title: reading a GML file with networkx (python) without labels for the nodes
body: <p>I have a long GML file (Graph Modelling Language) that i am trying to read with Networkx in Python.
  In the GML file, nodes don't have label, like this:</p>
  
  <pre><code>graph [
    node [
      id 1
    ]
    node [
      id 2
    ]
    edge [
      source 2
      target 1
      ]
    ]
  </code></pre>
  
  <p>I get an error when reading the file:
  G = nx.read_gml('simple_graph.gml')</p>
  
  <pre><code>---------------------------------------------------------------------------
  NetworkXError                             Traceback (most recent call last)
  &lt;ipython-input-39-b1b319a08668&gt; in &lt;module&gt;()
  ----&gt; 1 G = nx.read_gml('simple_graph.gml')
  
  &lt;decorator-gen-319&gt; in read_gml(path, label, destringizer)
  
  /usr/lib/python2.7/dist-packages/networkx/utils/decorators.pyc in _open_file(func, *args, **kwargs)
      218         # Finally, we call the original function, making sure to close the fobj.
      219         try:
  --&gt; 220             result = func(*new_args, **kwargs)
      221         finally:
      222             if close_fobj:
  
  /usr/lib/python2.7/dist-packages/networkx/readwrite/gml.pyc in read_gml(path, label, destringizer)
      208             yield line
      209 
  --&gt; 210     G = parse_gml_lines(filter_lines(path), label, destringizer)
      211     return G
      212 
  
  /usr/lib/python2.7/dist-packages/networkx/readwrite/gml.pyc in parse_gml_lines(lines, label, destringizer)
      407             raise NetworkXError('node id %r is duplicated' % (id,))
      408         if label != 'id':
  --&gt; 409             label = pop_attr(node, 'node', 'label', i)
      410             if label in labels:
      411                 raise NetworkXError('node label %r is duplicated' % (label,))
  
  /usr/lib/python2.7/dist-packages/networkx/readwrite/gml.pyc in pop_attr(dct, type, attr, i)
      397         except KeyError:
      398             raise NetworkXError(
  --&gt; 399                 "%s #%d has no '%s' attribute" % (type, i, attr))
      400 
      401     nodes = graph.get('node', [])
  
  NetworkXError: node #0 has no 'label' attribute
  </code></pre>
  
  <p>I see that it complains because the nodes don't have labels. From the documentation of GML i thought that labels were not obligatory (maybe i'm wrong?). Would there be a way to read such a file without labels? Or should i change my gml file?
  Thank you for your help!</p>