I assigned graph, vertex, and edge attributes to a graph and saved it to GraphML. When I read it in again, I get this message:
/usr/lib/python2.5/site-packages/python_igraph-0.6-py2.5-linux-x86_64.egg/igraph/__init__.py:1106: RuntimeWarning: Could not add vertex ids, there is already an 'id' vertex attribute at foreign-graphml.c:370
The GraphML file gets the 'id' data element for each node: