improve cutwidth.pyx

[dcoudert]

In this ticket, we:

• avoid recursive calls in method cutwidth
• avoid using .vertices() in all methods
• do some pep8 cleaning

[dcoudert]

New commits:

​303b896	avoid recursive calls in cutwidth
​6404223	avoid using .vertices() in cutwidth.pyx
​d57b5f4	pep8 cleaning in method width_of_cut_decomposition
​f5a479f	pep8 in method cutwidth
​656f8f3	pep8 in cutwidth_dyn
​f59a401	pep8 in exists
​b7abfc8	pep8 in cutwidth_MILP

[kdilks]

Everything tentatively looks good to me. Not sure why most recent patchbots are having build errors (builds and tests fine of this machine), will run patchbot on a different machine soon to double check.

[dcoudert]

See ​https://groups.google.com/forum/#!topic/sage-release/ULwr6YtMfnY for a discussion on the build errors with 8.7.beta0.

[tscrim]

Do we need this to explicitly be a list:

-    V = G.vertices()
+    V = list(G)
 
     # All vertices at different positions
     for v in V:

In particular, could we just do

-    V = G.vertices()
 
     # All vertices at different positions
-    for v in V:
+    for v in G:

[git]

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

​4d18612	trac #26829: Merged with 8.7.beta0
​3ca6bbe	trac #26829: avoid a creation of list of vertices

[dcoudert]

Perfectly right. It's better like that.

[tscrim]

Thank you. LGTM.

[dcoudert]

Thanks to both of you for the review !