combinat/posets/posets.py fails when the optional package dot2tex is installed

[strogdon]

As reported at ​https://groups.google.com/d/msg/sage-release/UdllnAJFufA/-e8ArqwFBAAJ there is often a failure when testing combinat/posets/posets.py

sage -t --long src/sage/combinat/posets/posets.py
**********************************************************************
File "src/sage/combinat/posets/posets.py", line 1742, in sage.combinat.posets.posets.FinitePoset.?
Failed example:
    L.plot(figsize=12, border=True, element_shape='s',
           element_size=400, element_color='white',
           element_colors={'blue': F, 'green': L.double_irreducibles()},
           cover_color='lightgray', cover_colors={'black': F_internal},
           title='The Frattini\nsublattice in blue', fontsize=10)
Exception raised:

    Traceback (most recent call last):
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/doctest/forker.py", line 509, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/doctest/forker.py", line 872, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.combinat.posets.posets.FinitePoset.?[12]>", line 5, in <module>
        title='The Frattini\nsublattice in blue', fontsize=Integer(10))
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/combinat/posets/posets.py", line 1834, in plot
        **kwds)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/misc/decorators.py", line 564, in wrapper
        return func(*args, **options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/generic_graph.py", line 18733, in plot
        return self.graphplot(**options).plot()
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/generic_graph.py", line 18384, in graphplot
        return GraphPlot(graph=self, options=options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/graph_plot.py", line 262, in __init__
        self.set_pos()
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/graph_plot.py", line 346, in set_pos
        self._pos = self._graph.layout(**self._options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/generic_graph.py", line 17847, in layout
        pos = getattr(self, "layout_%s"%layout)(dim = dim, **options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/digraph.py", line 2915, in layout_acyclic
        return self.layout_graphviz(rankdir=rankdir, **options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/generic_graph.py", line 18315, in layout_graphviz
        positions = dot2tex.dot2tex(self.graphviz_string(**options), format = "positions", prog = prog)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/misc/decorators.py", line 564, in wrapper
        return func(*args, **options)
      File "/usr/local/sage-8/local/lib/python2.7/site-packages/sage/graphs/generic_graph.py", line 19532, in graphviz_string
        "%s is not a valid format for edge"%(edge)
    AssertionError: [0, 4] is not a valid format for edge

The failure occurs because graphviz_string does not allow edges to be given as lists (exposed by #18545).

[tscrim]

This was not caused by #18545 but instead simply exposed.

[strogdon]

Replying to tscrim:

This was not caused by #18545 but instead simply exposed.

Yes, you are correct.

[strogdon]

That portion of the L.plot(...) call is cover_colors={'black': F_internal}. If it is removed then the posets.py test passes even with dot2tex installed.

[tscrim]

Okay, it might not be as simple as exposing a bug but bad input. If I use

F_internal = [(c[0],c[1],None) for c in F.cover_relations() if c in L.cover_relations()]

then I can see the plot.

[tscrim]

<self-hatred>It's amazing how useful error messages are...</self-hatred>

[tscrim]

Actually, this even works:

sage: F_internal = [tuple(c) for c in F.cover_relations() if c in L.cover_relations()]

Which makes sense considering the assertion is checking assert isinstance(edge, tuple). What I suggest is making it also allow list.

[tscrim]

Here's a branch with a MWE as a doctest.

---

New commits:

​12dc47d

Allow graphviz_string to take edges that are lists.

[jmantysalo]

I can check this.

[jmantysalo]

Works. Thanks!