When I run the command:
fab -H localhost host_type
[localhost] Executing task 'host_type'
[localhost] run: uname -s
Fatal error: Low level socket error connecting to host localhost: Connection refused
from fabric.api import run
The important part isn't the "low level error" part of the message - the important part is the "Connection refused" part. You'll get a "connection refused" message when trying to connect to a closed port.
The most likely scenario is that you are not running an ssh server on your machine at the time that Fabric is running. If you do
you'll probably get a message similar to
ssh: connect to host localhost: Connection refused
So you'll have to go out and set up an SSH server on your computer before you can proceed with Fabric from there.