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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Tue, 6 Feb 2018 19:44:22 +0900
Message-ID: <CAK7LNAS_29myncTw+6u7fH9BgjzZJfXSN1CJjOjK-qXjPRQ8hQ@mail.gmail.com>
Subject: Re: [PATCH 06/14] kbuild: define PYTHON2 and PYTHON3 variables
 instead of PYTHON
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "Luis R . Rodriguez" <mcgrof@suse.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Ulf Magnusson <ulfalizer@gmail.com>,
        Sam Ravnborg <sam@ravnborg.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tony Luck <tony.luck@intel.com>, linux-ia64@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>
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2018-02-06 18:34 GMT+09:00 Greg Kroah-Hartman <gregkh@linuxfoundation.org>:
> On Tue, Feb 06, 2018 at 09:34:46AM +0900, Masahiro Yamada wrote:
>> The variable 'PYTHON' allows users to specify a proper executable
>> name in case the default 'python' does not work.  However, this does
>> not address the case where both Python 2 and Python 3 scripts are
>> used in one system.
>>
>> PEP 394 (https://www.python.org/dev/peps/pep-0394/) provides a
>> convention for Python scripts portability.  Here is a quotation:
>>
>>   In order to tolerate differences across platforms, all new code
>>   that needs to invoke the Python interpreter should not specify
>>   'python', but rather should specify either 'python2' or 'python3'.
>>   This distinction should be made in shebangs, when invoking from a
>>   shell script, when invoking via the system() call, or when invoking
>>   in any other context.
>>
>> arch/ia64/scripts/unwcheck.py is apparently written in Python 2, so
>> it should be invoked by 'python2'.
>>
>> It is legitimate to use 'python' for scripts compatible with both
>> Python 2 and Python 3, but this is rare (at least I do not see the
>> case in kernel tree).  You do not need to make efforts to write your
>> scripts in that way.  Anyway, Python 2 will retire in 2020.
>>
>> This commit is needed for my new scripts written in Python 3.
>>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>> ---
>>
>>  Makefile           | 5 +++--
>>  arch/ia64/Makefile | 2 +-
>>  2 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 11aff0f..c4e935c 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -384,7 +384,8 @@ GENKSYMS  = scripts/genksyms/genksyms
>>  INSTALLKERNEL  := installkernel
>>  DEPMOD               = /sbin/depmod
>>  PERL         = perl
>> -PYTHON               = python
>> +PYTHON2              = python2
>> +PYTHON3              = python3
>
> Is this going to break any systems that were previous setting PYTHON?
>
> I like this change, and feel it is the correct thing to do, but having a
> "fallback" might be needed here.
>
> Could you do what the perf makefile does and do something like:
>         override PYTHON := $(call get-executable-or-default,PYTHON,$(PYTHON2))
> or is it really not an issue as only ia64 seems to care about this?
>

As far as I see, ia64 is the only instance that has used this ever.

(the perf Makefile defines PYTHON by itself, so should not be a problem.)


If people expect the backward-compatibility for this, I can do like follows:

# backward compatibility for 'PYTHON'
PYTHON2  := $(if $(PYTHON), $(PYTHON), python2)


Another (unlikely) possible breakage is
'python2' may not be installed on users' system.

I believe this is rare, but if needed, I could do like follows
at the cost of ugliness.


PYTHON2 := $(if $(PYTHON), $(PYTHON), \
        $(shell python2 --version 2>/dev/null && echo python2 || echo python))


-- 
Best Regards
Masahiro Yamada