Stefano C. - 4 months ago 47

Python Question

**What i Have:** a graph G imported in networkx whit nodes and egdes loaded by gml file.

**Problem** : How to add a new attribute to a selected edge E.

**What i want to do:** i want to add a new attribute 'type' for a particular edge E of my graph. Attention: the attribute 'type' doesn't exist for this edge E.

I read a lot of solutions proposed in Internet and here, but no one of those solutions solves my problem.

In fact my code is:

`G.edge[id_source][id_target]['type']= value`

But if i print all the edges of G, now i have n+1 edges, all the old edges of G, and a new edge p= (id_source, id_target, {'type'= value}). Furthermore, the old edge E (the one that i want modify) doesn't have the new attribute 'type'.

So my code have added a new edge (that i don't want).

Thank you for your help !

Thanks to Aric and some tricks i solved my problem:

`def add_attribute_to_edge(H,id_node_source,id_node_target,new_attr,value_attr):`

keydict =H[id_node_source][id_node_target]

key=len(keydict)

for k in keydict:

if 'type' not in H.edge[id_source][id_target][k]:

H.add_edge(id_node_source,id_node_target,key=k, new_attr= value_attr)

Answer

You may have a networkx MultiGraph instead of a graph and in that case the attribute setting for edges is a little tricker. (You can get a multigraph by loading a graph with more than one edge between nodes). You may be corrupting the data structure by assigning the attribute
`G.edge[id_source][id_target]['type']= value`

when you need
`G.edge[id_source][id_target][key]['type']= value`

.

Here are examples of how it works differently for Graphs and MultiGraphs.

For the Graph case attributes work like this:

```
In [1]: import networkx as nx
In [2]: G = nx.Graph()
In [3]: G.add_edge(1,2,color='red')
In [4]: G.edges(data=True)
Out[4]: [(1, 2, {'color': 'red'})]
In [5]: G.add_edge(1,2,color='blue')
In [6]: G.edges(data=True)
Out[6]: [(1, 2, {'color': 'blue'})]
In [7]: G[1][2]
Out[7]: {'color': 'blue'}
In [8]: G[1][2]['color']='green'
In [9]: G.edges(data=True)
Out[9]: [(1, 2, {'color': 'green'})]
```

With MultiGraphs there is an additional level of keys to keep track of the parallel edges so it works a little differently. If you don't explicitly set a key MultiGraph.add_edge() will add a new edge with an internally chosen key (sequential integers).

```
In [1]: import networkx as nx
In [2]: G = nx.MultiGraph()
In [3]: G.add_edge(1,2,color='red')
In [4]: G.edges(data=True)
Out[4]: [(1, 2, {'color': 'red'})]
In [5]: G.add_edge(1,2,color='blue')
In [6]: G.edges(data=True)
Out[6]: [(1, 2, {'color': 'red'}), (1, 2, {'color': 'blue'})]
In [7]: G.edges(data=True,keys=True)
Out[7]: [(1, 2, 0, {'color': 'red'}), (1, 2, 1, {'color': 'blue'})]
In [8]: G.add_edge(1,2,key=0,color='blue')
In [9]: G.edges(data=True,keys=True)
Out[9]: [(1, 2, 0, {'color': 'blue'}), (1, 2, 1, {'color': 'blue'})]
In [10]: G[1][2]
Out[10]: {0: {'color': 'blue'}, 1: {'color': 'blue'}}
In [11]: G[1][2][0]['color']='green'
In [12]: G.edges(data=True,keys=True)
Out[12]: [(1, 2, 0, {'color': 'green'}), (1, 2, 1, {'color': 'blue'})]
```