[Distutils] environment variables in buildout config files?

[Barry Warsaw]

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I just converted Mailman 3 to use zc.buildout.  Thanks to Gary Poster  
for providing some good examples and the impetus for my force vacation  
hacking. :)

I especially like being able to specify defaults in my ~/.buildout/ 
default.cfg file.  One thing that bugs me though is that I can't use  
environment variables like $HOME there.  This is a minor pain because  
I can't share my .buildout directory between Linux and OS X machines  
(say, via version control).  It appears as though I have to be  
explicit in the path, with /Users/barry on OS X and /home/barry on  
Linux.

Similarly, in my buildout.cfg file, if I include a develop field  
pointing to some source directories (other than .), I can't use $HOME/ 
projects/foo/bar here.

Any thoughts on supporting environment variables in .cfg files?  Or do  
they work and I'm just experiencing operator error? ;)

A couple of other minor questions.  In my ~/.buildout/default.cfg  
file, I have

[buildout]
eggs-directory=/Users/barry/.buildout/eggs
download-cache=/Users/barry/.buildout/download-cache

This is great because I can share that stuff between lots of  
development branches.  However buildout will create the eggs directory  
if needed, but complain that download-cache doesn't exist.  It seems  
like buildout could/should create download-cache on demand.

I'm also wondering, does it make sense to put develop-eggs and parts  
in say ~/.buildout?  I kind of dislike having artifacts like these  
live in my version controlled source directory (well, there's bin too,  
but I can kind of live with that).  What's the best way of doing that?

Thanks,
Barry

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