Python键错误-字典中的键：字典[键]

Question

我正在编写一个 Graph 类来处理游戏中的路径。图有一个字段，映射，它是一个将节点映射到边的字典。在我的一个函数中，我有以下代码：

for key in self.mapping:
    connected_edges = self.mapping[key]

在我的程序中，我调用了这段代码，但出现以下错误：

KeyError: <node.Node instance at 0x00000000026F3088>

这怎么可能？我只是遍历字典中的键，那么键怎么可能不在其中呢？为了进行完整性检查，我查看了字典的长度，发现它是 5，即：不是 0。有什么想法吗？谢谢！

我确实为 Node 和 Edge 实现了自己的 eq 和 hash 方法。以下是这些类的部分：

class Node:
    """
    A node connects to other nodes via Edges to form a graph

    Each node has a unique identifier (ID) so they can be mathematically distinct. Optionally, a node also has an x and
    y coordinate.
    """
    n_nodes = 0
    def __init__(self, x=0, y=0):
        """
        Create a node with its own unique identifier.

        :param x: x coordinate
        :param y: y coordinate
        :return: an initialized Node object
        """
        Node.n_nodes += 1
        self.ID = Node.n_nodes
        self.x = x  # x position
        self.y = y  # y position

    def __eq__(self, other):
        return (self.ID, self.x, self.y) == (other.ID, other.x, other.y)

    def __hash__(self):
        return hash((self.ID, self.x, self.y))



class Edge:
    """
    An edge is a connection between two nodes.

    Edges have a unique identifier and a list of two nodes. Optionally, an edge can have a numerical weight. Edges can
    also be directed, in which case it is only possible to traverse the edge in one direction, not both.
    """
    n_edges = 0
    def __init__(self, node1, node2, weight=1, directed=False):
        """
        Create an edge with its own unique identifier

        :param node1: node at first end of the edge
        :param node2: node at second end of the edge
        :param weight: numerical weight of the connection between node1 and node2
        :param directed: if True, the edge is oriented from node1 to node 2
        :return: an initialized Edge object
        """
        Edge.n_edges += 1
        self.ID = Edge.n_edges
        self.weight = weight
        self.nodes = frozenset([node1, node2])
        self.directed = directed

    def __eq__(self, other):
        return (self.ID, self.weight, self.nodes, self.directed) == (other.ID, other.weight, other.nodes, other.directed)

    def __hash__(self):
        return hash((self.ID, self.weight, self.nodes, self.directed))

Answer 1

如果您的对象实现了__eq__ 和__hash__，则必须确保在创建对象后哈希值不会改变。如果您在创建对象后对其进行变异，则可能会使它们进入不一致的状态，其中为它们存储在字典中的哈希值与其当前的哈希值不一致。这是一个例子：

class Foo(object):
    def __init__(self, x):
        self.x = x
    def __eq__(self, other):
        return self.x == other.x
    def __hash__(self):
        return self.x

a, b, c = Foo(1), Foo(2), Foo(3)

x = {a: 1, b: 2, c: 3}

for key in x:
    print(x[key])

a.x = 100

for key in x:
    print(x[key])

结果是与您收到的类似的 KeyError。大概你正在沿线某处改变你的 Node 对象。

可变对象不能被散列（或者至少，它们的散列值不能依赖于它们的任何可变状态）。