[Distutils] EasyInstall: installing from svn

[Phillip J. Eby]

At 03:56 PM 6/3/2005 -0500, Ian Bicking wrote:
>Now I'm feeling confused about how installation from svn should work.
>Or more generally, how deployment and development work.
>
>Usually in development, I like to install libraries directly out of svn
>(no distutil step).  Then I can fiddle with them if necessary, commit
>changes (or create diffs if I don't have commit access), etc.  This is
>all much easier to do when there's no distutil step.
>
>When deploying, that's not really what I want to do, even though I might
>very well be deploying from a repository.  What easy_install does now
>with svn repositories is good in that case.
>
>The problem is that the two are conflicting.  If I require a package,
>I'm not going to get the svn checkout, even if the checkout was on the
>path initially.  Since the egg package names don't map to Python
>packages, you can't even really tell that a package is already available
>without egg installation.  The only way I can think of right now to
>safely do this would be:
>
>try:
>      import wsgiutils
>except ImportError:
>      require('WSGI-Utils')
>
>Ick.  So how can I set up a working environment that is close, but not
>too close, to the deployment environment?  Maybe there's a way that, up
>front, I can specify which packages are made available through alternate
>methods?  E.g., fulfill_requirement('WSGI-Utils', 'checkouts/WSGIUtils')
>(where adding the second path to sys.path will fulfill the requirement,
>irrespective of any eggs).

Ah, you've encountered the part of the egg docs that I haven't written yet.

Basically, the solution is to have a PackageName.egg-info directory in the 
dependent package.  pkg_resources.require() will see the directory and use 
it for the metadata.  It will then know that the directory *contianing* the 
.egg-info directory is the directory that needs to be (and already is) on 
sys.path.

Here's how you do it: go to the library you want to use for development, 
and run setup.py bdist_egg.  This will create (as a side effect) a 
PackageName.egg-info subdirectory in the directory that you're going to be 
putting on sys.path.  pkg_resources automatically detects this "development 
egg" and knows what version is present.

This can work even for several packages installed in the same sys.path 
directory, you just have to have a PackageName.egg-info for each one, and 
you'll need to move or symlink them from their individual package locations.

Basically, you're not the first person to have this problem, or rather, you 
are the first person to have the problem, you're just not the first person 
to think of it.  :)  At PyCon, I was describing the egg system design to 
Fred Drake, and he pointed out the problem of require()-ing something 
you're developing in this fashion, so later I came up with the 
PackageName.egg-info trick as a fix.  (Originally, PackageName.egg-info was 
called EGG-INFO.in or something like that, and it lived in a different 
place; later I realized that the staging area could be reused to provide 
metadata for in-development packages, and renamed it to use the distro name 
so that more than one could live in the same parent directory on sys.path.)