Here is code to demonstrate the problem.  All asserts
succeed except the last, showing that pickle and
cPickle both handle a "classic" cycle correctly, but
only cPickle handles new-style cycles correctly.  It
would appear that the problem is that the pure-Python
pickle isn't putting the object into its 'memo' until
*after* the object's state has been pickled.  Thus, the
identity is not preserved on unpickling.  This may be
true for other object types that use __reduce__, but I
have not verified this.


import pickle, cPickle

class X: pass

x = X()
x.x = x

x2 = cPickle.loads(cPickle.dumps(x))
assert x2.x is x2

x2 = pickle.loads(pickle.dumps(x))
assert x2.x is x2

class Y(object): pass

y = Y()
y.y = y

y2 = cPickle.loads(cPickle.dumps(y))
assert y2.y is y2

# this fails
y2 = pickle.loads(pickle.dumps(y))
assert y2.y is y2   

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Compare bug 702858, which observes the same behaviour for 
copy.deepcopy.

The common parts of pickle and copy really ought to be merged.

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Another datapoint:  This above script runs fine in Py2.3b2.

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Runs fine in Py2.4.1 too, so should it be considered fixed?

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Runs find in 2.5cvs too.

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I think we can consider this fixed now.