Re: avoiding system's "easy-install.pth" with custom distribution

On 11 Sep, 2013, at 8:16, Ned Deily <[email protected]> wrote:

> In article 
> <CALzQQJbKh5cwRf6HPcLc7NurKmbajuTR3TyrO_WQXJZL9Vvr6w@mail.gmail.com>,
> Nat Echols <[email protected]> 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.

Personaly I'd just drop support for ~/Library/Python and /Library/Python in 3.4,
while keeping support for ~/.local (as used by the non-framework build and other
unix platforms).  This moves framework builds closer to normal unix builds,
and makes it less likely that a (hypothetical) Apple 3.4 install causes problems
with the Python.org provided install.  The disadvantage is that ~/.local is ugly
as hell and doesn't fit nicely in the OSX filesystem hierarchy standard but that's
something I'll have to learn to live with.

With built-in support for virtual environments even the per-user install
directory could be removed, but that is a much larger change that requires
a PEP and clear argumentation as to why this is a good idea (and even then it
probably wouldn't pass on python-dev).

Ronald

> 
> 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,
> [email protected]
> 
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