Is it possible to have bidirectional communication betweeen an Arduino Uno Rev 3 and a Raspberry Pi Modell B+ running Rasbian? They're connected ofer USB, and it would be good if I could use Python on the Pi.
Yes. The Arduino Uno implements a serial protocol inside the USB signal using hardware. This is how it is built. The Linux side is a bit more complex as there are many USB protocols. Here is a howto which talks about the Linux side of the Arduino / Linux connection.
To be clear, Rasbian is a derivative of Linux.
Yes, it is possible to have a USB serial communication between a raspberry pi and an Arduino. And it is much more easier when using Python on the raspberry pi.
Python has a serial library within it's standard library which you can import on the raspberry pi and then use it to send or receive data to/from the Arduino or the raspberry pi.(Just connected via USB cable).
Here is the link for a very similar requirement on YouTube.