[Python-ideas] Make py.exe default to Python 3

[Oscar Benjamin]

On 9 March 2016 at 21:25, Chris Angelico <rosuav at gmail.com> wrote:
> On Thu, Mar 10, 2016 at 8:20 AM, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
>> I do not believe that this needs a PEP, a tracker item should be fine IMHO.
>> We should just ensure that the original authors of the PEP are aware, as
>> missing out their opinions would be a disservice to the Python Windows
>> community, as small as that may be when compared to *nix.
>
> Given that this is basically a Windows-only change, it doesn't matter
> how large or small the Windows community is - they're the people who
> primarily matter :) If someone proposed a change to the way things are
> done on OS/2, it'd be worth getting the opinions of both of the Python
> OS/2 users... :P

Although it is a change to Windows-only I think it is important to
think about the consistency between the way that this works on Windows
compared with other operating systems. Inconsistencies here can be
awkward for novices. Allow me to spell out my scenario...

I teach a couple of University programming units in which students
learn to program in Python, C and Java. The cohort for these units
this year was 160 students. I often find myself giving the students
instructions such as you can run this script I've given you with

    $ python myscript.py

Now imagine that you've given that instruction to 160 students mostly
using Windows and OSX (and a few Linux users). I find it very annoying
that this could invoke Python 2 or 3 depending on the OS etc that the
students are using. py.exe should solve this since I can write py -3
and then (I assume) it would lead to an error if no Python 3 version
is available. However that doesn't work on OSX or Linux.

Okay so maybe we should just use the shebangs. I can explain to all my
students what shebangs are and instruct the OSX students to set the
executable bit. Then I can say so just run

    $ ./myscript.py

however on Windows this uses file associations which is flaky. In the
past I've seen bugs where e.g. input/output redirection didn't work
for scripts run in this way. It also may not work in myriad other
contexts where a command line may be used. One example is from
Makefiles. So when my students are working in C I can give them a
Python script that will test the output of their C program and I can
tell them to add this to their Makefile e.g.:

    test: myprog.exe
         ./myscript.py

At this point (on Windows) various failures are possible. If make
simply calls CreateProcess it fails since myscript.py isn't an exe
file (I believe it also accepts .bat files but does not observe file
associations). If make inspects the shebang it may come to a very
different conclusion from the one that py.exe would have (e.g. that
python3 does not exist since it is not on PATH). Over time I've seen
this go wrong in a number of ways and have ultimately concluded that
anything involving using file associations is fragile.

Personally I would really like it if we could have improved
consistency across OSes when it comes to invoking Python. So I would
really like it if we can either have py (and maybe py3) *everywhere*
or if that's not possible find some solution that does work
everywhere. Can we have a single command line that guarantees to run
Python 3 (or error out)?

--
Oscar