I need to make few rasberry pi communicate (but should be nice to have a system that can be easily scaled up). The boards could be at big distance from each other (from few centemetrs to 15 meters one from each others), and i need them to communicate. I think the best for my need should be a master-slaves communication, and a double-way communications (master sends commands to the slaves and recives a data back when the board has finished its job).

The communications has to be more realtimes as possible, thread-based (if a slave is doing some computationally heavy job it has to answer anyway), double way (master to slave and slave to master asynchronously), and the message has to be short (few numbers, some strings, no more).

I think i have some different possibilities, the first came up in my mind is ethernet (but i need the extra cost of a router).

Someone point me out that it could be more lower communication (talking about electronical protocols like SPI, i2c, etc). This could be more general considering to use as slave (or even the master) an arduino.

I would use any programming languages but i would like very much to use python :-)