[Pythonmac-SIG] Python virtual environments on OS X

[Ronald Oussoren]

On 19 Jul, 2011, at 13:52, Vinay Sajip wrote:

> Ronald Oussoren <ronaldoussoren <at> mac.com> writes:
> 
>> I'm not sure if I like this approach, I'll read the PEP before I start
> discussing that though. One thing I want
>> to mention beforehand: why is the file named 'env.cfg' instead of something
> starting with 'py'?  The
>> latter is basicly namespacing the filename and avoids clashes with other
> software (although I don't know
>> of any software that uses an env.cfg file).
> 
> Thanks for your feedback.
> 
> This file will only appear in the root folder of a virtual environment (i.e. the
> prefix location), so it's pretty unlikely to cause a clash in practice. However,
> the name change is easy to make at any point before release.
> 
> What is it that you don't like about the proposed approach?

I'll read the PEP before responding to this question.

> 
>> This should work fine with a framework build, with two caveats:
>> 
>> 1) bin/python in a framework build is not the interpreter, but a stub
> executable. The script that creates
>> the virtual environment should copy the executable in
> Resources/Python.app/MacOS instead.  
>> bin/python just execv-s the real interpreter.
>> 
>> 2) accessing GUI functionality requires that the executable is located in an
> .app bundle, that's  why
>> bin/python is a stub.
>> 
>> The latter point means that virualenvs will be slightly less functional than
> the framework install there
>> based on unless you add more OSX specific code.  It should be fairly easy to
> get this to work though, basicly
>> create a Python.app in a hidden folder in the virtualenv and copy the real
> interpreter into that, then add a
>> bin/python that execv's that hidden Python.app.
> 
> That raises a few more questions :-)
> 
> 1. Since we only use a copy of the executable so that argv[0] (and hence
> sys.executable) will point to the environment (and hence allow locating the
> environment config), mightn't the fact that a stub is used work to our
> advantage, in that it doesn't take up much disk space?

The 'env.cfg' file will be next to the stub executable, and hence not in a location where the real interpreter will look.

> 2. How does the stub locate the real executable?

The stub is linked to the framework and uses that to find the sys.prefix, the real executable is located in a fixed location relative to sys.prefix.

That's one other difference w.r.t. regular builds: framework builds use the location of the shared library to calculate sys.prefix. AFAIK this functionality was inherited from the NextStep port. 

> 3. What would the minimum contents of the Python.app folder be?

With some luck only Python.app/Contents/MacOS/Python needs to be present (the real executable), but it would be better to include the Info.plist file as well because some gui frameworks use the contents of that file

> 4. Does PyObjC support Python 3 yet? There's no mention of it on the PyObjC
> site, and from what I can tell this would need to be functional before Cocoa
> GUIs could be programmed in Python 3.

PyObjC supports Python 3, the site is very much out of date. I haven't found time to redesign the site yet, which needs to be done because the maintanence scripts no longer work due to some code reorganisations in PyObjC itself.

Ronald

> 
> Regards,
> 
> Vinay Sajip
> 
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