py3: fix doctests in vertex_separation.pyx

[dcoudert]

Several failing doctests were caused by the impossibility to sort of list of Set (already in Python2).

sage: from sage.graphs.graph_decompositions.vertex_separation import vertex_separation, linear_ordering_to_path_decomposition
sage: g = graphs.PathGraph(5)
sage: pw, L = vertex_separation(g, algorithm="BAB"); pw
1
sage: h = linear_ordering_to_path_decomposition(g, L)
sage: sorted(h)
[{0, 1}, {3, 4}, {2, 3}, {1, 2}]

Since path decomposition is a graph whose vertices are Set, we get different results in py2 and py3. => we mark doctests as # py2 and # py3.

Some doctests were due to a max(None, <number>). With a proper initialization of the cut_off variable, we don't have this issue anymore.

Another doctest is fixed in #27027.

[dcoudert]

The 10 failing doctests in py3 should be fixed with this ticket and #27027.

---

New commits:

​7fa6d9f

trac #27151: fix doctests in vertex_separation

[vklein]

Hi !

I don't think all the # py2 # py3 are necessary. For example for your two first modifications doing that :

sage: sorted(h.vertices(), key=str)
[{0, 1}, {1, 2}, {2, 3}, {3, 4}]
sage: sage: sorted(h.edges(labels=None), key=str)
[({0, 1}, {1, 2}), ({1, 2}, {2, 3}), ({2, 3}, {3, 4})]

give the same results in both py2 and py3.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​206f496

trac #27151: implement reviewer's comments

[dcoudert]

Very good idea. Thanks. I have changed 3 doctests. One pair of py2/py3 tags remains as I don't know how to avoid it.

[vklein]

Ok this looks fine.