[Distutils] virtual-python.py, sys.prefix, and Mac

Ronald Oussoren ronaldoussoren at mac.com
Mon Sep 17 19:15:31 CEST 2007

On 16 Sep, 2007, at 21:55, Ian Bicking wrote:

> Ronald Oussoren wrote:
>> On 16 Sep, 2007, at 21:44, Ian Bicking wrote:
>>> Ronald Oussoren wrote:
>>>> On 15 Sep, 2007, at 18:09, Ian Bicking wrote:
>>>>> Hi all.  I'm kind of giving up on workingenv, and have started  
>>>>> working
>>>>> from virtual-python as a basis instead
>>>>> (http://pypi.python.org/pypi/virtualenv/).
>>>>> So the basic technique here is to copy the executable into
>>>>> /ENV/bin/python, and then sys.prefix will be '/ENV'.  The standard
>>>>> Python installed on a Mac doesn't seem to do this -- the prefix  
>>>>> remains
>>>>> '/opt/local/Library/Frameworks/Python.framework/Versions/2.4'
>>>>> regardless.  (Custom built Python's on Mac work like normal.)
>>>> All framework builds behave as you describe, Modules/getpath.c  
>>>> special-cases calculation of sys.prefix for framework builds of  
>>>> Python (the prefix is inside the framework regardless of where  
>>>> the executable is).
>>> Is there any way to effect that calculation?  I.e., in a normal  
>>> build that calculation is based on the location of the executable,  
>>> so virtualenv moves the executable to effect that.
>> Move the framework.
> I don't really know what that means...?  What exactly is the  
> framework?

The python framework, that is /Library/Frameworks/Python.framework  
(or /System/... if you use Apple's build of Python).  getpath.c uses  
some API calls to determine the absolute path of the python framework  
linked into the current executable and sets sys.prefix to a directory  
inside that framework.

As background info for anyone that's not used to the mac way: OSX  
supports the usual unix organisation of shared libraries but also has  
a different method: frameworks. A framework is basicly a directory  
containing the shared library, header files and resources (the last  
two are optional) and also supports versioning, although Apple's  
development tools don't offer full support for that.

I should be possible to coax a framework install into support virtual- 
python by creating a directory structure for a python.framework inside  
the virtual-python environment and using a simular mechanism as you  
have now for adding the real stdlib to sys.path. You will have to  
modify the copied python executable to load this mini-framework  
because the OSX linker adds absolute paths to shared libraries and  
frameworks to the executable.

The macholib library can be used to do this last task, it is used by  
py2app for rewriting the linker commands in shared libraries that are  
used in application bundles. I don't have an example for that handy,  
but it should be easy enough to extract code from macho_standalone.

> -- 
> Ian Bicking : ianb at colorstudy.com : http://blog.ianbicking.org
>            : Write code, do good : http://topp.openplans.org/careers

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