Have you ever written a Fabric script that work wonderfully well with remote hosts only to realise that it has a problem running locally? If that is the case, then env.host is your friend.

#! /usr/bin/env python
# save this as smartfab.py
from fabric.api import local
from fabric.api import run
from fabric.api import env

def run_that_job():
    """get some info about your host"""
    cmd = "uname -a"
    if env.host:
        run(cmd)
    else:
        local(cmd)

If you run this script with the -H option, it will execute cmd using run(), but if you omit -H, cmd will be executed using local(). To see the difference for yourself, run

$ fab -f smartfab.py -H some_remote_host run_that_job

and compare results with:

$ fab -f smartfab.py run_that_job