为什么我会收到此密钥错误？ （Python）

Question

I am trying to run my Dijkstra function twice so that I can figure out the shortest distance between 3 points. It works fine when I run it once, but two times returns a key error on line 38 and 44.

Code:

graph = {'c1': {'c2':4, 'L1':3}, 'c2':{'c1':4, 'c3':3, 'L1':2.5}, 'c3':{'c2':3, 'L1':2}, 'L1':{'c1':3, 'c2':2.5, 'c3':2}}

def dijkstra(graph, start, goal):
    shortest_distance = {}
    predecessor = {}
    unseenNodes = graph
    infinity = float('inf')
    path = []

    for node in unseenNodes:
        shortest_distance[node] = infinity
    shortest_distance[start] = 0
    #print(shortest_distance)

    while unseenNodes:
        minNode = None
        for node in unseenNodes:
            if minNode is None:
                minNode = node
            elif shortest_distance[node] < shortest_distance[minNode]:
                minNode = node
        for childNode, weight in graph[minNode].items():
            if weight + shortest_distance[minNode] < shortest_distance[childNode]:
                shortest_distance[childNode] = weight + shortest_distance[minNode]
                predecessor[childNode] = minNode
        unseenNodes.pop(minNode)

    currentNode = goal
    while currentNode != start:
        try:
            path.insert(0, currentNode)
            currentNode = predecessor[currentNode]
        except KeyError:
            print('Path not reachable')
            break
    path.insert(0, start)

    if shortest_distance[goal] != infinity:
        print('Shortest distance: ' + str(shortest_distance[goal]))
        print('Path:' + str(path))

dijkstra(graph, 'L1', 'c2')
dijkstra(graph, 'c2', 'c3')

Here is the error I get:

Path not reachable
    dijkstra(graph, 'c2', 'c3')
  File "E:\Work\Delivery_API\Delivery.py", line 38, in dijkstra
    if shortest_distance[goal] != infinity:
KeyError: 'c3'

Answer 1

When you do unseenNodes = graph you bind the variable unseenNodes to the same dict referenced by graph .

That means that any changes done on unseenNodes will reflect also on graph .

So, in the first dijkstra(graph, 'L1', 'c2') all goes good and well.

BUT , during that run you executed some unseenNodes.pop(minNode) which again, are equivalent to doing graph.pop(minNode) . So by the time you do dijkstra(graph, 'c2', 'c3') your graph has changed and doesn't look like you think it does. This is why it works when running only once.

The easy remedy is to work on a copy of graph to make sure it stays intact. All you need to do is change the assignment to: unseenNodes = dict(graph) . This creates a copy of graph ( unseenNodes now references another separate dict, and not the one referenced by graph ).