On Fri, Sep 19, 2014 at 04:44:26AM -0400, Donald Stufft wrote:

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On Sep 19, 2014, at 3:31 AM, Bohuslav Kabrda <bkabrda@redhat.com> wrote:

Hi, as Fedora is getting closer to having python3 as a default, I'm being more and more asked by Fedora users/contributors what'll "/usr/bin/python" invoke when we achieve this (Fedora 22 hopefully). So I was rereading PEP 394 and I think I need a small clarification regarding two points in the PEP: - "for the time being, all distributions should ensure that python refers to the same target as python2." - "Similarly, the more general python command should be installed whenever any version of Python is installed and should invoke the same version of Python as either python2 or python3."

The important word in the second point is, I think, *whenever*. Trying to apply these two points to Fedora 22 situation, I can think of several approaches:

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- /usr/bin/python will always point to python3 (seems to go against the first mentioned PEP recommendation)

Definitely not that. Arch Linux pointed /usr/bin/python at Python 3 some years ago, and I understand that this has caused no end of trouble for the folks on #python. I haven't seen any sign of this being an issue on the tutor@ or python-list@python.org mailing lists, but the demographics are quite different so that's not surprising.

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- /usr/bin/python will always point to python2 (seems to go against the second mentioned PEP recommendation, there is no /usr/bin/python if python2 is not installed)

My understanding is that this is the intention of the PEP, at least until such time as Python 2 is end-of-lifed. My interpretion would be that the second recommendation in the PEP is just confused :-) Perhaps the PEP author could clarify what the intention is.

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- /usr/bin/python will point to python3 if python2 is not installed, else it will point to python2 (inconsistent; also the user doesn't know he's running and what libraries he'll be able to import - the system can have different sets of python2-* and python3-* extension modules installed)

Likely to cause all sorts of problems, and I understood that this was not the intention. Perhaps it was added *only* as a "grand-father clause" so that people don't yell at Arch Linux "See, the PEP says you're doing it wrong!".

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- there will be no /usr/bin/python (goes against PEP and seems just wrong)

Seems like the least-worst to me. If you think of "python == Python 2.x" (at least for the next few years), then if Python 2.x isn't installed, there should be no /usr/bin/python either.

I don’t know for a fact, but I assume that as long as Python 2.x is installed by default than ``python`` should point to ``python2``. If Python 3.x is the default version and Python 2.x is the “optional” version than I think personally it makes sense to switch eventually. Maybe not immediately to give people time to update though?

Agreed. Once Python 2 is finally end-of-lifed in 2023 or thereabouts, then we can reconsider pointing /usr/bin/python at Python 3 (or 4, whatever is current by then). If Arch Linux jumped the gun by a decade or so, that's their problem :-) -- Steven

----- Original Message -----

On 19 Sep 2014 17:38, "Bohuslav Kabrda" < bkabrda@redhat.com > wrote:

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- "Similarly, the more general python command should be installed whenever any version of Python is installed and should invoke the same version of Python as either python2 or python3."

The important word in the second point is, I think, *whenever*. Trying to apply these two points to Fedora 22 situation, I can think of several approaches: - /usr/bin/python will always point to python3 (seems to go against the first mentioned PEP recommendation) - /usr/bin/python will always point to python2 (seems to go against the second mentioned PEP recommendation, there is no /usr/bin/python if python2 is not installed)

I think this is what should happen, and the PEP is currently wrong. When writing the PEP, I don't think we accounted properly for the case where the "system Python" has migrated to Python 3, but the "default Python for end user scripts that don't specify otherwise" is still Python 2 (which is the migration strategy both Fedora and Ubuntu are adopting). Thanks, that was my thinking, too.

How does this sound as a possible revised recommendation (keep in mind I haven't checked this against the larger context yet):

"The more general python command should only be installed whenever the corresponding version of Python is installed (whether python2 or python3)." It seems to me that it is a bit unclear what "corresponding" is. Would it make sense to explicitly say that "python" command should be installed whenever the distro-chosen default system Python is installed?

