When where and how can i change the __class__ attr of an object in Python?

Question

I'd like to be able to do:

>>> class a(str):
...     pass
...
>>> b = a()
>>> b.__class__ = str
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: only for heap types

Answer 1

Only classes that were defined with a class keyword could be used for __class__ attribute assignment:

>>> class C:
    pass

>>> class D:
    pass

>>> C().__class__ = D
>>>

Answer 2

I've solved it in this way:

>>> class C(str):
...     def __getattribute__(self, name):
...         if name == '__class__':
...             return str
...         else:
...             return super(C, self).__getattribute(name)
...         
>>> c = C()
>>> c.__class__
<type 'str'>