How I would expect to be able to write flatten

def flatten(obj):
    """ flattens a nested data structure into a list of (value, path) tuples """
    vals = []
    def rec(obj, path):
        try:
            for k,v in obj.iteritems(): vals.append((v, path + (k,)))
        except:
            vals.append((obj, path))
    rec(obj, ())
    return vals