Regards, Nick.

Thanks a lot -- Regards, Slavek Kabrda

On Sep 19, 2014, at 10:23 AM, Donald Stufft wrote:

My biggest problem with ``python3``, is what happens after 3.9.

FWIW, 3.9 by my rough calculation is 7 years away.

I know Guido doesn’t particularly like two digit version numbers and it’s been suggested on this list that instead of 3.10 we’re likely to move directly into 4.0 regardless of if it’s a “big” change or not.

If that is the case, then all of the user education, ui, etc around ``python3`` would then need to be again updated to ``python4`` *OR* we’d need a ``python3`` bin which points to ``python4``. If there’s a call to action for at some point moving ``python`` to invoke Python 3.x at some point then hopefully at that point Python 4.x could just be ``python``.

All of this assuming of course that 4.0 isn’t a major break like 3.0 and that we do 4.0 instead of 3.10 as has been suggested.

I seem to recall Guido saying that *if* there's a 4.0, it won't be a major break like Python 3, whatever that says about the numbering scheme after 3.9. Is 7 years enough to eradicate Python 2 the way we did for Python 1? Then maybe Python 4 can reclaim /usr/bin/python. Cheers, -Barry

"python" should always be the same as "python2". On Fri, Sep 19, 2014 at 8:03 AM, Steven D'Aprano <steve@pearwood.info> wrote:

On Fri, Sep 19, 2014 at 10:41:58AM -0400, Barry Warsaw wrote:

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On Sep 19, 2014, at 10:23 AM, Donald Stufft wrote:

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My biggest problem with ``python3``, is what happens after 3.9.

FWIW, 3.9 by my rough calculation is 7 years away.

That makes it 2021, one year after Python 2.7 free support ends, but two years before Red Hat commercial support for it ends.

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I seem to recall Guido saying that *if* there's a 4.0, it won't be a major break like Python 3, whatever that says about the numbering scheme after 3.9.

Is 7 years enough to eradicate Python 2 the way we did for Python 1? Then maybe Python 4 can reclaim /usr/bin/python.

I expect not quite. Perhaps 10 years though.

-- Steven _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/guido%40python.org

-- --Guido van Rossum (python.org/~guido)

On Sep 19, 2014 8:36 AM, "Antoine Pitrou" <solipsis@pitrou.net> wrote:

On Fri, 19 Sep 2014 08:20:48 -0700 Guido van Rossum <guido@python.org> wrote:

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"python" should always be the same as "python2".

"Always" as in "eternally"?

Until I say so. Which will happen in the distant future.

Barry Warsaw wrote:

On Sep 19, 2014, at 08:40 AM, Guido van Rossum wrote:

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Until I say so. Which will happen in the distant future.

I'm gonna hid your time machine keys so you didn't find them.

Hiding someone's time machine keys never works. Chances are he's already taken a trip to the future in which you get kidnapped and tortured until you reveal where you hid them, and then nipped over there to take them back. Which means he *might* be able to avoid carrying out the actual torture now, as long as it doesn't create too big a temporal paradox. -- Greg

On 20 September 2014 00:23, Donald Stufft <donald@stufft.io> wrote:

On Sep 19, 2014, at 10:16 AM, Barry Warsaw <barry@python.org> wrote:

If the user wants to invoke Python 3, it's not hard to type 'python3' and I think that's the message we should be spreading. That already seems pretty ingrained in user habits afaict.

My biggest problem with ``python3``, is what happens after 3.9. I know Guido doesn’t particularly like two digit version numbers and it’s been suggested on this list that instead of 3.10 we’re likely to move directly into 4.0 regardless of if it’s a “big” change or not.

