[Pythonmac-SIG] Python PyGame GTK SQLAlchemy on Leopard

[Ned Deily]

In article 
<5465ee790903220622m7897dfcch7e8d838e6b4292bc at mail.gmail.com>,
 Chris Van Bael <chris.van.bael at gmail.com> wrote:
> some simple questions:
> - if I installed another Python, how can I start it?  Whenever I open
> a terminal and type "python", I get Python 2.5.1, which I assume is
> the Python from Apple.
> - On Windows there is the directory /Lib/site-packages, I can't find
> this one on my Mac.

For python.org installers, each python major version lives in its own 
framework sub-tree rooted at:

/Library/Frameworks/Python.framework/Versions/m.n/

where m.n is 2.5, 2.6, 3.0, etc.

At the top level of each version's subtree, there are bin, lib, include 
and other directories.   You'll find a pythonm.n executable and a python 
symlink to it in the bin directory.

Site packages for each version reside within its subtree lib directory 
but normally you don't need to manipulate those directly.  By default, 
distutils (and its users, easy_install, pip, virtualenv et al) will 
install extensions to the right site-library by running under the 
setup.py script or easy_install or pip under the appropriate version of 
the python executable.  Extension scripts will also be installed in the 
corresponding bin directory.

So to select which python you want to start, you can invoke it directly 
with:

/Library/Frameworks/Python.framework/Versions/2.5/bin/python

or modify your shell path to ensure that bin directory comes before 
/usr/bin where the Apple-supplied python resides.  Or create a shell 
alias to it.  Or use /usr/local/bin/pythonm.n because the python.org 2.x 
installers create a link there by default - but beware because (1) that 
doesn't help for installed scripts, (2) can be confusing with multiple 
versions, and (3) by default the 3.x installers do not create that link.

For development environments with multiple versions, a good solution 
these days is to use virtualenv.  Jesse Noller has a very good overview 
of how to do that here:

<http://jessenoller.com/2009/03/16/so-you-want-to-use-python-on-the-mac/>

Again, all of the above applies to python.org installers.  For the 
record, the Apple-supplied python uses a more elaborate framework 
scheme, split between /System/Library/Frameworks and /Library/Python, 
with different defaults.  macports uses a framework scheme rooted at 
/opt/local/Library/Frameworks. And fink python installs are more 
debian-y style non-framework layouts in /sw/{bin,lib,...}.

-- 
 Ned Deily,
 nad at acm.org