But you can usually subclass builtins and get what you want.

PythonWin 2.3.2 (#49, Oct 2 2003, 20:02:00) [MSC v.1200 32 bit (Intel)] on win32.
Portions Copyright 1994-2001 Mark Hammond (mhammond@skippinet.com.au) - see 'Help/About PythonWin' for further copyright information.
>>> "thing".x = 0
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
AttributeError: 'str' object has no attribute 'x'
>>> a = "thing"
>>> a.x = 0
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
AttributeError: 'str' object has no attribute 'x'
>>> class Thing(str):
... \tpass
...
>>> a = Thing()
>>> a.x = 0
>>> a.x
0