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Thanks for your battle-tested feedback, Mr. Bicking! I reply inline.
> The biggest problem is that the environmental variable is inherited by
> subprocesses. [...] Another problem is that scripts aren't really
> sticky with respect to the environment. [...]
> These are real-world problems I encountered with workingenv, and
> virtualenv has resolved them very reliably by instead using
> sys.executable to select the environment.
Excellent points. PYTHONPREFIXES is not a virtualization cure-all; for
a complete solution you clearly need an executable file for folks to
hang their hat on.
However: wouldn't PYTHONPREFIXES greatly simplify virtualenv? All you
should need is an executable and a "sitecustomize" module. Your
executable would set PYTHONPREFIXES as makes sense and run Python. Your
usercustomize would:
 * set sys.executable,
 * set sys.prefix (and maybe sys.exec_prefix),
 * *unset* PYTHONPREFIXES, and
 * run the user's real sitecustomize if present.
If this works reliably it would obviate most of the work virtualenv
currently has to do.
In fact I just tried this. A two-line shell script for "python3", a
twenty-line "sitecustomize.py", installed lib/python3.1/config/Makefile
and include/python3.1 in my virtualized environment, and I was able to
install HeapDict for python3 using its setup.py. (PyPI lies, though;
HeapDict doesn't work unmodified in python3. It calls callable(). But
once I fixed that it ran fine.) So it passes a smoke-test at least.
> Also, it's hard to mix and match environments in this system.
How so? I thought this proposal made it far easier to mix and match
environments. PYTHONPREFIXES is a stack; push and pop environments as
you like.
> Also with respect to the patch, for consistency there needs to be
> changes to distutils to make use of this variable. PYTHONUSERBASE
> included changes so that you can install based on that variable.
Good call. I found it infuriating that setuptools didn't (still doesn't
iirc) understand PYTHONUSERBASE, and would complain that the --prefix
directory doesn't support .pth files. If this patch goes further I'll
fix up distutils.
Thanks again for your feedback!