[Python-ideas] Policy for altering sys.path

[Thomas Güttler]

Am 07.05.2015 um 08:48 schrieb Florian Bruhin:
> * Thomas Güttler <guettliml at thomas-guettler.de> [2015-05-07 08:00:09 +0200]:
>> Am 06.05.2015 um 17:07 schrieb Paul Moore:
>>> On 6 May 2015 at 15:05, Thomas Güttler <guettliml at thomas-guettler.de> wrote:
>>>> I am missing a policy how sys.path should be altered.
>>>
>>> Well, the docs say that applications can modify sys.path as needed.
>>> Generally, applications modify sys.path in place via sys.path[:] =
>>> whatever, but that's not mandated as far as I know.
>>>
>>>> We run a custom sub class of list in sys.path. We set it in sitecustomize.py
>>>
>>> Can you explain why?
>>
>> I forgot to explain the why I use a custom class. Sorry, here is the background.
>>
>> I want sys.path to ordered:
>>
>>   1. virtualenv
>>   2. /usr/local/
>>   3. /usr/lib
>>
>> We use virtualenvs with system site-packages.
>>
>> There are many places where sys.path gets altered.
>>
>> The last time we had sys.path problems I tried to write a test
>> which checks that sys.path is the same for cron jobs and web requests.
>> I failed. Too many places,  I could not find all the places
>> and the conditions where sys.path got modified in a different way.
>
> It looks like you explained *how* you do what you do, but not *why* -
> what problem is this solving? Why can't you just invoke the
> virtualenv's python and let python take care of sys.path?

I want the sys.path be ordered like it, since I want that packages of the inner
environment are tried first.

Here "inner" means "upper" in the above sys.path order.

Example: If a package is installed in the virtualenv with version 2.2 and
in global site packages with version 1.0, then I want the interpreter to
use the version from virtualenv.

Does this explain the *why* enough? If not, please tell me what you want to know.

Regards,
   Thomas Güttler