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schol[53]:ccd>cat uss.py
import os,UserString
a=UserString.UserString('/etc/passwd')
print os.path.exists(a)
schol[54]:ccd>python uss.py
Traceback (most recent call last):
 File "uss.py", line 5, in ?
 print os.path.exists(a)
 File "/usr/local/nonius/lib/python2.0/posixpath.py", line 166, in exists
 st = os.stat(path)
TypeError: stat, argument 1: expected string, instance found

Alas, this is true. The same holds for UserDict and UserList.
There is no easy fix.
Fixing this will be a major project, probably for Python 3000.
I'm adding a reminder to PEP 42, and closing the bug report.

When I submitted the bug, I thought it was very difficult. But
I think there is a bit hackish way to solve it for the UserString.
PyArg_ParseTuple could check for UserString objects if 
a "s" variable is asked for, and return the .data attribute.
Sorry for even proposing it. I'll wait for 3000....

This still seems to be the case in 3.2.
Python 3.2.3 (default, Apr 23 2012, 23:35:30) 
[GCC 4.7.0 20120414 (prerelease)] on linux2
>>> from collections import UserString
>>> a = UserString("foooooop")
>>> import os.path
>>> os.path.exists(a)
Traceback (most recent call last):
 File "<input>", line 1, in <module>
 File "/usr/lib/python3.2/genericpath.py", line 18, in exists
 os.stat(path)
TypeError: Can't convert 'UserString' object to str implicitly