The nanpy library allows Python programmers to communicate with an Arduino connected via a serial line connected to a host computer (in our case, a Raspberry Pi).
It is similar to pyFirmata. The nanpy library is under development and has a long way to go to get up to the level of that other library, but it looks promising and simple to use.
One big thing that is missing at the moment, however, is a guide on what the Arduino needs to be running in order for it to work! I will contact the authors and find out!
Edit: It seems that there is a firmware folder in the nanpy package that you upload to the Arduino. I find it slightly surprising (and a bit confusing) that this isn’t spelt out in the ‘How to use documentation’ but, hey, it is under development!