python - 从 NetworkX MultiDiGraph 中删除自循环会导致运行时错误

Question

我有一个包含自环的 NetworkX MultiDiGraph。根据documentation ，这是 MultiDiGraph 的有效属性。

A MultiDiGraph holds directed edges. Self loops are allowed.

但是当我尝试使用 MG.remove_edges_from(MG.selfloop_edges()) 从 MultiDiGraph 中删除自循环时，生成了以下警告:

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-13-ff3391f2296f> in <module>()
      1 # remove selfloop edges from the graph
----> 2 MG.remove_edges_from(MG.selfloop_edges())

~/Program_Files/miniconda3/envs/py36/lib/python3.6/site-packages/networkx/classes/multigraph.py in remove_edges_from(self, ebunch)
    603         []
    604         """
--> 605         for e in ebunch:
    606             try:
    607                 self.remove_edge(*e[:3])

~/Program_Files/miniconda3/envs/py36/lib/python3.6/site-packages/networkx/classes/function.py in <genexpr>(.0)
   1154                 return ((n, n)
   1155                         for n, nbrs in G.adj.items()
-> 1156                         if n in nbrs for d in nbrs[n].values())
   1157         else:
   1158             return ((n, n) for n, nbrs in G.adj.items() if n in nbrs)

~/Program_Files/miniconda3/envs/py36/lib/python3.6/_collections_abc.py in __iter__(self)
    759 
    760     def __iter__(self):
--> 761         for key in self._mapping:
    762             yield self._mapping[key]
    763 

RuntimeError: dictionary changed size during iteration

在您从 MultiDiGraph 中删除自循环的方式中我是否遗漏了什么或者这是 NetworkX 的错误？

演示意外错误的可重现示例:

import networkx as nx

# create an empty MultiDiGraph
MG = nx.MultiDiGraph()

# add some edges to the graph
MG.add_edges_from([(1, 2), (2, 3), (3, 1), (1, 2), (2, 1), (2, 2)])

# check the edges in the graph
MG.edges()

# remove selfloop edges from the graph
MG.remove_edges_from(MG.selfloop_edges())

此方法与有向图的预期效果一致，如下所示:

# create an empty MultiDiGraph
G = nx.DiGraph()

# add some edges to the graph
G.add_edges_from([(1, 2), (2, 3), (3, 1), (1, 2), (2, 1), (2, 2)])

# check the edges in the graph
G.edges
OutEdgeView([(1, 2), (2, 3), (2, 1), (2, 2), (3, 1)])

# remove selfloop edges from the graph
G.remove_edges_from(G.selfloop_edges())

# check the edges in the graph
G.edges()
OutEdgeView([(1, 2), (2, 3), (2, 1), (3, 1)])

Answer 1

问题在于 MG.selfloop_edges() 是图中自循环边上的迭代器(更准确地说是生成器)，通过移除边，您在迭代期间更改了迭代器的边。

根据documentation :

Parameters: ebunch (list or container of edge tuples) - ...

这意味着 ebunch 参数应该是一个容器，而 MG.selfloop_edges() 返回一个生成器。您可以阅读更多关于两者之间区别的信息 here .

可以通过将 list(MG.selfloop_edges()) 传递给 MG.remove_edges_from 来解决这个问题(而不是传递 MG.selfloop_edges() 直接)。