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Thy are visible by calling gc.get_referents(). High-level function can use this to count recursive size of objects.
>>> import sys, gc, itertools
>>> def gettotalsizeof(*args, seen=None):
... if seen is None:
... seen = {}
... sum = 0
... for obj in args:
... if id(obj) not in seen:
... seen[id(obj)] = obj
... sum += sys.getsizeof(obj)
... sum += gettotalsizeof(*gc.get_referents(obj), seen=seen)
... return sum
... 
>>> a, b = tee(range(10000))
>>> sum(next(a) for i in range(1000))
499500
>>> gettotalsizeof(a)
750
>>> gettotalsizeof(b)
18734