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

Chris McDonough chrism at plope.com
Sat Oct 29 01:10:08 CEST 2011

This is really very comprehensive, thank you!

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.base_prefix to indicate the base prefix (as opposed to sys.site_prefix 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.

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