python - 不可排序的类型 : Vertex() < Vertex()

Question

我在使用 dijkstra 时遇到了这个错误:TypeError:不可排序的类型:Vertex() < Vertex()

整个错误日志是:

Traceback (most recent call last):
  File "C:/Users/Dimitar/PycharmProjects/Dijkstra/Dijkstra.py", line 165, in <module>
    dijkstra(g, g.get_vertex('a'))
  File "C:/Users/Dimitar/PycharmProjects/Dijkstra/Dijkstra.py", line 101, in dijkstra
    heapq.heapify(unvisited_queue)
TypeError: unorderable types: Vertex() < Vertex()

这是我的代码:

import sys


class Vertex:
    def __init__(self, node):
        self.id = node
        self.adjacent = {}
        # Set distance to infinity for all nodes
        self.distance = sys.maxsize
        # Mark all nodes unvisited        
        self.visited = False
        # Predecessor
        self.previous = None

    def add_neighbor(self, neighbor, weight=0):
        self.adjacent[neighbor] = weight

    def get_connections(self):
        return self.adjacent.keys()

    def get_id(self):
        return self.id

    def get_weight(self, neighbor):
        return self.adjacent[neighbor]

    def set_distance(self, dist):
        self.distance = dist

    def get_distance(self):
        return self.distance

    def set_previous(self, prev):
        self.previous = prev

    def set_visited(self):
        self.visited = True

    def __str__(self):
        return str(self.id) + ' adjacent: ' + str([x.id for x in self.adjacent])


class Graph:
    def __init__(self):
        self.vert_dict = {}
        self.num_vertices = 0

    def __iter__(self):
        return iter(self.vert_dict.values())

    def add_vertex(self, node):
        self.num_vertices = self.num_vertices + 1
        new_vertex = Vertex(node)
        self.vert_dict[node] = new_vertex
        return new_vertex

    def get_vertex(self, n):
        if n in self.vert_dict:
            return self.vert_dict[n]
        else:
            return None

    def add_edge(self, frm, to, cost=0):
        if frm not in self.vert_dict:
            self.add_vertex(frm)
        if to not in self.vert_dict:
            self.add_vertex(to)

        self.vert_dict[frm].add_neighbor(self.vert_dict[to], cost)
        self.vert_dict[to].add_neighbor(self.vert_dict[frm], cost)

    def get_vertices(self):
        return self.vert_dict.keys()

    def set_previous(self, current):
        self.previous = current

    def get_previous(self, current):
        return self.previous


def shortest(v, path):
    ''' make shortest path from v.previous'''
    if v.previous:
        path.append(v.previous.get_id())
        shortest(v.previous, path)
    return


import heapq


# noinspection PyArgumentList
def dijkstra(aGraph, start):
    print('''Dijkstra's shortest path''')
    # Set the distance for the start node to zero 
    start.set_distance(0)

    # Put tuple pair into the priority queue
    unvisited_queue = [(v.get_distance(), v) for v in aGraph]
    heapq.heapify(unvisited_queue)

    while len(unvisited_queue):
        # Pops a vertex with the smallest distance 
        uv = heapq.heappop(unvisited_queue)
        current = uv[1]
        current.set_visited()

        for next in current.adjacent:
            # if visited, skip
            if next.visited:
                continue
            new_dist = current.get_distance() + current.get_weight(next)

            if new_dist < next.get_distance():
                next.set_distance(new_dist)
                next.set_previous(current)
                print
                ('updated : current = %s next = %s new_dist = %s' \
                % (current.get_id(), next.get_id(), next.get_distance()))
            else:
                print
                ('not updated : current = %s next = %s new_dist = %s' \
                % (current.get_id(), next.get_id(), next.get_distance()))

        # Rebuild heap
        # 1. Pop every item
        while len(unvisited_queue):
            heapq.heappop(unvisited_queue)
        # 2. Put all vertices not visited into the queue
        unvisited_queue = [(v.get_distance(), v) for v in aGraph if not v.visited]
        heapq.heapify(unvisited_queue)


if __name__ == '__main__':

    g = Graph()

    g.add_vertex('a')
    g.add_vertex('b')
    g.add_vertex('c')
    g.add_vertex('d')
    g.add_vertex('e')
    g.add_vertex('f')

    g.add_edge('a', 'b', 7)
    g.add_edge('a', 'c', 9)
    g.add_edge('a', 'f', 14)
    g.add_edge('b', 'c', 10)
    g.add_edge('b', 'd', 15)
    g.add_edge('c', 'd', 11)
    g.add_edge('c', 'f', 2)
    g.add_edge('d', 'e', 6)
    g.add_edge('e', 'f', 9)

    print
    ('Graph data:')
    for v in g:
        for w in v.get_connections():
            vid = v.get_id()
            wid = w.get_id()
            print
            ('( %s , %s, %3d)' % ( vid, wid, v.get_weight(w)))

    dijkstra(g, g.get_vertex('a'))

    target = g.get_vertex('e')
    path = [target.get_id()]
    shortest(target, path)
    print('The shortest path : %s' % (path[::-1]))

有人能解释一下为什么会出现这种错误吗？我是一个自学者，我将不胜感激。

导致错误的代码行是:heapq.heapify(unvisited_queue)

提前感谢所有对此主题发表评论的人。

最好的，迪米塔尔

Answer 1

自定义类型不会隐式定义其实例之间的顺序:

>>> v1 = Vertex(1)
>>> v2 = Vertex(2)
>>> v1 < v2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: Vertex() < Vertex()

你需要告诉 python 如何比较你的顶点。您需要实现 rich comparison这样做的方法。自 heapify需要 < ，你必须至少实现 __lt__ .

您还可以查看 @total_ordering 装饰器以避免实现所有这些。

类似的东西:

from functools import total_ordering

@total_ordering
class Vertex:
    # Your current class code

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self.distance == other.distance
        return NotImplemented

    def __lt__(self, other):
        if isinstance(other, self.__class__):
            return self.distance < other.distance
        return NotImplemented

如果您需要将该类用作字典键，则需要添加 __hash__方法。大概是这样的:

    def __hash__(self):
        return id(self)