[Python-Dev] Python install layout and the PATH on win32 (original) (raw)

VanL van.lindberg at gmail.com
Tue Mar 20 20:22:55 CET 2012

On 3/20/2012 1:56 PM, Paul Moore wrote:

This is covered (better, IMO) by PEP 397 - Python Launcher for Windows. Step 2, just run "py". If you prefer a particular version, run "py -2" or "py -3" or "py -3.2".

Interesting. I haven't played around with that at all, so can't comment. I will have to try it out.

Adding python.exe to PATH actually makes this message worse as it confuses the issue. In a post-PEP 397 world, I would say that we should be telling Windows users not to run python.exe at all. (Unless they are using virtualenvs - maybe PEP 397 could do with an extra option to run the Python from the currently active virtualenv, but that's a side issue).

I think that having the PATH manipulation be optional might address this issue. I also think that the PEP 397 launcher should respect virtualenvs

For your steps 3 and 4, there is certainly user intervention required as things stand. It would indeed be nice if "regetron" just worked as expected. But I'd argue a few points here:

1. On Windows, if regetron was not explicitly an application for working with my Python installation (like pip, easyinstall, nose, etc) then I'd prefer it to be packaged as a standalone application using cxFreeze or py2exe. I've had too many "applications" break because I accidentally uninstalled a dependency from my Python installation to want anything that is an end-user application installed into my Python scripts/bin directory.

Maybe so - and I would probably agree that for any packaged application, bundling it into its own environment (or at least its own virtualenv) is the best practice. But what about pip, easy_install, nose, cython, pygments, PIL, etc, that do this and are meant to be associated with a particular python version? Substitute "nose" for "regetron" if you want, and there is still a problem.

The problem with your example is that it depends on the package/executable. I looked at regetron (I thought it was a made up example at first!)

...! I got the name from the feedback I received. I thought it was made up too.

I have no idea what proportion of Python users on Windows have multiple versions installed. I also have no idea how many such users work on the command line. My guess would be "not that many" and "not that many of the first group" respectively... But there are big groups like scientists and web developers who could sway these figures a lot.

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.

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.

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