Bringing the viewport into focus worked... wow, I had a feeling it was something simple like that!

Above is a quick screen grab of me trying to run it.

So I am in the process of learning how Python and scripting in C4D works.
Working with this interesting tutorial series, but have falled down at the first hurdle: nothing seems to run from the expression editor.
I have run scripts successfully from the Script Manager, but nothing so far from the Expression Editor.

All I have is:
a cube with a python tag
The following code:

import c4d
import sys
#Welcome to the world of Python

def main():
    print sys.version

It runs fine in the command line and in the script manager, but not the expression editor looking at the python tag on the cube.

Any suggestions where I may have gone wrong?

R21.207 (Build RB303831)