If it's a global variable, then you can do:
>>> a = 5
>>> globals()['a']
5
A note about the various "eval" solutions: you should be careful with eval, especially if the string you're evaluating comes from a potentially untrusted source -- otherwise, you might end up deleting the entire contents of your disk or something like that if you're given a malicious string.
(If it's not global, then you'll need access to whatever namespace it's defined in. If you don't have that, there's no way you'll be able to access it.)
