I was working on the leetcode problem Count Sub Islands and encountered a very odd Python behavior.

I had what I believed to be a working solution but no matter what I did, it would not run. Exasperated, I ChatGPT’d the answer, however, the ChatGPT answer was logically the same as mine but passed all test cases.

Tinkering around with it, I realized that I was setting a flag using flag = flag and dfs() which would result in an error, but ChatGPT had replaced the line with flag &= dfs().

I am returning booleans so this seemed logically the same to me. Flipping the order and setting a variable to equal the result of dfs will also end up with a correct solution.

Not sure what is causing this, anyone have an idea?

Code

def countSubIslands(grid1, grid2):
    for i in range(len(grid2)):
        for j in range(len(grid2[0])):
            if grid2[i][j] == 1 and grid1[i][j] == 1:
                grid2[i][j] += 1


    def dfs(i, j):
        if i < 0 or i >= len(grid2):
            return True

        if j < 0 or j >= len(grid2[0]):
            return True

        if grid2[i][j] == 0:
            return True

        if grid2[i][j] == 1:
            return False

        grid2[i][j] = 0

        flag = True
        flag1 = True

        dirs = [(1,0), (-1, 0), (0, 1), (0, -1)]

        for d in dirs:
            x = d[0]
            y = d[1]

            # DOES NOT WORK
            flag = flag and dfs(i + x, y + j)

            # ALL THREE OF THESE WORK
            # a.
            # flag &= dfs(i + x, y + j)
            # b.
            # flag = dfs(i + x, y + j) and flag
            # c.
            # res = dfs(i + x, y + j)
            # flag1 = flag and res

        return flag

    counter = 0
    for i in range(len(grid2)):
        for j in range(len(grid2[0])):
            if grid2[i][j] == 2:
                flag = dfs(i, j)
                if flag:
                    counter += 1
                print('------------')

    return counter

grid1 = [[1,0,1,0,1],[1,1,1,1,1],[0,0,0,0,0],[1,1,1,1,1],[1,0,1,0,1]]
grid2 = [[0,0,0,0,0],[1,1,1,1,1],[0,1,0,1,0],[0,1,0,1,0],[1,0,0,0,1]]

countSubIslands(grid1, grid2)

Expected

The following four code snippets to evaluate the same:
a. flag = flag and dfs(i + x, y + j)
b. flag &= dfs(i + x, y + j)
c. flag = dfs(i + x, y + j) and flag
d.

res = dfs(i + x, y + j)
flag1 = flag and res

Result

a. does not evaluate correctly, however b, c, and d do.