#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 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.
--
Ticket URL: <http://trac.sagemath.org/ticket/20499#comment:2>
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