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

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

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'>

Answer 2

Python 2 doesn't have a unified object hierarchy (ie. not everything is descended from the object class). Anything that is part of this hierarchy can be played with via __class__ , but those that aren't cannot be modified in this way (or at all, really). These are called Python's "types", and they're hard-coded in C. Examples of types are str , int , float , list , tuple , etc. This means that you cannot use types in the same ways as classes, for example you cannot change the class of an instance of a type, you cannot add, remove or modify methods of types, etc. The following transcript shows the difference in behaviour between types such as str (hard-coded, non-dynamic C constructs) and classes I've called A and B (changeable, dynamic, Python constructs):

>>> str
<type 'str'>
>>> class A:
...     pass
... 
>>> a = A()
>>> A
<class __main__.A at 0xb747f2cc>
>>> a
<__main__.A instance at 0xb747e74c>
>>> type(a)
<type 'instance'>
>>> type(A)
<type 'classobj'>
>>> type(str)
<type 'type'>
>>> type(type(a))
<type 'type'>
>>> type(type(A))
<type 'type'>
>>> A.foo = lambda self,x: x
>>> a.foo(10)
10
>>> A().foo(5)
5
>>> str.foo = lambda self,x: x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't set attributes of built-in/extension type 'str'
>>> 'abc'.foo(5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'foo'
>>> class B:
...     pass
... 
>>> a.__class__
<class __main__.A at 0xb747f2cc>
>>> a.__class__ = B
>>> a
<__main__.B instance at 0xb747e74c>
>>> 'abc'.__class__
<type 'str'>
>>> 'abc'.__class__ = B
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ must be set to new-style class, not 'classobj' object
>>> class B(object):
...     pass
... 
>>> 'abc'.__class__ = B
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __class__ assignment: only for heap types

Answer 3

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 4

I tried this way!

>>> class C(str):
...     __class__ = str
...
>>> c = C()
>>> c.__class__
<class 'str'>