[Pythonmac-SIG] avoiding system's "easy-install.pth" with custom distribution

[Ned Deily]

In article 
<CALzQQJbKh5cwRf6HPcLc7NurKmbajuTR3TyrO_WQXJZL9Vvr6w at mail.gmail.com>,
 Nat Echols <nathaniel.echols at gmail.com> wrote:
> I'm distributing a large cross-platform Python-based application, for which
> we bundle Python 2.7 and a number of dependencies (wxPython, matplotlib,
> etc.).  We've tried to make it as self-contained as possible, to prevent
> either a) polluting the user's environment and potentially breaking other
> programs, or b) breaking our program because other software pollutes the
> environment.  This means wiping out PYTHONPATH at runtime, which has worked
> quite well for us.
> 
> What I've now discovered is that at least on OS 10.7.5, if a user installs
> packages for the system python using easy_install (possibly other
> mechanisms as well, but that's what "worked" for me), these will now end up
> in the search path for our app's python interpreter, and override many of
> the packages we distribute because they appear relatively early in
> sys.path.  The problem appears to be traceable to this file:
> 
> /Library/Python/2.7/site-packages/easy-install.pth
> 
> which modifies sys.path directly.  Removing or renaming that file leaves
> our custom distribution's path alone.  So, two questions:
> 
> 1) Why is this happening?  It seems incredibly broken - but I have no idea
> who's fault it is, for all I know we're doing something foolish when we
> compile Python on our build system (10.6, if it matters).
> 
> 2) Is there any way we can modify our distribution to avoid this?

The behavior is the result of a feature added in 2.7 for Issue4865 which 
appends the site-packages directory for the Apple-supplied system Python 
to sys.path of a Python framework build.  With hindsight, I don't think 
this was a good idea and you are not the first to complain about it 
(see, for instance, Issue15048).  OTOH, as Ronald pointed out in that 
issue, there isn't a complete separation between Python frameworks 
anyway: they all share the user site-packages directory 
(~/Library/Python for a framework build) and configuration files (like 
for Distutils and IDLE).  It is late in the game to make a change like 
this in Python 2.7.x which is nearing the end of its supported life.  
It's not too late, IMO, to change current Python 3 before it becomes an 
issue;  it's not an issue yet because Apple has not yet shipped anything 
newer than Python 2.7.x.  Feel free to chime in on the bug tracker.

The path appends occur in the standard library "site" module.  You could 
patch it in your own interpreter to not include /Library/Python; you 
might be also be able to do it by supplying a "sitecustomize" module.  
See site.py for details.

http://bugs.python.org/issue4865
http://bugs.python.org/issue15048
http://hg.python.org/cpython/file/2.7/Lib/site.py#l305

-- 
 Ned Deily,
 nad at acm.org