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

[Christian Tismer]

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.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.
>

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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