Trouble getting to windows My Documents directory

[Mark Storkamp]

I'm just learning Python, and I've run into trouble trying to change 
directory to the windows My Documents directory. There's likely a better 
way to do this, but this is what I've tried so far:

---------------------------------------------
from tkinter import Tk
from tkinter.filedialog import askopenfilename

import os

Tk().withdraw()

sourcedir = os.environ['HOME']+"/Documents/"
os.chdir(sourcedir)
src = askopenfilename()
if src == '' :
    sys.exit()
fin = open(src, mode='r')
## do stuff
fin.close()
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When this is run from IDLE, it works fine. But when I double click on 
the saved file to run it, it quits without ever showing the open file 
dialog box, and doesn't show any error message.

The problem is with the os.environ['HOME'] call. If I comment it out 
(and fix up the surrounding code with an absolute path) then it runs. 
But then it won't work properly for other users.

Interestingly enough, when I moved this to a Mac so I could post to 
Usenet, I discovered it works fine on the Mac. Only Windows seems to be 
the problem. Windows 7.

Any thoughts or suggestions?