How to access environment variable values?

Question

I set an environment variable that I want to access in my Python application. How do I get its value?

Answer 1

The original question (first part) was "how to check environment variables in Python."

Here's how to check if $FOO is set:

try:  
   os.environ["FOO"]
except KeyError: 
   print "Please set the environment variable FOO"
   sys.exit(1)