Re: [Boost-users] [BGL] python bindings build graph from edge list

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2006-06-02 10:55:06

Hi John,

On Jun 1, 2006, at 4:02 AM, John Reid wrote:
> I was trying to build a graph from an edge list and ran into some
> problems. The following code throws an exception on write_graphviz 
> (). I
> think it is because there's no property map called node_id in the  
> graph.
>
> import boost.graph as bgl
> edges = [ ( 'a', 'b' ), ( 'b', 'c' ), ( 'c', 'a' ) ]
> g = bgl.Graph( edges )
> g.write_graphviz( 'bind-copy.dot' )
>
> I get the following:
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
> KeyError: 'node_id'
>
>
> I couldn't manage to generate the node_id property map  
> automatically. So
> after you have built the graph with the edge list constructor how  
> do you
> know which vertex is which?

Your code above should work properly, but doesn't due to several bugs  
in the BGL-Python bindings. I have now fixed these bugs in  
Subversion. In particular:

- There is now a name_map parameter for the edge list constructor,  
which defaults to "node_id". If name_map is not the empty string, the  
constructor creates a vertex property map g.vertex_properties 
[name_map] that gives the name of each vertex, e.g., 'a', 'b', or  
'c', in your example.

- If write_graphviz cannot find the node_id map, it will just use the  
vertex indices as identifiers in the GraphViz output file.

Thanksf or reporting these problems!

        Doug