[Python-Dev] PEP 405 (built-in virtualenv) status (original) (raw)

Christian Tismer tismer at stackless.com
Sat Jun 2 19:33:20 CEST 2012

On 21.03.12 14:35, Kristján Valur Jónsson wrote:

-----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.execprefix, 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 (ignoreenv) 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.

As an old windows guy, I very much agree with Kristjan. The venv approach is great. Windows is just a quite weird situation to handle in some cases, and a super-simple way to get rid of any built-in behavior concerning setup would be great.

The idea of moving path setup stuff into the python.exe stub makes very much sense to me. This would make pythonxx.dll a really useful library to be shared.

Kristjan can then provide his own custom python.exe and be assured the python dll will not try to lurk into something unforeseen. I think this would also be a security aspect: The dll can be considered really safe for sandboxing if it does not even have the ability to change the python behavior by built-in magic.

Besides that, I agree with Ethan that explicit is better than implicit, again. I am missing even more explicitness:

Python has IMHO too much behavior like this: 'by default, look into xxx, but if a yyy exists, behave differently'. I don't like this, because the absense of a simple file changes the whole system behavior. I would do it the other way round: As soon as you introduce the venv.cfg file, enforce its existence completely! If that file is not there, then python exits with an error message. This way you can safely ensure its existence, and the file can be made read-only and so on. A non-existent file is just a bad thing and is hard to make read-only ;-) So please let's abandon the old 'if exists ...' pattern, at least this one time. By the explicit cfg file, the file can clearly say if there is a virtual env or not.

Together with removing magic from the .dll, the situation at least for windows would greatly improve.

ciao - chris

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