[Python-Dev] PEP 405 (built-in virtualenv) status
Kristján Valur Jónsson
kristjan at ccpgames.com
Wed Mar 21 14:35:50 CET 2012
> -----Original Message-----
> From: Carl Meyer [mailto:carl at oddbird.net]
> Sent: 19. mars 2012 19:19
> To: Kristján Valur Jónsson
> Cc: Python-Dev (python-dev at python.org)
> Subject: Re: [Python-Dev] PEP 405 (built-in virtualenv) status
> Hello Kristján,
> I think there's one important (albeit odd and magical) bit of Python's current
> behavior that you are missing in your blog post. All of the initial sys.path
> directories are constructed relative to sys.prefix and sys.exec_prefix, and
> those values in turn are determined (if PYTHONHOME is not set), by walking
> up the filesystem tree from the location of the Python binary, looking for the
> existence of a file at the relative path "lib/pythonX.X/os.py" (or "Lib/os.py"
> on Windows). Python takes the existence of this file to mean that it's found
> the standard library, and sets sys.prefix accordingly. Thus, you can achieve
> reliable full isolation from any installed Python, with no need for
> environment variables, simply by placing a file (it can even be empty) at that
> relative location from the location of your Python binary. You will still get
> some default paths added on sys.path, but they will all be relative to your
> Python binary and thus presumably under your control; nothing from any
> other location will be on sys.path. I doubt you will find this solution
> satisfyingly elegant, but you might nonetheless find it practically useful.
Right. Thanks for explaining this. Although, it would appear that Python also
has a mechanism for detecting that it is being run from a build environment
and ignore PYTHONHOME in that case too.
> Beyond that possible tweak, while I certainly wouldn't oppose any effort to
> clean up / document / make-optional Python's startup sys.path-setting
> behavior, I think it's mostly orthogonal to PEP 405, and I don't think it would
> be helpful to expand the scope of PEP 405 to include that effort.
Well, it sounds as this pep can definitely be used as the basis for work to
completely customize the startup behaviour.
In my case, it would be desirable to be able to completely ignore any
PYTHONHOME environment variable (and any others). I'd also like to be able
to manually set up the sys.path.
Perhaps if we can set things up that one key (ignore_env) will cause
the environment variables to be ignored, and then, an empty home
key will set the sys.path to point to the directory of the .cfg file.
Presumably, this would then cause a site.py found at that place
to be executed and one could code whatever extra logic one
wants into that file.
Possibly a "site" key in the .cfg file would achieve the same goal, allowing
the user to call this setup file whatever he wants.
With something like this in place, the built in behaviour of python.exe
to realize that it is running from a "build" environment and in that case
ignore PYTHONPATH and set a special sys.path, could all be removed
from being hardcoded into being coded into some buildsite.py in the
cpython root folder.
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