Problem solved. "sys.platform is 'whatever'" never gives True. Don't know why,
but tested it on console. For these problems so called 'minimal working examples'
are nice. You did not give one on stackexchange and I think this is why it
was so hard to find it. Hard for you and ... well, it cost a bit of time to
ask for informations, download the code, understand your descriptions on stack etc.
etc. ...

But for me it was nice. Thanks for the training.