From the project homepage:
r³ is a map-reduce engine written in python using redis as a backend
r³ is a map reduce engine written in python using a redis backend. It’s purpose is to be simple.
r³ has only three concepts to grasp: input streams, mappers and reducers.
You need to visit this project. It is simple, efficient and effective.
I found this following r³ – A quick demo of usage, which I found at: Demoing the Python-Based Map-Reduce R3 Against GitHub Data, Alex Popescu’s myNoSQL.