我有一个包含自循环的NetworkX MultiDiGraph。根据文档，这是MultiDiGraph的有效属性。

MultiDiGraph保存有向边。允许自循环。

但是，当我尝试使用从MultiDiGraph中删除自环时MG.remove_edges_from(MG.selfloop_edges())，会生成以下警告：

---------------------------------------------------------------------------
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

我是否从MultliDiGraph删除自环的过程中缺少某些东西，或者这是NetworkX的错误？

可重现的示例演示了意外错误：

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())

此方法可与DiGraph一起使用，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)])

解决方案

问题是MG.selfloop_edges()图形上的自环边缘上有一个迭代器（更确切地说是生成器），并且通过删除边缘，您可以在迭代过程中更改迭代器的边缘。

根据文档：

参数：ebunch（边缘元组的列表或容器）-...

这意味着该ebunch参数应该是一个容器，同时MG.selfloop_edges()返回一个生成器。您可以在此处详细了解两者之间的区别。

可以通过传递list(MG.selfloop_edges())给MG.remove_edges_from（而不是MG.selfloop_edges()直接传递）来解决此问题。