在递归python函数中，如何到达调用自身的代码行之后的代码行

Question

this is my code that I copied from the sudoku solver video computerphile made:

grid = [ [4, 0, 0, 0, 0, 5, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 1, 9, 8],
[3, 0, 0, 0, 8, 2, 4, 0, 0],
[0, 0, 0, 1, 0, 0, 0, 8, 0], 
[9, 0, 3, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 3, 0, 6, 7, 0], 
[0, 5, 0, 0, 0, 9, 0, 0, 0], 
[0, 0, 0, 2, 0, 0, 9, 0, 7], 
[6, 4, 0, 3, 0, 0, 0, 0, 0], ]

def possible(x, y, n):
    for i in range(0, 9):
        if grid[i][x] == n and i != y:
            return False
    for i in range(0, 9):
        if grid[y][i] == n and i != x:
            return False            
    x0 = (x // 3) * 3
    y0 = (y // 3) * 3
    for X in range(x0, x0 + 3):
        for Y in range(y0, y0 + 3):
            if grid[Y][X] == n:
                return False    
    return True


def Print(matrix):
    for i in range(9):
        print(matrix[i])


def solve():
    global grid
    for y in range(9):
        for x in range(9):
            if grid[y][x] == 0:
                for n in range(1, 10):
                    if possible(x, y, n):
                        grid[y][x] = n
                        solve()
                        grid[y][x] = 0
                return
    Print(grid)
    input("")

solve()

I want to know how the grid[y][x] = 0 located inside the solve() function ever gets reached? Wouldn't Python go back to the beginning of the solve() function every time it reaches the line above, which is solve() ?

So how does the function solve() run?

Answer 1

somewhere in solving the puzzle the answer to possible(x, y, n) will be false for every instance of the for-loop, in this case the code jumps to return statement for solve method. in this time, the program runs the grid[y][x] = 0 part!

you can see more example of recursive functions here .