[Python-ideas] Migration of /usr/bin/python to python3

[M.-A. Lemburg]

On 11.03.2015 23:45, Andrew Barnert wrote:
> On Mar 11, 2015, at 1:38 PM, M.-A. Lemburg <mal at egenix.com> wrote:
>>
>>> On 11.03.2015 21:03, David Mertz wrote:
>>> https://www.python.org/dev/peps/pep-0394/
>>
>> I think the migration should start with modifying scripts to use
>>
>> #!/usr/bin/env python2
>>
>> when they are Python 2 only (*) and
>>
>> #!/usr/bin/env python3
>>
>> when they are Python 3 only and
>>
>> #!/usr/bin/env python
>>
>> only when they support both Python 2 and 3.
> 
> Except that you need a step 0: Not every system out there is PEP 394-compliant. Most notably, the built-in Python on OS X has nothing named python2, and python is a wrapper executable that selects between python2.5, python2.6, and python2.7. Some of the popular third-party Python-with-extra-batteries installs still give you only python and maybe python2.7, but not python2. And most linux distros' built-in and standard repo Pythons are PEP 394-compliant, but "most Linux distros" is not the same thing as "all non-OS X Unix-like systems".
> 
> So if you want to move this forward, I think someone needs to create a central place with all the info on how far we are from 100% PEP 394 compliance (including but reports to each vendor, etc.) and start pressuring the vendors who don't comply.
> 
> In the case of Apple, unless you can get them to add a python2 executable in bug fix releases of 10.9 and 10.10, rather than just in 10.11, it'll be a long time before python2 becomes a portable command. (Even if you could, would dropping OS X 10.6-10.8 be acceptable today for most scripts?)

I won't comment on the Mac Python installations. There's a reason
why we've been using our own installations since the early days of
Max OS X :-)

In any case, if those binaries don't exist on the target system,
the user will get a nice error message saying e.g.:

python2: Command not found.

and even better: modern OSes will then point the user to the
missing packages on their system to make the script run again.

I think that's a lot more informative than

SyntaxError: Missing parentheses in call to 'print'

:-)

Anyway, the above is just what I recommend.

I'm not a big fan of doing setuptools like magic all over the
place when there's a clean solution right in front of you (ok,
one may need to take off those cool Python monkey-patching and
meta-class sunglasses occasionally to see it ;-)).

>> "Explicit is better than implicit" and all that Zen :-)
>>
>> Once that's done, switching the symlink is really a no-brainer.
>>
>> The recipe for this is easy too:
>>
>> 1. replace all "#!/usr/bin/env python" with "#!/usr/bin/env python2"
>> 2. migrate your scripts one by one to either Python 3.x or
>>   to Python 2.7 + 3.4+
>> 3. after migration replace "#!/usr/bin/env python2" with
>>   "#!/usr/bin/env python3" or "#!/usr/bin/env python" resp.
>>
>> (*) Some OSes may require to use python2.7, if they don't come
>> with a symlink from python2 -> python2.7.

-- 
Marc-Andre Lemburg
eGenix.com

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