subgraph creates duplicate edges

[watson ix]

I cannot figure out how to create a subgraph off an existing graph without creating duplicate edges. For instance:

G = GV.AGraph(strict=False,directed=True)

G.add_edge('a','b')

G.add_edge('a','c')

G.add_edge('c','a')

G.to_string()

G.subgraph('a','b'),rank='same')

print G.to_string()

digraph  {
	{
		graph [rank=same];
		a -> b;
	}
	a -> b;
	a -> c;
	c -> a;
}

Which graphviz draws as: 

Whereas I intended it to wind up with something which looks like this:

This can be done if instead the output were:

digraph  {
	{
		graph [rank=same];
		a;b;
	}
	a -> b;
	a -> c;
	c -> a;
}

[Aric Hagberg]

Adding a subgraph that way makes a node-induced subgraph (i.e. adds the edges again).

You can do it this way to without adding the edges.

import pygraphviz as pgv                                                        

G = pgv.AGraph(strict=False,directed=True)                                      

G.add_edge('a','b')                                                             

G.add_edge('a','c')                                                             

G.add_edge('c','a')                                                             

G.to_string()                                                                   

S = G.subgraph(rank='same')                                                     

S.add_nodes_from(['a','b'])                                                     

print G.to_string() 

Aric

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