Cannot add attributes to existing edges

[tlarock]

I want to add attributes to edges after I have created a hypergraph with H = xgi.Hypergraph(incoming_data=hyperedge_list) and I do not think it is possible right now (please correct if I am wrong).

Note that this issue is only relevant when the hypergraph is constructed first, then edge attributes added later. I could instead create an empty hypergraph and add edges one at a time with the relevant attributes (this is what I will do for now). In fact this would save me looping over the edges a second time after construction. However, in my particular use case I would prefer the semantics I describe because I think (1) they are more clear/general and (ii) would allow me to add attributes without constructing a new hypergraph. But the functionality is not sctrictly necessary.

It is possible to add or update attributes of existing nodes with H.add_node(existing_node, **attr), since per the docs

If node is already in the hypergraph, its attributes are still updated.

However, the same does not work for edges, because H.add_edge(existing_edge_members, **attr) creates a multi-edge, and H.add_edge(edge_id, **attr) fails because H.add_edge expects an iterable representing a hyperedge as its first argument, not an edge ID.

The simplest fix I can think of is to allow H.add_edge to accept an edge ID, check if the edge exists and if so modify provided attributes, or raise an error if the edge ID is not present. However, I think that overloads add_edge in a confusing way (already the case with add_node), so it may be better to define new functions for attribute addition/updates, i.e.,

H.update_node_attributes(node, **attr) and H.update_edge_attributes(edge_id, **attr),

with the edge function accepting an edge ID rather than the hyperedge itself. These functions might return an error if the node/edge does not exist and/or a warning if the attribute does not already exist for the node/edge, both of which could be managed with boolean arguments to the functions. This setup pushes the multi-edge issue onto the user, since whether multi-edges should have the same attributes is generally ambiguous.

I think this is related to discussions in #126 and #175.

[leotrs]

I believe you can do this with H.add_edges_from to add all attributes to all existing edges using "format 4" here.

I agree this is far from optimal though and I kinda hate that right now we have to use add_node to add attributes for existing nodes, and yes, for edges it's even worse :(.

[nwlandry]

Would xgi.set_edge_attributes work for what you're trying to do?
https://github.com/ComplexGroupInteractions/xgi/blob/57af2b43b8e2da9fd970e764f3ba27b9a61cd46c/xgi/classes/function.py#L505

[tlarock]

Yes @nwlandry I think that is very close to what I want, only difference is I was looking for something that operated on a single edge at a time rather than a dictionary of edges, but that is fine.

Perhaps this should be added to "See Also" for xgi.Hypergraph.add_edge etc.? I was looking through the doucmentation for Hypergraph and never saw this function, looks like it only appears in the documentation of xgi.Hypergraph.merge_duplicate_edges right now.

[nwlandry]

These are great suggestions. Let's keep this issue open until I implement your suggestions.

[nwlandry]

I added See Also links to the docs and we moved the add/remove attribute functionality to the Hypergraph class.