Comment 3 for bug 1012434

GraphML has no "list" data type and the default attribute extension of the GraphML specification supports numeric and string types (and maybe boolean). According to the GraphML primer, the preferred way to extend GraphML with custom types is to redefine parts of the XML schema definition to describe the custom types one would like to add. See http://graphml.graphdrawing.org/primer/graphml-primer.html, Chapter 4.2.

Also, I believe that GML lists are more like "ordered key-value pairs", which would correspond to Python dicts (and named lists in R?).

Regarding your initial concern (the relevance of writing complex attributes with a native serialization format): theoretically, nothing forces the higher level interface to use a _native_ serialization format. The higher level interface could allow the user to hook into the serialization process and specify custom serializers/deserializers for data types, falling back to the platform native serialization format if nothing else succeeds.