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

[Andrew Barnert]

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?)

> "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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