Pickling and copying instances of subclasses of some basic classes pickles and copies instance attributes. Example:

>>> class BA(bytearray):
...     pass
... 
>>> b = BA(b'abc')
>>> b.x = 10
>>> c = copy.copy(b)
>>> c.x
10
>>> c = pickle.loads(pickle.dumps(b))
>>> c.x
10

But this doesn't work if attributes are saved not in instance dictionary, but in slots.

>>> class BA(bytearray):
...     __slots__ = ('x',)
... 
>>> b = BA(b'abc')
>>> b.x = 10
>>> c = copy.copy(b)
>>> c.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x
>>> c = pickle.loads(pickle.dumps(b))
>>> c.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x

Since using __slots__ is implementation detail, this failure can be considered as a bug.

Proposed patch adds support of pickling and copying slots in subclasses of all classes that already support pickling and copying non-slot attributes. It is backward compatible, classes with slots can be unpickled on older Python versions without slots. Affected classes: bytearray, set, frozenset, weakref.WeakSet, collections.OrderedDict, collections.deque, datetime.tzinfo.

The patch adds the copyreg._getstate() function for Python classes and exposes the _PyObject_GetState() function for extension classes. An alternative (and simpler for end user) solution would be to add default implementation as object.__getstate__(). But this is not easy to reject non-pickleable classes (issue22995) in this case, since __getstate__ is looked up as instance attribute, not as other special methods.