(disjoint) Union of graphs with multiple edges

[nathanncohen]

Err... Right now, the disjoint union of two graphs with multiedges/loops is the disjoint union of those two graphs without loops nor multiple edges. That's clearly missing from the code.

Here is the new behaviour

Multiple edges and loops::                                                                                                                                                             
                                                                                                                                                                                               
            sage: g = Graph(multiedges=True, loops=True)                                                                                                                                       
            sage: g.add_edges(graphs.PetersenGraph().edges())                                                                                                                                  
            sage: g.add_edges(graphs.PetersenGraph().edges())                                                                                                                                  
            sage: g.add_edge(0,0)                                                                                                                                                              
            sage: g.add_edge(0,0,"Hey")                                                                                                                                                        
            sage: g.add_edge(0,9)                                                                                                                                                              
            sage: g.add_edge(0,9)                                                                                                                                                              
            sage: g.add_edge(0,9)                                                                                                                                                              
            sage: (2*g.size()) == (2*g).size()                                                                                                                                                 
            True                                                                                                                                                                               
                                                                                                                                                                                               
        Immutable input ? Immutable output::                                                                                                                                                   
                                                                                                                                                                                               
            sage: g = g.copy(immutable=True)                                                                                                                                                   
            sage: (2*g)._backend                                                                                                                                                               
            <class 'sage.graphs.base.static_sparse_backend.StaticSparseBackend'>  

I hate multiedges. I hate loops. I hate labels.

Nathann

Depends on #15623

CC: @simon-king-jena @fchapoton

Component: graph theory

Author: Nathann Cohen

Branch: 9f1716e

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/15627

[nathanncohen]

Changed dependencies from #15603 to #15623

[nathanncohen]

Branch: u/ncohen/15627

[sagetrac-git]

Commit: 6cb609d

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

6cb609d	trac #15627: (disjoint) Union of graphs with multiple edges
b6ed038	trac #15623: Immutable graph backend for Posets
0f57805	trac #15622: Immutable graphs must not be relabeled
c441178	trac #15619: bug in the former definition; exception to avoid it in the future
0229348	trac #15619: Pickling of immutable graphs
7860f39	trac #15603: "immutable=True" for Graph/Digraph `__init__` and copy()
2525d22	Trac 15278: Fix syntax error in doc test
fcf9e30	Trac 15278: Only graphs that use the static backend are identical with their copy
8fc9c94	Merge branch 'develop' into ticket/15278
51d6328	Trac 15278: Error messages explain how to create (im)mutable graph copy

[fchapoton]

comment:3

There is a typo "afinite projective plane"

I propose to replace "No other value is available for this parameter." by

"For the moment, no other value is available for this parameter."

[fchapoton]

comment:4

oops, wrong ticket !

[fchapoton]

comment:6

There is a failing doctest about the immutability :

sage: (2*g)._backend
    Traceback (most recent call last):
      ...
    NotImplementedError: An immutable graph does not change name

I suggest also to add a reference to :trac:`15627`

[sagetrac-git]

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

a79d1b0	trac #15627: rebase on 6.2.beta3
9f1716e	trac #15627: Fixed doctest

[sagetrac-git]

Changed commit from 6cb609d to 9f1716e

[nathanncohen]

comment:9

Here it is !

Nathann

[fchapoton]

Reviewer: Frédéric Chapoton

[fchapoton]

comment:10

Ok, good enough for me (even if not fully pep8 or python3)

:)

[nathanncohen]

comment:11

Ahahah. Thanks for the review :-)

Nathann

[vbraun]

Changed branch from u/ncohen/15627 to 9f1716e

[fchapoton]

Changed commit from 9f1716e to none