How to make an executable .py available everywhere on OS X?
You need to add /foo/bar to your $PATH environment variable. Navigate to your home directory by typing cd at the prompt, then type nano .profile. Inside this file, add the following line:

export PATH=$PATH:/foo/bar


then save and quit. Exit Terminal.app (or whichever term program you're using) and restart it. /foo/bar should now be in your search path, to make sure type echo $PATH and see if it's at the end. You should now be able to run myfile.py from anywhere.