PropertyMap._get_data

Hi,

I have a code that accesses pmap.a for a PropertyMap pmap attached to
vertex-filtered GraphView quite frequently. The function
PropertyMap._get_data calls the GraphView constructor every time, simply to
construct a non-filtered graph view from which it obtains e.g. the number of
vertices for vertex properties.

Quite some time is spent in this constructor cumulatively, since it is
called millions of times from _get_data in my case. It seems to me that
there could be a simpler solution if the API is extended a bit.

See in graph.cc

Actually, all we might want to do, is to use SoftNumVertices despite working
on a vertex-filtered graph, right? So, we don't need to construct a
non-filtered graph. Is that correct? It could be kind of a
"GetUnfilteredNumberOfVertices".

BR, Dominik

I have fixed this now in git:

  https://git.skewed.de/count0/graph-tool/commit/a49b19f3f94cc71919a4b2e685b0cadda85b3a9c

Best,
Tiago