[Fab-user] How is config file merged into environment?

[Steve Steiner (listsin)]

I'm wanting to move my hosts off into config files so that I can apply  
the same sets of commands in my fabfile to different hosts by  
switching config files instead of editing fabric.py.

I have a simple test fabfile.py:

# fabfile.py
# ------------
from fabric.api import env, run, sudo
from fabric.context_managers import cd
from fabric.operations import put
from os import path

env.user = "root"
env.hosts = ["xxx.yyy.zzz.kkk"]

# NOTE: LIB
def ls():
    """ls whatever directory you're in"""
    return sudo("ls -Gla")

I want to just move the env.host declaration out to the config file  
like so:

my.cfg
----------------
hosts = ["xxx.yyy.zzz.kkk"]

So, I comment out the env.hosts line in fabfile.py, and call fabric  
like so:

fab --config=my.cfg ls

After the annoying deprecation warnings from Parimiko I get:

No hosts found. Please specify (single) host string for connection:

I've tried rearranging the assignment in my.cfg in all the obvious  
ways but I either I get a message about not being able to open "'["1"  
or some other fragment, or am just prompted for a host to connect to.

I looked through the code and it seems to just parse the config file  
with a nested list comprehension, dense but not particularly obtuse,  
then send the resulting dict back to the env dictionary for updating.

I'm running of the master just pulled from github a few minutes ago.

What idiotically simple thing am I missing?

Clues anyone?

Thanks,

S