[Python-Dev] draft PEP: virtual environments (original) (raw)

Carl Meyer carl at oddbird.net
Mon Oct 31 15:06:26 CET 2011

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On 10/28/2011 05:10 PM, Chris McDonough wrote:

Why not modify sys.prefix? - --------------------------

As discussed above under Backwards Compatibility, this PEP proposes to add sys.siteprefix as "the prefix relative to which site-package directories are found". This maintains compatibility with the documented meaning of sys.prefix (as the location relative to which the standard library can be found), but means that code assuming that site-packages directories are found relative to sys.prefix will not respect the virtual environment correctly. Since it is unable to modify distutils/sysconfig, virtualenv is forced to instead re-point sys.prefix at the virtual environment. An argument could be made that this PEP should follow virtualenv's lead here (and introduce something like sys.baseprefix to point to the standard library and header files), since virtualenv already does this and it doesn't appear to have caused major problems with existing code. Another argument in favor of this is that it would be preferable to err on the side of greater, rather than lesser, isolation. Changing sys.prefix to point to the virtual environment and introducing a new sys.baseprefix attribute would err on the side of greater isolation in the face of existing code's use of sys.prefix. It would seem to make sense to me to err on the side of greater isolation, introducing sys.baseprefix to indicate the base prefix (as opposed to sys.siteprefix indicating the venv prefix). Bugs introduced via a semi-isolated virtual environment are very difficult to troubleshoot. It would also make changes to existing code unnecessary. I have encountered no issues with virtualenv doing this so far.

I'm convinced that this is the better tradeoff. I'll begin working on a branch of the reference implementation that does things this way. Thanks for the feedback.

Carl -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/

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