#15706: Graph built from their edges are simple by default
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Reporter: | Owner:
ncohen | Status: needs_review
Type: | Milestone: sage-6.4
enhancement | Resolution:
Priority: major | Merged in:
Component: graph | Reviewers:
theory | Work issues:
Keywords: | Commit:
Authors: | f6087ffcf2370bdfe45a1162c7d7106c9829ad5c
Nathann Cohen | Stopgaps:
Report Upstream: N/A |
Branch: |
u/ncohen/15706 |
Dependencies: |
#15704 |
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Comment (by ncohen):
Yo !
> Well, like I said, I'm not sure I want to change that behavior or not.
It seems like the kind of thing that should be discussed at some length.
Well... I would be glad to discuss it at length with anybody but this
ticket has been here for 10 months already....
> I do agree there should be a flag to avoid the runtime cost, that seems
clear - but again, I am agnostic on the ''default''.
Well... Again, all graph books usually begin by saying that all graphs
considered are simple, i.e. no loops nor multiple edges. There is such a
thing as "multigraph", too. And I really don't like this behaviour:
{{{
sage: G=Graph([(1,2),(1,2),(1,1)]); G.edges()
[(1, 1, None), (1, 2, None), (1, 2, None)]
sage: G=Graph(); G.add_edge(1,2); G.add_edge(1,2); G.add_edge(1,1);
G.edges()
[(1, 2, None)]
}}}
Somehow the current behaviour of `add_edge` depends on non-explicit
information, and considers simple graphs as the default. I would feel
better if everybody got used to state clearly what they want to work with
and dealt with the consequences. A guy who wants to build multigraphs and
initializes them from a list of edges will experience seriously
inconsistent behaviour depending on whether the initial information
contains multiple edges and/or loops !
> As a constructive comment, maybe one ticket could be to have a flag (how
that is named, I don't know) and then this one for making defaults,
assuming there is something approaching consensus.
Hmmmmm.... Well, what do you expect consensus to look like when it seems
after 10 months that only one person cares ? `:-P`
Nathann
--
Ticket URL: <http://trac.sagemath.org/ticket/15706#comment:19>
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