>>> os.environ['FOO']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/UserDict.py", line 22, in __getitem__
    raise KeyError(key)
KeyError: 'FOO'
>>> os.environ.get('FOO')
>>> os.environ.get('FOO', 'bar')
'bar'


% export FOO=
% python2.5
>>> import os
>>> os.environ['FOO']
''

An environment variable set to '' is valid and is still considered set
(thats how the environment works).