title: igraph set edge width by EID python
body: <p>This is a silly question, but I just can't see how to do it in the igraph documentation. It will be very simple I imagine, but my python isn't good enough to work it out.</p>
  
  <p>using igraph, I am finding the edge I want to play with using select. It returns a reference to the edge object. When I try to change the edge_width attribute it isn't being updated in the plot.</p>
  
  <p>My code example is looking for the edge between vertices A and B.</p>
  
  <pre><code> source = g.vs.find(name = 'A')
   sink   = g.vs.find(name = 'B')
   edge   = g.es.select(_source = source, _target= sink)
   edge["edge_width"] = 20
  </code></pre>
  
  <p>But when I plot the graph, all edges are the same. What am I doing wrong?</p>
  
  <p>EDIT: To make life easier, here is a full code example that generates the issue. It simply creates a graph with 5 nodes, A to E, fully connected to each other and plots it to screen. </p>
  
  <pre><code>import string
  import igraph as ig
  
  num_nodes = 5
  
  alpha_list = list(string.ascii_uppercase)
  alpha_list = alpha_list[:num_nodes]
  
  g = ig.Graph()
  g.add_vertices(alpha_list)
  
  for x in range (0, num_nodes + 1):
      for y in range (x, num_nodes):
          print "x: "+str(x)+", y: "+str(y)
          if (x != y):
              g.add_edge(x, y)
  
  g.vs["label"] = g.vs["name"]
  
  source = g.vs.find(name = 'A')
  sink   = g.vs.find(name = 'B')
  
  edge = g.es.select(_source = source, _target= sink)
  
  edge["edge_width"] = 20
  
  print edge.attributes()
  
  layout = g.layout("circle")
  ig.plot(g, layout = layout)
  </code></pre>