Python igraph cant find edges in an undirected graph

Question

I have this simple code in Python 3 using igraph to return an edge which I have added to an undirected graph.

from igraph import *
g = Graph()
g.add_vertices(3)
g.add_edges([(0,1), (1,2)])
# display(plot(g))
edge=g.es.find(_source=1, _target=0)

But when I use the function graph.es.find(), it gives me an error instead of returning the edge. It says that there is "no such edge".

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-4182287b1986> in <module>
      4 g.add_edges([(0,1), (1,2)])
      5 # display(plot(g))
----> 6 edge=g.es.find(_source=1, _target=0)

E:\Softwares\Anaconda\lib\site-packages\igraph\__init__.py in find(self, *args, **kwds)
   3615         if es:
   3616             return es[0]
-> 3617         raise ValueError("no such edge")
   3618 
   3619     def select(self, *args, **kwds):

ValueError: no such edge

However it does return the edge if I set the source and destination in the same order as I entered them in. In this case, source = 0, target = 1
My guess is that it is not really an undirected graph.

And my question is, how do I get a truly undirected graph which returns the edge even if I switch the source and target nodes in the g.es.find() function, as it should be for an undirected graph?

seralouk seralouk · Accepted Answer · 2019-11-22T19:23:28

Use this to get the edge:

from igraph import *
g = Graph()
g.add_vertices(3)
g.add_edges([(0,1), (1,2)])

#get the ID
g.get_eid(0,1)
0

#get the edge list
g.get_edgelist()
[(0, 1), (1, 2)]

#get the first edge from the edge list
g.get_edgelist()[0]
(0, 1)

Finally, to verify that it is not directed use g.is_directed()

Python igraph cant find edges in an undirected graph

1 Answers