[Python-Dev] Python install layout and the PATH on win32

[Paul Moore]

On 20 March 2012 19:22, VanL <van.lindberg at gmail.com> wrote:
> There are a number of casual users that probably only have one version
> installed, but every python user/dev on windows that I know has one python
> that they consider to be "python," and everything else needs to be launched
> with a suffix (e.g., python26.exe). This is usually put earlier on the PATH
> so that it gets picked up first. For example, right now I have 2.6, 2.7,
> 3.2, jython, and pypy all installed, and I have "python" pointing to 2.7.

But no Python I am aware of *has* a suffixed version (python26.exe).
Renaming/copying is (in my view) a far more invasive change than
simply modifying PATH (and it doesn't help the whole nose/regetron
situation either).

Serious question: Given a brand new PC, if you were installing Python
2.7, 3.2, 3.3a1, jython, and pypy, what would you do (beyond simply
running 5 installers) to get your environment set up the way you want?

For me, I'd
1. Install the Python launcher (only until 3.3 includes it)
2. Edit py.ini to tailor py.exe to my preferred defaults for Python and Python3.
3. Install my powershell module which allows me to switch which Python
is on PATH

Done. (That doesn't cater for pypy or jython, as I don't use them. But
I'd probably use a couple of aliases for the rare uses I'd make of
them)

>>  I'd be curious as to how much PEP 397's py.exe would have helped
>>  those people. But yes, it's an issue. Although someone at some point
>>  will have to introduce those beginners to the question of Python 2 vs
>>  Python 3, and PATH pain will hit them then, anyway.
>
> I would imagine that it would help steps 1 and 2, but 3 and 4 would be
> problematic (how can you pip install something using py?) unless you were in
> a virtualenv, and then (unless py respected the virtualenv) the whole thing
> would be problematic, because there wouldn't be one clear way to do it.

There isn't one clear way right now. And adding one particular version
to PATH only helps if you only *have* one version.

My current preference is as follows:

1. If you only ever have one Python on your machine, add it (and its
scripts dir) to PATH and be done with it. Unfortunately, we're in the
throes of the Python 2-3 transition, and not many people can manage
with the one-Python restriction (I certainly can't). Also the Python
installer can't detect if that's what you want.
2. Otherwise, use virtualenvs for anything that isn't being packaged
up as a standalone environment. Activate as needed.
3. To access your system python(s) use py.exe with a version flag if
needed. Never (or nearly never) install packages in the system Python.
4. To run scripts, use #! lines and the py.exe association (and set
PATHEXT if you want) to associate the precise Python you want with the
script.

I have to say, I've recently discovered virtualenv, so the above is
the opinion of a newly-converted zealot - so take with a pinch of salt
:-)

Paul.