> The second part changes based on what settings have been programmed
> in the radio. I've seen at least 2 different values in addition to
> the original 'x'.

Ah, I should have looked at the code, I thought you were making an
example. If there are three known values, I'd just check for them. If
there end up being lots, then maybe we should do something more like
what you had where we try to capture the first one. Checking for any of
the three is best done like this:

  ack = radio.pipe.read(1)
  if ack not in ('x', 'y', 'z'):
     raise ...