Ah, you're correct.  I missed that it was being decommissioned (I thought it was just down).

> That the python driver fallback to RoundRobin suggests that it doesn't properly update it's token range definitions when a node is decommissioned.

It looks like it should be updating the token metadata, but you're correct, something in that chain of events is probably broken.  I'll figure out what's going on.

> Note that by design, if the routing key is correct, then in a single DC setup, if TokenAware fallback to the child policy then that must imply that the query fail with an unavailable exception (unless maybe for write at CL.ANY). If no replica for the query are live, how could the query work?

In theory, the replicas could simply be unreachable by the driver (say, due to network problems) but still be live and reachable from the rest of the cluster.  So, I do think it is best for the driver to continue with the child policy's query plan.  The java and python drivers both do that right now.