Learn Algorithm Design by Building a Shortest Path Algorithm - Step 47

Please help. I have checked the article on how to copy a list using slice syntax. The example given uses my_ list and my_duplicate_list. Okay let me try duplicate_paths[node] Done so but the code does not pass

Your code so far

my_graph = {
    'A': [('B', 3), ('D', 1)],
    'B': [('A', 3), ('C', 4)],
    'C': [('B', 4), ('D', 7)],
    'D': [('A', 1), ('C', 7)]

def shortest_path(graph, start):
    unvisited = list(graph)
    distances = {node: 0 if node == start else float('inf') for node in graph}
    paths = {node: [] for node in graph}
    while unvisited:
        current = min(unvisited, key=distances.get)
        for node, distance in graph[current]:
            if distance + distances[current] < distances[node]:
                distances[node] = distance + distances[current]

/* User Editable Region */

                if paths[node] and paths[node][-1] == node:
                    paths[node] = paths[current]
                    duplicate_paths[node] = paths[current][:]

/* User Editable Region */

    print(f'Unvisited: {unvisited}\nDistances: {distances}\nPaths: {paths}')
shortest_path(my_graph, 'A')

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Learn Algorithm Design by Building a Shortest Path Algorithm - Step 47

Hey Zack, you don’t need to add other lines of code. Here you can use the slice syntax as you did on paths[current] without saving it into another variable.

1 Like

Hello Zack,

You are very close, but instead of making a new variable, it wants you to assign the copy to paths[node]. So if you use the logic on line 2 but give it to paths[node] in line 1 instead you should be good.

We do this to fix the bug that the question mentions:

“This means both variables point to the same list. Since lists are mutable, when you append the neighbor node to its path, both paths[node] and paths[current] are modified because they are the same list.”


@Dario_DC , @HungryBee , thank you for your patience and your suggestions. I really appreciate it.


just add [:] to your previous line
so the code will be
---------solution removed

Hi and welcome to the forum.

This was already solved a month ago, and please don’t post solution code. Thank you!