title: Assigning networkx edge attributes/weights to a dict
body: <p>I run this program:</p>
  
  <pre><code>import networkx as nx
  from pylab import *
  g=nx.Graph()
  
  g.add_edge('a','b', weight=3)
  
  g.add_edge('a','c', weight=2)
  
  g.add_edge('b','c', weight=7)
  
  w=nx.get_edge_attributes(g, 'weight')
  
  w[('b','a')]=3
  
  w[('c','a')]=2
  
  w[('c','b')]=7
  </code></pre>
  
  <p>and I do not understand why the <code>w</code> dict contains only 5 instead of 6 items:</p>
  
  <pre><code>{('b', 'a'): 3, ('c', 'a'): 2, ('a', 'b'): 3, ('c', 'b'): 7, ('a', 'c'): 2}
  </code></pre>
  
  <p>the <code>('b','c')</code> entry is gone...</p>
  
  <p>What am I doing wrong here?</p>