[Python-Dev] Another update for PEP 394 -- The "python" Command on Unix-Like Systems

[Matthias Klose]

On 16.02.19 00:15, Gregory P. Smith wrote:
> On Thu, Feb 14, 2019 at 9:29 AM Barry Warsaw <barry at python.org> wrote:
> 
>> On Feb 13, 2019, at 23:08, Matěj Cepl <mcepl at cepl.eu> wrote:
>>
>>> Is this relevant to the discussion at hand? We are talking about
>>> the binary /usr/bin/python3 which will be surely be provided
>>> even by Python 4, won't it?
>>
>> Why would it be?  Since this is all hypothetical anyway <wink>, I’d more
>> likely expect to only ship /usr/bin/python.
>>
> 
> Because nobody can use 'python' and expect that to be anything but a 2and3
> compatible interpreter for the next 5+ years given we live in a world where
> people routinely have a very real need to write #! lines that works with
> obsolete distributions.  python3 implies >=3.x, thus python 4, 5, 6, 2069,
> 3001, and 90210 should all have python3 point to them.  realistically
> people will stop referring to python3 by 2069 so we could consider removing
> the recommendation at that point.
> 
> 2020 is not the end of use or end of importance for Python 2.  It is merely
> the end of bugfixes applied by python-dev.
> 
> A thing I want to make sure we _don't_ do in the future is allow future
> pythonN binaries.  python4, python90210, etc.  those should never exist.
> python, python3, and pythonX.Y only.  If we were ever to go back on our
> promise and create another world-breaking python version, it could get its
> own canonical binary name.  But we're specifically planning _not_ to make
> that mistake again.
> 
> I suspect most of my opining below will be contentious to multiple people
> because I describe a state of the world that is at conflict with decisions
> multiple independent distros have already made.  Accept their mistakes and
> move on past it to the hack in that case:
> 
> A new "py" launcher isn't going to solve this problem - it is separate and
> should be its own PEP as it has its own set of requirements and reasons to
> be considered (especially on platforms with no concept of a #!).  Recommend
> "py" today-ish and nobody can rely on it for at least 10+ years in a wide
> support cross platform scripting type of situation because it won't be
> present on the obsolete or long term supported things that people have a
> need for such #!s to run on.
> 
> Not our problem?  Well, actually, it is.  Matthias speaking for Debian
> suggesting they don't want to have "python" at all if it isn't a synonym
> for "python2" because it'll break software is... wrong.  If software is not
> 3 compatible and uses "python", it'll also break when python is python3.
> Just in a different manner.  "python" should point to python3 when a distro
> does not require python2 for its core.  It should not _vary_ as to which of
> 2.7 or 3.7 it will point to within a given stable distribution (installing
> python2.7 should never suddenly redirect it back to python2).  But "python"
> itself should always exist when any python interpreter is core to the OS.
> That means if a distro no longer requires python2 as part of its base/core
> but does require python3... "python" must point to "python3".  If a posixy
> OS no longer requires python at all (surely there are some by now?) the
> question of what python should point to when an OS distro supplied optional
> python package gets installed is likely either "nothing at all" or ">=3.x"
> but should never be decided as "2.7" (which sadly may be what macOS does).

There is no notion of a "core" for Debian. So "core" applies to the whole
distro, as long as there are python shebangs found.

For Ubuntu, you don't have a python command on the default desktop install, just
python3.  Trying to invoke python, command-not-found tells you:

  $ python

  Command 'python' not found, but can be installed with:

  [...]

  You also have python3 installed, you can run 'python3' instead.

That tells you which way to go.

> Do we already have LTS _stable_ distributions making that mistake today?
> If so they've done something undesirable for the world at large and we're
> already screwed if that distro release is deemed important by masses of
> users: There is no way to write a *direct* #! line that works out of the
> box to launch a working latest version Python interpreter across all
> platforms.

If you count the above example towards this "mistake", probably yes. But there
is *no* way to have a sane way to have what you want.

Matthias