[Linux-SIG] Introducing a Python launcher for *nix?

[Nick Coghlan]

On 8 Oct 2015 08:09, "Fred Drake" <fred at fdrake.net> wrote:
>
> On Wed, Oct 7, 2015 at 4:17 PM, Piotr Ozarowski <ozarow at gmail.com> wrote:
> > If it's about letting users invoke any Python available (is pypy
> > included here, BTW?), then why do you need -2 or -3 or worse: -X.Y?
>
> I'm not sure what the original motivation for this proposal is; it
> appears to have been on a different list.

The first motive is consistent cross platform invocation - due to the way
Python invocation on Windows is handled, commands like "python3 -m pip" and
"python3.4 -m pip" don't work, and never will, and the differences in file
system layouts exclude the use of absolute paths.

The Python launcher for Windows *does* exist, though, so introducing a "py"
script on the *nix side of things offers the opportunity to provide a truly
cross platform Python experience modelled on the way that works:
https://docs.python.org/3/using/windows.html#python-launcher-for-windows

The second motive is to make it easy for individual users to switch to an
alternative implementation like PyPy as their default interactive shell
without impacting shebang lines and other automated invocations. I also
want to allow users to make that switch without needing admin rights on
their machine.

The third motive is simply to make -m invocations shorter ("py -m pip"
rather than "python -m pip").

>
> The notion that a script is going to figure out which Python is the
> right one just seems pretty strange to me.  When developing an
> application, I know exactly which Python I want to use, and build that
> application with that Python to begin with.

You're already a professional developer, though, rather than someone
learning about the command line and filesystems as part of learning to
program in Python.

> Any user-facing scripts
> get the right sh-bang line to begin with. If I'm building a library,
> I'm going to do things with several versions (tests can be driven with
> tox, for example), but library users are going to know what version of
> Python their application calls for, and they'll use it.

No, they don't necessarily know this. If folks don't want to take my word
for that, volunteering as a helper at a Software Carpentry bootcamp, a
Python for Young Coders workshop or another community event that introduces
children or adults to Python programming highlights just how radically
wrong these kinds of assumptions are in the context of people learning to
program, especially when instructors are needing to cope with a mixed
audience of Windows, OS X, and Linux users.

> I may be missing something pretty obvious to many, though.  My
> approaches to building an application don't appear to be commonly
> applied in Python environments.

Yeah, I failed to convey that the main intended beneficiaries are future
newcomers to Python, and the folks that are themselves using Linux or OS X,
but need to provide instructions that also work on Windows.

Cheers,
Nick.

>
>
>   -Fred
>
> --
> Fred L. Drake, Jr.    <fred at fdrake.net>
> "A storm broke loose in my mind."  --Albert Einstein
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