[Python-Dev] Proposed: add an environment variable, PYTHONPREFIXES

[Larry Hastings]

I've submitted a patch to implement a new environment variable, 
PYTHONPREFIXES.  The patch is here:

    http://bugs.python.org/issue5819

PYTHONPREFIXES is similar to PYTHONUSERBASE: it lets you add "prefix 
directories" to be culled for site packages. It differs from 
PYTHONUSERBASE in three ways:

* PYTHONPREFIXES has an empty default value. PYTHONUSERBASE has a
default, e.g. ~/.local on UNIX-like systems.

* PYTHONPREFIXES supports multiple directories, separated by the
site-specific directory separator character (os.pathsep). Earlier
directories take precedence. PYTHONUSERBASE supports specifying
at most one directory.

* PYTHONPREFIXES adds its directories to site.PREFIXES, so it reuses
the existing mechanisms for site package directories, exactly
simulating a real prefix directory. PYTHONUSERBASE only adds a
single directory, using its own custom code path.


This last point bears further discussion. PYTHONUSERBASE's custom code
to inspect only a single directory has resulted in at least one bug, if
not more, as follows:

* The bona-fide known bug: the Debian package mantainer for Python
decided to change "site-packages" to "dist-packages" in 2.6,
for reasons I still don't quite understand. He made this change in
site.addsitepackages and distutils.sysconfig.get_python_lib, and
similarly in setuptools, but he missed changing it in
site.addusersitepackages. This meant that if you used setup.py to
install a package to a private prefix directory, PYTHONUSERBASE had
no hope of ever finding the package. (Happily this bug is fixed.)

* I suspect there's a similar bug with PYTHONUSERBASE on the "os2emx"
and "riscos" platforms. site.addsitepackages on those platforms
looks in "{prefix}/Lib/site-packages", but
site.addusersitepackages looks in
"{prefix}/lib/python{version}/site-packages" as it does
on any non-Windows platform. Presumably setup.py on those two
platforms installs site packages to the directory site.addsitepackages
inspects, which means that PYTHONUSERBASE doesn't work on those
two platforms.

PYTHONUSERBASE's custom code path to add site package directories seems 
unnecessary to me. I cannot fathom why its implementors chose this 
approach; in any case I think reusing site.addsitepackages is a clear 
win. I fear it's too late to change PYTHONUSERBASE so it simply called 
site.addsitepackages, as that would change its established semantics.  
Though if that idea found support I'd be happy to contribute a patch.


A few more notes on PYTHONPREFIXES:

* PYTHONPREFIXES is gated by the exact same mechanisms that shut off
PYTHONUSERBASE.
* Specifying "-s" on the Python command line shuts it off.
* Setting the environment variable PYTHONNOUSERSITE to a non-empty
string shuts it off.
* If the effective uid / gid doesn't match the actual uid / gid it
automatically shuts off.

* I'm not enormously happy with the name. Until about an hour or two
ago I was calling it "PYTHONUSERBASES". I'm open to other
suggestions.

* I'm not sure that PYTHONPREFIX should literally modify site.PREFIXES.
If that's a bad idea I'd be happy to amend the patch so it didn't
touch site.PREFIXES.

* Reaction in python-ideas has been reasonably positive, though I gather
Nick Coughlan and Scott David Daniels think it's unnecessary. (To
read the discussion, search for the old name: "PYTHONUSERBASES".)

* Ben Finney prefers a counter-proposal he made in the python-ideas
discussion: change the existing PYTHONUSERBASE to support multiple
directories. I don't like this approach, because:
a) it means you have to explicitly add the local default if you
want to use it, and
b) PYTHONUSERBASE only inspects one directory, whereas PYTHONPREFIX
inspects all the directories Python might use for site packages.
I do admit this approach would be preferable to no change at all.


The patch is thrillingly simple and works fine. However it's not ready 
to be merged because I haven't touched the documentation. I figured I'd 
hold off until I see which way the wind blows.

I'd also be happy to convert this into a PEP if that's what's called for.


/larry/