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Hi,
I was recently playing with txAmpoule & Twisted on win32. When Twisted spawns processess, the 
environment is checked for unicode variables (and an exception is raised in case of). Then it 
came to my attention, that importing Tkinter on win32 sets an environment variable, which value 
is Unicode. Just have a look:
C:\>python
ActivePython 2.6.2.2 (ActiveState Software Inc.) based on
Python 2.6.2 (r262:71600, Apr 21 2009, 15:05:37) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import Tkinter
>>> import os
>>> os.environ['TK_LIBRARY']
u'C:\\Python26\\tcl\\tk8.5'
>>> ^Z
Why is it Unicode? Does it really have to be? Do we need that environment variable at all?
On Linux, it is different:
root@foo:~# python
Python 2.6.2 (release26-maint, Apr 19 2009, 01:56:41)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import Tkinter
>>> import os
>>> os.environ['TK_LIBRARY']
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python2.6/UserDict.py", line 22, in __getitem__
 raise KeyError(key)
KeyError: 'TK_LIBRARY'

I cannot reproduce it with the python.org version:
Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit 
(Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
py> import Tkinter
py> import os
py> os.environ['TK_LIBRARY']
'D:\\apps\\Python26\\tcl\\tk8.5'
py> any(isinstance(value,unicode) for value in os.environ.values())
False
This might be an ActiveState issue.

I'm running the exact same version as Gabriel (on Windows 7, 32 bit)
from the python.org installer, and have the same behavior as Michał. 
Checking FixTk.py it appears that on Vista and above, it calls the Win32
API GetFinalPathNameByHandleW to expand symbolic links, but this is a -W
function, not a -A so it deals with unicode.
The blame shows this function was added in response to #3881
For my system, the paths are all lower ascii so changing the end of
convert_path to return str(s) works. I don't know anything about
filesystem encodings on Windows to do a more correct conversion.

This patch may solve this issue, but I don't have a Vista install to 
test it.

This patch works OK for me (Vista Home Premium + Python 2.6), thanks!

The bug on the Twisted side may be of interest:
http://twistedmatrix.com/trac/ticket/3964 

I hit that bug with Twisted too - I tried to use AMP: http://twistedmatrix.com/trac/ticket/3931 

This bug also breaks code that uses the subprocess module, e.g.:
env = os.environ.copy()
env['MY_VARIABLE'] = 'MY_VAL'
subprocess.Popen(... , env=env)
Fails on Windows 7 with an error that the environment can only contain strings. The offending variables are TK_LIBRARY and TCL_LIBRARY, which have Unicode strings as their values. I'm using Python 2.6.2.
(The subprocess module should probably be fixed to use CreateProcessW and handle Unicode, but that's a separate issue).

confirmed on Win7 buildbot, when the tcl or tk test is run before test_wsgiref:
======================================================================
FAIL: test_simple_validation_error (test.test_wsgiref.IntegrationTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "D:\cygwin\home\db3l\buildarea\trunk.bolen-windows7\build\lib\test\test_wsgiref.py", line 159, in test_simple_validation_error
 "AssertionError: Headers (('Content-Type', 'text/plain')) must"
AssertionError: "AssertionError: Environmental variable TCL_LIBRARY is not a string: <type 'unicode'> (value: u'D:\\\\cygwin\\\\home\\\\db3l\\\\buildarea\\\\trunk.bolen-windows7\\\\tcltk\\\\lib\\\\tcl8.5')" != "AssertionError: Headers (('Content-Type', 'text/plain')) must be of type list: <type 'tuple'>"
http://www.python.org/dev/buildbot/all/builders/x86%20Windows7%20trunk/builds/171
This error was introduced with patch for #3881.

Fixed with r78722 on trunk. Pending backport to 2.6.

Ported to 2.6 with r79336