For Python:
import boto.utils
region=boto.utils.get_instance_metadata()['local-hostname'].split('.')[1]
which boils down to the one-liner:
python -c "import boto.utils; print boto.utils.get_instance_metadata()['local-hostname'].split('.')[1]"
Instead of local_hostname you could also use public_hostname, or:
boto.utils.get_instance_metadata()['placement']['availability-zone'][:-1]