AccessSelections failing when the component is a sub-assembly

Dear All,

The problem is that I'm using pyhton to convert a VB macro topython. The document is a part file not an assembly. As theSolidWorks api document says, in AccessSelections ( topDoc,component ), topDoc is an instance of ModelDoc2 and component isset to NULL when document is a Part file. When I want to convertthis code to python, I replace 'Nothing' with 'None' for thecomponent argument but it raises error and does not accespt Nonevariable.
Can any body help me to resolve the error?

Thanks in advance,
ArtmanSolidworksApi macros