Later I had got a crash.

>>> class S(str): __slots__ = ()
... 
>>> 'a'.__class__ = S
>>> 
>>> def f(a): pass
... 
Fatal Python error: non-string found in code slot

Current thread 0xb7583700 (most recent call first):
Aborted (core dumped)

The stdlib is full of implicit caches. Virtually any hashable object can be cached and shared. Why __class__ assignment is allowed at all? There are only two uses of __class__ assignment in the stdlib besides tests (in Lib/importlib/util.py and in Lib/xml/sax/saxutils.py), and in both cases it looks as optimization trick.