Hello everybody! I hope you can help figure out the reason of the following problem.

This solution works in my local editor (PyCharm 2019.1.3, Win10, 64-bit) but not in the online editor:

from itertools import product


def valid_neighbours(max_length, start, path, grid_dim):
    row, col = start
    vertices = set()
    for i, j in product([-1, 0, 1], [-1, 0, 1]):
        if any([row+i<0, col+j<0]) or any([row+i>grid_dim, col+j>grid_dim]):
            continue
        if grid_dim - min([row+i, col+j]) + 1 <= max_length - len(path):
            vertices.add((row+i, col+j))
    return vertices


def dfs_g_key(grid, start, goal, max_length, path=None):
    if path == None:
        path = set([start])
    if start == goal:
        yield path
    vertices = valid_neighbours(max_length, start, path, len(grid)-1) - path
    for vertex in vertices:
            yield from dfs_g_key(grid, vertex, goal, max_length, path | set([vertex]))


def g_key(grid, N):
    if N == len(grid) ** 2:
        return sum([sum(x) for x in grid])

    max_sum = float("-inf")
    for path in dfs_g_key(grid, (0, 0), (len(grid)-1, len(grid)-1), N):
        max_sum = max(max_sum, sum([grid[i][j] for i, j in path]))
    return max_sum

Once I change it to this one, it works in both, local and online editors:

from itertools import product


def valid_neighbours(max_length, start, path, grid_dim):
    row, col = start
    vertices = set()
    for i, j in product([-1, 0, 1], [-1, 0, 1]):
        if any([row+i<0, col+j<0]) or any([row+i>grid_dim, col+j>grid_dim]):
            continue
        if grid_dim - min([row+i, col+j]) + 1 <= max_length - len(path):
            vertices.add((row+i, col+j))
    return vertices


def dfs_g_key(grid, start, goal, max_length, path=None):
    if path == None:
        path = set([start])
    if start == goal:
        yield path
    vertices = valid_neighbours(max_length, start, path, len(grid)-1) - path
    if not vertices:
        yield None
    for vertex in vertices:
        if vertex == goal:
            yield path | set([vertex])
        else:
            yield from dfs_g_key(grid, vertex, goal, max_length, path | set([vertex]))


def g_key(grid, N):
    if N == len(grid) ** 2:
        return sum([sum(x) for x in grid])

    max_sum = float("-inf")
    for path in dfs_g_key(grid, (0, 0), (len(grid)-1, len(grid)-1), N):
        if path is None:
            continue
        max_sum = max(max_sum, sum([grid[i][j] for i, j in path]))
    return max_sum

Thank you all in advance!