In the py3k-struni branch the str() constructor doesn't use __str__ when the argument is an instance of a subclass of str. A user defined string can't change __str__().

It works in Python 2.5 and in the p3yk branch.

Python 3.0x (py3k-struni:56245, Jul 10 2007, 23:34:56) 
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Mystr(str):
...     def __str__(self): return 'v'
... 
>>> s = Mystr('x')
>>> s
'x'
>>> str(s)
'x' # <- SHOULD RETURN 'v'

Python 2.5.1 (r251:54863, May  2 2007, 16:56:35) 
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Mystr(str):
...      def __str__(self): return 'v'
... 
>>> s = Mystr('x')
>>> s
'x'
>>> str(s)
'v'

Python 3.0x (p3yk:56180, Jul  6 2007, 23:35:08) 
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class Mystr(str):
...     def __str__(self): return 'v'
... 
>>> s = Mystr('x')
>>> s
'x'
>>> str(s)
'v'