#20499: Graphs: docstring of _add_ conflicts with function
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Reporter: jmantysalo | Owner:
Type: defect | Status: needs_info
Priority: major | Milestone: sage-7.2
Component: graph theory | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by mcognetta):
Replying to [comment:2 dcoudert]:
> The current implementation of the `add` method is
> {{{
> if isinstance(other_graph, GenericGraph):
> return self.disjoint_union(other_graph, labels='integers')
> }}}
> So it forces to relabel vertices as integer in `[0..n-1]`.
>
> At the least, we should raise an error for cases such as
`Graph({0:[]})+'junk'`. Indeed, the other ordering raises an error
> {{{
> sage: 'junk'+Graph({0:[]})
>
---------------------------------------------------------------------------
> TypeError Traceback (most recent call
last)
> <ipython-input-99-2e66a308aaff> in <module>()
> ----> 1 'junk'+Graph({Integer(0):[]})
>
> TypeError: cannot concatenate 'str' and 'Graph' objects
> }}}
>
> Now, the semantic of `+` is the disjoint union, and I believe this is
the right choice.
> We should however ensure that the doc of `__add__`, `disjoint_union`,
`union`, `__mul__` and may be `join`, are clear enough and without
ambiguity for users.
>
> Concerning the relabel to integers part, I agree that this is often
painful. We could propose and intermediate behavior like: relabel only if
some vertices have same label. But again some people will complain.
>
> David.
I like the idea of only relabeling (the entire graph) if there are common
vertices. NetworkX automatically relabels everything to integers when
adding graphs and they get along fine without people complaining. We could
possibly think about doing something with recording the previously named
values when relabeling them. That way people could choose to invert the
labeling if it was really necessary. Our add function could pass in a
dictionary of the previous labels as an attribute in the newly created
graph.
In the end, I think that relabeling to integers is best. If people know
they need to preserve vertex names, they can take precautions beforehand.
--
Ticket URL: <http://trac.sagemath.org/ticket/20499#comment:7>
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