Question

对于我的生活，我不能弄清楚为什么我的代码会抛出一个KeyError。我觉得值应该在那里 - 我在第一个for loop中添加了它们，而我添加它们的列表不是空的（我用print语句检查过）。那么为什么第54行会不停地抛出一个KeyError？我确信我只是忽略了一些东西，但是在整天工作之后，我很困惑。

使用的函数（graph（）和shortest_path（）等）是here。

edgeData具有以下结构：

{string value: [list of string values that are groupings of the values in the dictionary's keys]}

tagUsers具有以下结构：

{string value: [grouped lists of string values found in edgeData]}

提前致谢。

from collections import defaultdict, deque
import json

class Graph(object):
    def __init__(self):
        self.nodes = set()
        self.edges = defaultdict(list)
        self.distances = {}

    def add_node(self, value):
        self.nodes.add(value)

    def add_edge(self, from_node, to_node, distance):
        self.edges[from_node].append(to_node)
        self.edges[to_node].append(from_node)
        self.distances[(from_node, to_node)] = distance


def dijkstra(graph, initial):
    visited = {initial: 0}
    path = {}

    nodes = set(graph.nodes)

    while nodes:
        min_node = None
        for node in nodes:
            if node in visited:
                if min_node is None:
                    min_node = node
                elif visited[node] < visited[min_node]:
                    min_node = node
        if min_node is None:
            break

        nodes.remove(min_node)
        current_weight = visited[min_node]

        for edge in graph.edges[min_node]:
            try:
                weight = current_weight + graph.distances[(min_node, edge)]
            except:
                continue
            if edge not in visited or weight < visited[edge]:
                visited[edge] = weight
                path[edge] = min_node

    return visited, path


def shortest_path(graph, origin, destination):
    visited, paths = dijkstra(graph, origin)
    full_path = deque()
    _destination = paths[destination]

    while _destination != origin:
        full_path.appendleft(_destination)
        _destination = paths[_destination]

    full_path.appendleft(origin)
    full_path.append(destination)

    return visited[destination]
if __name__ == '__main__':
    edgeData = {'a': ['c', 'd'], 'b': ['d'], 'c': ['d'], 'd': ['a', 'b']}
    tagUsers = {'hashtag1': ['a', 'c', 'd'], 'hashtag2': ['b'], 'hashtag3': ['b', 'd']}
    shortestpaths = {}

    graph = Graph()

    users = []


    # calls function, builds graph with data in edgeData
    for key, value in edgeData.items():
        users.append(key)
        graph.add_node(key)

        for each in value:
            graph.add_edge(key, each, 1)

    # determines how many users used each hashtag
    hashtags = {}
    for key, value in tagUsers.items():
        tmpTags = key
        count = len(value)
        hashtags[key] = count

    # normally determines which hashtag was used the most
    # Here, it's pre-set
    topTag = ['hashtag1']

    # calculates the shortest path from each user to another user
    # that uses the most-used hashtag
    count = 0
    if count < 1:
        for key, value in edgeData.items():
            tmpDict = {}
            for tag in topTag:
                shortest = 10000
                for k, v in tagUsers.items():
                    if k == tag:
                        for each in v:
                            flag = False
                            if key != each
                                flag = True
                                tmpShort = shortest_path(graph, key, each)
                                if tmpShort < shortest:
                                    shortest = tmpShort
                if flag:
                    tmpDict[tag] = shortest
            shortestpaths[key] = tmpDict
            count += 1

目标是shortestpaths中的数据为每个用户包含使用顶部主题标签的另一个用户的最短距离

函数调用引用此代码，由github上的mdsrosa提供。

具体来说，错误会在shortest_path()的`_destination = paths [_destination]

中抛出

Answer 1

向shortest_path添加一些日志记录会显示问题：

def shortest_path(graph, origin, destination):
    print 'shortest_path     Origin:%s  Destination:%s' % (origin, destination)
    visited, paths = dijkstra(graph, origin)
    full_path = deque()
    print 'paths: %s' % paths
    _destination = paths[destination]

结果：

shortest_path     Origin:a  Destination:a
paths: {'c': 'a', 'b': 'd', 'd': 'a'}
Traceback (most recent call last):
  File "e.py", line 43, in <module>
    tmpShort = dj.shortest_path(graph, key, each)
  File "E:\kiribati\dijkstra.py", line 61, in shortest_path
    _destination = paths[destination]
KeyError: 'a'

您需要处理原点和目的地相同的边缘情况

一种选择是在致电if key == each

之前添加支票shortest_path

    for k, v in tagUsers.items():
        if k == tag:
            for each in v:
                if key == each:
                    continue
                tmpShort = dj.shortest_path(graph, key, each)
                if tmpShort < shortest:
                    shortest = tmpShort
    tmpDict[tag] = shortest

同时将您的循环变量从k，v，key，value，each更改为描述实际数据的内容

为什么我的代码不断抛出KeyError？

1 个答案: