parse multiple digraph in one dot file

Question

I'm trying to process and render some graph in DOT format. The dot file I have is large (~300MB), and it contains multiple digraphs

digraph 1 {...}
digraph 2 {...}
digraph 3 {...}

I have got 2 questions:
1. Is it possible to use render only 1 digraph instead of the whole graph?
Something like dot -3 -Tps mygraph.dot -o out.ps to render digraph 3 only?

2.What's the best Python library to process dot format?(other languages are also acceptable)
Here are two I tried, but not good enough
pydot It gives me a digraph list after importing, which is good, but it doesn't handle "." in the node name. For example nd.nd [label="nd_node"] will fail

pygraphviz It does handle ".", but only imports digraph 1 when given multiple graph definition in a file :(

Answer 1

Since you have dot, you should also have gvpr, a graph-processing tool. You can print the third graph with a simple gvpr script, like so:

BEGIN { int count = 0; }
BEG_G {
    count = count + 1;
    if(count == 3) {
        write($G);
    }
}

Then, you can use this as a source filter:

cat mygraph.dot | gvpr -f thirdgraph.gv | dot -Tps -o out.ps