AttributeError: Edge instance has no attribute 'vto'

I'm trying to pickle a class instance containing two lists of another instances. The instances in the two lists have attributes that refer instances of each other. Here are the classes.

class Graph:
    def __init__(self):
        self.vertices = {}
        self.edges = set()
    def __repr__(self):
        return "\n".join(map(str, sorted(self.vertices, key=lambda v:v.id)))

class Edge:
    def __init__(self, vfrom, vto):
        self.vfrom = vfrom
        self.vto = vto
    def __hash__(self):
        return hash(tuple(map(hash, (self.vto, self.vfrom))))
    def __repr__(self):
  开发者_开发技巧      return str(self.vto.id)

class Vertax:
    def __init__(self, id):
        self.id = id
        self.incoming = set()
        self.outgoing = set()
    def __repr__(self):
        return "Vertax %d -> %s"%(self.id, ", ".join(map(str, self.outgoing)))
    def __hash__(self):
        return hash(self.id)

When I tried to pickle a simple graph, unpickling gives an error.

>>> v0 = Vertax(0)
>>> v1 = Vertax(1)
>>> e0to1 = Edge(v0, v1)
>>> v0.outgoing.add(e0to1)
>>> v1.incoming.add(e0to1)
>>> g = Graph()
>>> g.vertices[v0] = v0
>>> g.vertices[v1] = v1
>>> g.edges.add(e0to1)
>>> print g
Vertax 0 -> 1
Vertax 1 -> 
>>> 
>>> import pickle
>>> p = pickle.dumps(g)
>>> pickle.loads(p)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/pickle.py", line 1374, in loads
    return Unpickler(file).load()
  File "/usr/lib/python2.6/pickle.py", line 858, in load
    dispatch[key](self)
  File "/usr/lib/python2.6/pickle.py", line 1133, in load_reduce
    value = func(*args)
  File "<stdin>", line 6, in __hash__
AttributeError: Edge instance has no attribute 'vto'

I found that the error disappears if one comments out the __hash__ function of the Edge class. I need your help to understand why this happens.

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This Python issue may be the cause: http://bugs.python.org/issue1761028

"pickle - cannot unpickle circular deps with custom __hash__"