[Python-Dev] PEP 250, site-python, site-packages

[Guido van Rossum]

> Speaking as someone who uses Python day-to-day and occasionally worries
> about compatibility across Python versions: yes, it would be really nice
> if Python better supported multiple versions installed at the same time.
> lib/python1.5, lib/python2.0, and lib/python2.1 just don't cut it: I
> remember running 1.5.1, 1.5.2, and alpha/beta versions of 1.6
> simultaneously.  I had to install each to a separate prefix, which was
> ugly but workable.  It would be nice if Python (and, yes, the Distutils
> now) had better native support for multiple simultaneous versions.

That was mostly because we were abusing the version numbering scheme
to roll out feature releases with a micro version number (1.5.1,
1.5.2).  We don't do that any more -- feature releases have a minor
(middle) version number change.

If you really need to distinguish Python 2.0 and 2.0.1 on the same
system, you're a Python developer by definition. :-)

> Speaking as the main perpetrator of the Distutils: AAUUGGHGHHHH!!!!
> NOOOOO!!!  Please, don't make me look at this stuff AGAIN!!!!
> Aiiieeee!!

BTW, Greg, there's this bug I've found in Distutils, but the margin of
this email isn't wide enough to describe it. :-)

> But seriously: I think I once attempted to convince Guido that a
> revamped organization of the library directories would be a good idea,
> and that the Distutils would be a good way to introduce that scheme.
> Obviously, I didn't convince him, so we still have the same system.

Which I think isn't so bad given that we now have a well-behaved
versioning policy in place.

> The one glimmer of good news is that the Distutils "install" command
> is insanely flexible; if you can manage to wrap your head around the
> 17,000 levels of indirection, it should be a simple matter of
> changing a few hard-coded dictionaries (there are two for Unix, and
> one each for Windows and Mac OS) to introduce a completely new
> installation scheme.  I probably had some expectation that someday
> this discussion would open up again.
> 
> BTW, I'm skeptical about keeping .py and .pyc code in a
> non-Python-version-specific directory (ie. site-python).  Debian's
> bytecode-recompilation at installation time scheme sounds cool, but the
> desire/need to have multiple Python versions available kind of nixes it.
> Bummer.

Yes, good point.  Bytecode is generally not compatible between
versions -- its specification is considered an internal detail of the
implementation (again, it can't vary with a micro-version, but it can
and usually does vary with the minor version number).

> Oh yeah, another thing I vaguely recall from the pre-Distutils-0.1 era:
> Guido doesn't (didn't?) like site-python and wanted to deprecate it.
> Perhaps the above paragraph explains why.

Indeed, /usr/local/lib/python<major>.<minor>/site-packages/ is where
site packages should go.

--Guido van Rossum (home page: http://www.python.org/~guido/)