Unusual behavior in networkx node coloring

Question

I'm drawing to draw a simple undirected graph with networkx and I'm having trouble coloring the nodes according to a specific attribute. I created a dictionary, in which case the key references a list of nodes that contain that attribute. It looks like so: ``` { 1 : [1, 2, 3, 4], 3 : [9, 11, 10, 8], 2 : [7, 5, 6] } ``` I would like to render the graph so that each set of nodes is colored differently. The keys are used to access a specific color. I draw the graph like so: ``` colors = [(random.random(), random.random(), random.random()) for i in xrange(0, 3)] pos = nx.circular_layout(G) for k,v in node_list_dict.iteritems(): nc = colors[int(k)-1] nx.draw_networkx_nodes(G, pos, nodelist=v, node_color=nc) nx.draw_networkx_edges(G, pos) nx.draw_networkx_labels(G, pos) ``` This almost works, but results in an image like: So it appears that the first two sets render correctly, but the last one does not. Any idea as to why? I looked at the documentation but I don't see why it's failing. Also…

Accepted Answer

You seem to have discovered an interesting broadcasting problem.

The trigger for this issue is when `len(v) == len(nc)`, as according to the documentation:

```
node_color : color string, or array of floats
   Node color. Can be a single color format string (default='r'),
   or a  sequence of colors with the same length as nodelist.
   If numeric values are specified they will be mapped to
   colors using the cmap and vmin,vmax parameters.  See
   matplotlib.scatter for more details.
```

so when `len(v) != 3` `nc` is interpreted as RGB triple, when `len(v) == 3`, the floats in `nc` are mapped to RGB via the default color map (which is `jet`).

A possible work-around for this is

```
for k,v in node_list_dict.iteritems():
   nc = colors[int(k)-1]
   if len(v) == 3:
       nc = nc +  (1,)
   nx.draw_networkx_nodes(G, pos, nodelist=v, node_color=nc)
```

which turns `nc` into a RGBA tuple, which won't trigger the usage of the color map.

If you want to track this down, the main work of `draw_network_nodes` is done here by a call to `scatter`. There is a comment in the `scatter` function (here) that marks this as a known problem and states how the ambiguity is broken. If you feel really strongly about this, you can create an issue on github, but I don't think it will get traction, as this is an explicit design decision.

Unusual behavior in networkx node coloring

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