#10139: Problem found in graphs.is_planar() when the graph has no edges.
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Reporter: edward.scheinerman | Owner: tbd
Type: defect | Status: new
Priority: major | Milestone:
Component: PLEASE CHANGE | Keywords:
Author: | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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The is_planar method for graphs includes a set_embedding argument that
saves the combinatorial embedding of the graph (if it's planar). In this
example, it works properly:
sage: g = graphs.WheelGraph(6)
sage: g.is_planar(set_embedding=True)
True
sage: g.get_embedding()
{0: [1, 5, 4, 3, 2], 1: [2, 5, 0], 2: [0, 3, 1], 3: [0, 4, 2], 4: [0, 5,
3], 5: [0, 1, 4]}
However, is we apply this method to a graph with no edges, a problem
arises:
sage: g = graphs.EmptyGraph()
sage: g.add_vertices(range(6))
sage: g.is_planar(set_embedding=True)
---------------------------------------------------------------------------
AttributeError Traceback (most recent call
last)
/home/ers/<ipython console> in <module>()
/usr/public/share/sage/local/lib/python2.6/site-
packages/sage/graphs/generic_graph.pyc in is_planar(self, on_embedding,
kuratowski, set_embedding, set_pos)
2438 self._pos = G._pos
2439 if set_embedding:
-> 2440 self._embedding = G._embedding
2441 return planar
2442
AttributeError: 'Graph' object has no attribute '_embedding'
Once we add an edge to this graph, all is well:
sage: g.add_edge(0,1)
sage: sage: g.is_planar(set_embedding=True)
True
sage: g.get_embedding()
{0: [1], 1: [0], 2: [], 3: [], 4: [], 5: []}
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10139>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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