Re: [Python-Dev] Clarification of PEP 394 for scripts that run under Python 2 and 3

[Nick Coghlan]

On 14 November 2015 at 08:57, Barry Warsaw  wrote:
> On Nov 13, 2015, at 10:32 PM, Damien George wrote:
>
>>1. What is the true intent of PEP 394 when only Python 3 is installed?  Is
>>"python" available or not to run scripts compatible with 2.x and 3.x?
>>
>>2. Is it possible to write a shebang line that supports all variations of
>>Python installations on *nix machines?
>>
>>3. If the answer to 2 is no, then what is the recommended way to support
>>all Python installations with one standalone script?
>
> It's important to remember that PEP 394 is an informational PEP, still under
> active (if dormant) discussion.  It is also just a recommendation, and can't
> force any downstream redistributors to do one thing or another.  Still, the
> intent is to provide a set of guidelines for the majority of *nix distributors
> to (eventually) adopt.
>
> I'll also point you to the recently created linux-sig where this topic is
> being discussed.
>
> https://mail.python.org/pipermail/linux-sig/2015-October/00.html
>
> As you've noticed, Arch took a particular approach, but speaking as part of
> the Debian/Ubuntu community, don't expect that ecosystem to go down the same
> path.  It's highly unlikely /usr/bin/python will ever point to anything other
> than Python 2, even when only Python 3 is installed by default.  That might
> change once Python 2.7 is actually EOL'd, and that won't happen for quite a
> long time.  PEP 373 currently says there will be bug fix releases until 2020,
> and I'd expect security-only releases for some time after that.

This is the same situation for the Fedora/RHEL/CentOS ecosystem - for
the time being, we think failure -> "yum install /usr/bin/python" is
the cleanest user experience we can readily offer for Python 3 only
systems. However, we don't think it's an acceptable long term answer,
and something *else* needs to be done with /usr/bin/python than just
making it a symlink to Python 3 (since the latter will lead to cryptic
error messages for Python 2 only scripts).

> In general, the discussions on linux-sig and elsewhere is coalescing around a
> launcher-type approach, where you'd put something like /usr/bin/py in your
> shebang if your script really is bilingual.  Then `py` can try to figure out
> what's available on your system, what your preference is, etc. and then
> execute your script using that version-specific binary.  I would expect the
> launcher to eventually be provided by the upstream Python development
> community.

A couple of other links folks may find worth reading for background:

* LWN summary of the discussion at this year's language summit:
https://lwn.net/Articles/640296/
* Geoffrey Thomas's launcher concept:
https://ldpreload.com/blog/usr-bin-python-23

This thread prompted me to do an update pass over
https://wiki.python.org/moin/Python3LinuxDistroPortingStatus, adding
these background links to the Python symlink section, and updating the
Fedora porting status link to refer to the recently published porting
DB: http://portingdb-encukou.rhcloud.com/

Regards,
Nick.

-- 
Nick Coghlan   |   ncogh...@gmail.com   |   Brisbane, Australia
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Re: [Python-Dev] Clarification of PEP 394 for scripts that run under Python 2 and 3

[Barry Warsaw]

On Nov 13, 2015, at 10:32 PM, Damien George wrote:

>1. What is the true intent of PEP 394 when only Python 3 is installed?  Is
>"python" available or not to run scripts compatible with 2.x and 3.x?
>
>2. Is it possible to write a shebang line that supports all variations of
>Python installations on *nix machines?
>
>3. If the answer to 2 is no, then what is the recommended way to support
>all Python installations with one standalone script?

It's important to remember that PEP 394 is an informational PEP, still under
active (if dormant) discussion.  It is also just a recommendation, and can't
force any downstream redistributors to do one thing or another.  Still, the
intent is to provide a set of guidelines for the majority of *nix distributors
to (eventually) adopt.

I'll also point you to the recently created linux-sig where this topic is
being discussed.

https://mail.python.org/pipermail/linux-sig/2015-October/00.html

As you've noticed, Arch took a particular approach, but speaking as part of
the Debian/Ubuntu community, don't expect that ecosystem to go down the same
path.  It's highly unlikely /usr/bin/python will ever point to anything other
than Python 2, even when only Python 3 is installed by default.  That might
change once Python 2.7 is actually EOL'd, and that won't happen for quite a
long time.  PEP 373 currently says there will be bug fix releases until 2020,
and I'd expect security-only releases for some time after that.

In general, the discussions on linux-sig and elsewhere is coalescing around a
launcher-type approach, where you'd put something like /usr/bin/py in your
shebang if your script really is bilingual.  Then `py` can try to figure out
what's available on your system, what your preference is, etc. and then
execute your script using that version-specific binary.  I would expect the
launcher to eventually be provided by the upstream Python development
community.

AFAIK, there have been a few stabs at writing such a launcher, but nothing
that's far enough to be usable.

So for now, you don't have great options.

Cheers,
-Barry
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[Python-Dev] Clarification of PEP 394 for scripts that run under Python 2 and 3

[Damien George]

Hi python-dev,

We have a Python script that runs correctly under Python 2.6, 2.7 and 3.3+.
It is executed on a *nix system using the "python" executable (ie not
python2 or python3 specifically). This works just fine for systems that
have Python 2 installed, or 2 and 3, or just 3 and symlink "python" to
"python3" (eg Arch Linux).

But it fails for systems that have only Python 3 and do not create a
"python" symlink, ie only "python3" exists as an executable.

We thought that PEP 394 would come to the rescue here but it seems to be
unclear on this point. In particular it says:

- 4th point of the abstract: "so python should be used in the shebang line
only for scripts that are source compatible with both Python 2 and 3"

- 6th point of the recommendation section: "One exception to this is
scripts that are deliberately written to be source compatible with both
Python 2.x and 3.x. Such scripts may continue to use python on their
shebang line without affecting their portability"

- 8th point in the migration notes: "If these conventions are adhered to,
it will become the case that the python command is only executed in an
interactive manner as a user convenience, or to run scripts that are source
compatible with both Python 2 and Python 3."

Well, that's pretty clear to me: one can expect the "python" executable to
be available to run scripts that are compatible with versions 2.x and 3.x.

The confusion comes because there are systems that install Python 3 without
creating a "python" symlink (hence breaking the above).  And Guido said
that "'python' should always be the same as 'python2'" (see
https://mail.python.org/pipermail/python-dev/2014-September/136389.html).
Further, Nick Coghlan seemed to agree that "when there's only python3
installed, there should be no /usr/bin/python" (see
https://mail.python.org/pipermail/python-dev/2014-September/136527.html).

My questions are:

1. What is the true intent of PEP 394 when only Python 3 is installed?  Is
"python" available or not to run scripts compatible with 2.x and 3.x?

2. Is it possible to write a shebang line that supports all variations of
Python installations on *nix machines?

3. If the answer to 2 is no, then what is the recommended way to support
all Python installations with one standalone script?

Thanks!

Regards,
Damien.
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