Guidelines for Global Variables and Coding

This question is pretty general, but I like to understand the good coding methods in python more ...

I would like to define a constant that I can use internally in any function without passing it as an argument, and without having to change it after the declaration - it might be nice to make it impossible, but not sure if I can do it.

What would be the best way to do this in terms of programming style (coding, naming convention, etc.).

i.e., GLOBAL_CONSTANT = 'this_variable_will_not_be_changed'

My python code will have this form, good style too?

#!/usr/bin/env python

import sys
import os

GLOBAL_CONSTANT='this_variable_will_not_be_changed'

def test():
    print GLOBAL_CONSTANT
    return 0

def main():
    test()
    return 0

if __name__ == "__main__":
    main()