Remove selfloops from NetworkX MultiDiGraph causes runtime error

Question

I have a NetworkX MultiDiGraph that contains selfloops. According to the documentation, this is a valid property of a MultiDiGraph.

A MultiDiGraph holds directed edges. Self loops are allowed.

But when I try to remove the selfloops from the MultiDiGraph using MG.remove_edges_from(MG.selfloop_edges()), the following warning in generated:

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

Am I missing something in the way you remove a selfloop from a MultliDiGraph or is this a bug with NetworkX?

Reproducible example demonstrating the unexpected error:

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())

This method works as expected with a DiGraph, as seen below:

# 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

The problem is that MG.selfloop_edges() is an iterator (more precisely generator) over the selfloop edges in the graph, and by removing the edges you are changing the iterator's edges during the iteration.

According to the documentation:

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

That means that the ebunch parameter should be a container, while MG.selfloop_edges() returns a generator. You can read more about the difference between the two here.

The issue can be solved by passing list(MG.selfloop_edges()) to MG.remove_edges_from (instead of passing MG.selfloop_edges() directly).