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Not sure where to put example code, but here it goes:
import weakref
class MyObj(object):
	def __init__(self):
		self.ref = weakref.ref(self)
	def __del__(self):
		print "HERE123", self.ref()
o = MyObj()
o = None

The behavior is documented in the C code. I don't know how it is
described in the docs.
Objects/typeobject.c around line 820 says:
/* If we added a weaklist, we clear it. Do this *before* calling
 the finalizer (__del__), clearing slots, or clearing the instance
 dict. */

Georg, can you update the docs? Weak references *to* an object are
cleared before the object's __del__ is called, to ensure that the weak
reference callback (if any) finds the object healthy.

Done in r61733.