converting between Graph and DiGraph naturally

[jasongrout]

This should work. At the very minimum, there should be a sensible error message:

sage: DiGraph(graphs.PathGraph(4))                                             
---------------------------------------------------------------------------
NetworkXError                             Traceback (most recent call last)

/home/grout/.sage/<ipython console> in <module>()

/home/grout/sage-3.2.3/local/lib/python2.5/site-packages/sage/graphs/graph.pyc in __init__(self, data, pos, loops, format, boundary, weighted, implementation, sparse, vertex_labels, **kwds)
   8362             else:
   8363                 if implementation == 'networkx':
-> 8364                     self._backend = NetworkXGraphBackend(networkx.XDiGraph(data, selfloops=loops, **kwds))
   8365                 elif implementation == 'c_graph':
   8366                     if data is None:

/home/grout/sage-3.2.3/local/lib/python/networkx/xdigraph.py in __init__(self, data, name, selfloops, multiedges)
    118         self.multiedges=multiedges
    119         if data is not None:
--> 120             convert.from_whatever(data,create_using=self)
    121         self.name=name
    122 

/home/grout/sage-3.2.3/local/lib/python/networkx/convert.py in from_whatever(thing, create_using)
    114 
    115     raise networkx.NetworkXError, \
--> 116           "Input is not a known data type for conversion."
    117 
    118     return 

NetworkXError: Input is not a known data type for conversion.
sage: Graph(DiGraph({0:[1,2],1:[0,3]}))
---------------------------------------------------------------------------
NetworkXError                             Traceback (most recent call last)

/home/grout/.sage/<ipython console> in <module>()

/home/grout/sage-3.2.3/local/lib/python2.5/site-packages/sage/graphs/graph.pyc in __init__(self, data, pos, loops, format, boundary, weighted, implementation, sparse, vertex_labels, **kwds)
   7022                         self.add_vertices(xrange(data))
   7023                     else:
-> 7024                         self._backend = NetworkXGraphBackend(networkx.XGraph(data, selfloops=loops, **kwds))
   7025                 elif implementation == 'c_graph':
   7026                     if data is None:

/home/grout/sage-3.2.3/local/lib/python/networkx/xgraph.py in __init__(self, data, name, selfloops, multiedges)
    111         self.multiedges=multiedges
    112         if data is not None:
--> 113             self=convert.from_whatever(data,create_using=self)
    114         self.name=name
    115 

/home/grout/sage-3.2.3/local/lib/python/networkx/convert.py in from_whatever(thing, create_using)
    114 
    115     raise networkx.NetworkXError, \
--> 116           "Input is not a known data type for conversion."
    117 
    118     return 

NetworkXError: Input is not a known data type for conversion.

Component: graph theory

Issue created by migration from https://trac.sagemath.org/ticket/5046

[jasongrout]

comment:1

Currently, there are to_directed and to_undirected methods, but the above is the natural way to convert things in Sage.

[jasongrout]

comment:2

Oh, and don't forget that you can specify whether loops are allowed or not. That replaces the to_simple function.

[rlmill]

comment:3

Fixed by the patch at #5171, I believe.

[rlmill]

comment:4

I just checked this example:

sage: DiGraph(graphs.PathGraph(4))
Path Graph: Digraph on 4 vertices

[sagetrac-mabshoff]

comment:5

Fixed via #5171.

Cheers,

Michael