [field[j][i] for j in range(9) if field[j][i] != 0]
        if len(a) != len(set(a)):
            return False

    for x in range(3):
        for y in range(3):
            a = []
            for addX in range(3):
                for addY in range(3):
                    spot = field[x * 3 + addX][y * 3 + addY]
                    if spot != 0:
                        a.append(spot)
            if len(a) != len(set(a)):
                return False

    return True

def findEmpty(field):

    for i in range(9):
        for j in range(9):
            if field[i][j] == 0:
                return i, j


def backtracking(field):

    find = findEmpty(field)
    if not find:
        return True, field
    else:
        x, y = find
 
    for i in range(1, 10):
        print(f"Trying {i} at {x} {y}")
        field[x][y] = i
        if isValid(field):
            s = backtracking([e for e in field])
            if s[0]:
                return s
        else:
            print("Not valid")
            for row in field:
                print(row)

    return False, None


field = [[0, 0, 0, 0, 1, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0],
         [0, 0, 0, 0, 0, 0, 0, 0, 0]]

solution = backtracking(field)
if solution[0]:
    print("There was a solution. The field is:")
    for row in solution[1]:
        print(row)
else:
    print("No solution was found")

Answer 1

I did some research and apparently it really is a reference error. For me importing pythons copy library and assigning each new field saying f = copy.deepcopy(field) fixes the issue (this also works for the complex example).