Backtracking algorithm for Sudoku in python

Question

I'm trying to use backtracking to make a Sudoku solver. When I work through the code by hand it works perfectly but when I run it I get empty cells in my return solution. I've spent such a ridiculous amount of time trying to fix it but I can't even figure out where exactly its going wrong. I'm fairly certain my function for checking cell solutions is correct. Here's the code:

import numpy as np

board = np.array([[0, 0, 0, 8, 4, 0, 0, 0, 3],
                  [0, 7, 5, 0, 0, 0, 0, 0, 0],
                  [0, 4, 3, 0, 0, 6, 0, 0, 0],
                  [0, 0, 7, 0, 0, 8, 4, 9, 0],
                  [0, 0, 0, 9, 3, 1, 0, 0, 0],
                  [0, 5, 2, 7, 0, 0, 8, 0, 0],
                  [0, 0, 0, 2, 0, 0, 3, 4, 0],
                  [0, 0, 0, 0, 0, 0, 6, 2, 0],
                  [2, 0, 0, 0, 7, 3, 0, 0, 0]])

#check if input is viable solution for a cell
def isSolution(row, col, n):
    #return 0 for false (not a possible solution)
    #return 1 for true (possible solution)
    a = 0
    for i in range(0,9):
        if(board[row,i] == n):
            a += 1
        if(board[i,col] == n):
            a += 1
    h = (1 - (2 * ((row % 3) != 0)))
    i = (1 - (2 * ((col % 3) != 0)))
    j = (2 - (row % 3)**2)
    k = (2 - (col % 3)**2)
    if(board[row + h, col + i] == n):
        a += 1
    elif(board[row + h, col + k] == n):
        a += 1
    elif(board[row + j, col + i] == n):
        a += 1
    elif(board[row + j, col + k] == n):
        a += 1
    if(a == 0):
        return 1
    else:
        return 0

def solve():
    for row in range(0, 9):
        for col in range(0,9):
            if(board[row,col] == 0):
                for n in range(1, 10):
                    if(isSolution(row, col, n) == 1):
                        board[row,col] = n
                        print(board)
                        solve()
                        board[row,col] = 0

                return


#main
solve()

Please help if you can I'm trying to get better at python and feel like I've hit a wall here

Answer 1

Expanding comment of Thierry Lathuille

def solve():
    for row in range(0, 9):
        for col in range(0,9):
            if(board[row,col] == 0):
                for n in range(1, 10):
                    if(isSolution(row, col, n) == 1):
                        board[row,col] = n
                        # print(board)
                        if (board != 0).all():
                            raise StopIteration
                        solve()
                        board[row,col] = 0

                return


#main
try:
    solve()
except StopIteration:
    print(board)