how to get the environment variable??

[ed_tsang at yahoo.com]

hi i am trying to set a global variable called tharn from the 
environemtn variable in my .cshrc file ... 
I used the folowing code:
try:
    tharn = os.environ['THARN']
except KeyError:
    try:
       tharn = os.environ['CTF']
    except KeyError:
       print "\nEnvironment variable THARN must be set"
       print "before starting the Test Harness."
       sys.exit(1)
The problem I am facing is tharn can be set by either environment 
varible CTF or THARN. Only one of CTF and THARN should  exist, or 
they can be both missing then user get a wanring message...

I amnot sure if my code is archieveing that....
or any potential pittfall in this ... wondering if I may intoduce 
error in having another try .. except block in KeyError...

thanks