title: Select network nodes with a given attribute value
body: <p>I'd like to select and perform operations on nodes within a graph with particular attributes. <strong>How would you select nodes with a given attribute value?</strong> For example:</p>
  
  <pre><code>P=nx.Graph()
  P.add_node('node1',at=5)
  P.add_node('node2',at=5)
  P.add_node('node3',at=6)
  </code></pre>
  
  <p>Is there a way to select only the nodes with  at == 5?.</p>
  
  <p>I'm imagining something like (this doesn't work):</p>
  
  <pre><code>for p in P.nodes():
      P.node[p]['at'==5]
  </code></pre>