To give an example of what the test could check:

>>> class C(object):
...  def __init__(self):
...    self.some_long_attribute_name = 5
... 
>>> c = C()
>>> c.__dict__
{'some_long_attribute_name': 5}
>>> sorted(map(id, c.__dict__))
[140371243499696]
>>> import pickle
>>> d = pickle.loads(pickle.dumps(c, -1))
>>> d
<__main__.C object at 0x7faaba1b0390>
>>> d.__dict__
{'some_long_attribute_name': 5}
>>> sorted(map(id, d.__dict__))
[140371243501232]

The `sorted(map(id, d.__dict__))` should have been the same before and
after pickling.