An empty undirected graph object

UndirectedGraph.py

class undirected_graph(dict):
    """An undirected graph edge list

    In an undirected graph edges are symmetric, so vertex order doesn't matter.
    Store lists or dictionaries to represent attributes.

    >>> a = undirected_graph()
    >>> a[4,5] = True
    >>> a[5,4]
    True
    >>> pair = (1,8)
    >>> a[pair] = {"weight":5,"color":"red"}
    >>> a[8,1]['color']
    'red'
    """

    def __getitem__(self, key, second=None):
        return super(undirected_graph, self).__getitem__(tuple(sorted(key)))

    def __setitem__(self, key, value):
        super(undirected_graph, self).__setitem__(tuple(sorted(key)), value)

if __name__=='__main__':
    import doctest
    doctest.testmod()

The earlier implementation worked fine for me when edges were specified out of order. If you copy and pasted the code into the python repl the line spaces caused parsing problems and it would break... maybe that was what you encountered?

In any event, I've cleaned it up a bit and included tests. It's much smaller now.

Simmered on it for a bit, and I like your solution better. I've changed the class to reflect it. Thanks!