FWIW, I think we actually do this better on Windows these days, where PEP 397 made "py" a switchable entry point. I'd like to bring the same scheme to POSIX systems at some point, but that's a *long* way down the todo list (like, so far down I can't even see it any more). I'd be willing to review proposals, though :) Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia

On 30 September 2014 20:13, Bohuslav Kabrda <bkabrda@redhat.com> wrote:

----- Original Message -----

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On 20 September 2014 00:23, Donald Stufft <donald@stufft.io> wrote:

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On Sep 19, 2014, at 10:16 AM, Barry Warsaw <barry@python.org> wrote:

If the user wants to invoke Python 3, it's not hard to type 'python3' and I think that's the message we should be spreading. That already seems pretty ingrained in user habits afaict.

My biggest problem with ``python3``, is what happens after 3.9. I know Guido doesn’t particularly like two digit version numbers and it’s been suggested on this list that instead of 3.10 we’re likely to move directly into 4.0 regardless of if it’s a “big” change or not.

FWIW, I think we actually do this better on Windows these days, where PEP 397 made "py" a switchable entry point. I'd like to bring the same scheme to POSIX systems at some point, but that's a *long* way down the todo list (like, so far down I can't even see it any more). I'd be willing to review proposals, though :)

We did a similar thing with Ruby in Fedora, it's called Rubypick [1]. Is that a direction worth pursuing?

Yes, I think so. The general idea would be to take the PEP 397 launcher CLI and propose a similar API for *nix systems: https://docs.python.org/3/using/windows.html#python-launcher-for-windows

Also, what is actually result of this thread? AFAICS the PEP still hasn't changed. IMO when there's only python3 installed, there should be no /usr/bin/python, which also seems to be aligned with what Guido says. Would it be possible to update the PEP accordingly?

Thanks for the prompt. I just pushed an update [1,2] to merge the previously incorrect bullet point with the one following it, so it now reads: * The more general ``python`` command should be installed whenever any version of Python 2 is installed and should invoke the same version of Python as the ``python2`` command (however, note that some distributions have already chosen to have ``python`` implement the ``python3`` command; see the `Rationale`_ and `Migration Notes`_ below). I also pushed a few tweaks to account for the extension of Python 2.7 maintenance, and to change the verb tense to reflect the fact this was implemented ages ago [3]. Cheers, Nick. [1] https://hg.python.org/peps/rev/3d16b0cd10bc [2] https://hg.python.org/peps/rev/32b6619e9259 [3] https://hg.python.org/peps/rev/0418f146b50f -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia

Donald Stufft wrote:

My biggest problem with ``python3``, is what happens after 3.9.

Python2 technically includes 1.x versions as well, so it wouldn't be unprecedented for python3 to imply versions beyond 3.x. It would be a bit confusing, though. -- Greg

On Fri, 19 Sep 2014 10:16:20 -0400, Barry Warsaw <barry@python.org> wrote:

The way I look at it is that "/usr/bin/python" is user interface. Distributions are completely free to choose whichever Python they want for system scripts, and it's great to see that Fedora is well on their way to making Python 3 the default for system scripts. Debian is also making good progress, though we likely won't complete the transition until Jessie+1.

Yep, user interface. Therefore I think the distribution should let the user choose :) Most users will chose python2, because that's what other scripts they might download and install will expect. So that should be the default. But I'll choose python3 (and have, on most of my gentoo linux systems, which supports having both quite well, for system installed packages). On the gripping hand, given what Fedora is doing, it is not that hard to change (or create) the symlinks as an end user to point to the python3 programs even if Fedora doesn't support it directly, so I don't see a problem with the proposed strategy.

If the user wants to invoke Python 3, it's not hard to type 'python3' and I think that's the message we should be spreading. That already seems pretty ingrained in user habits afaict.

As Donald pointed out, that presents a problem for the future. But since there is no good alternative to putting /usr/bin/python3 in the shebang of scripts that require python3, I don't think there is a solution and we're just going to have to cross that further bridge when we come to it. --David