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

Oscar Benjamin oscar.j.benjamin at gmail.com
Tue Mar 15 08:38:48 EDT 2016 
On 15 March 2016 at 12:01, Paul Moore <p.f.moore at gmail.com> wrote:
> On 15 March 2016 at 11:25, Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:
>> AFAICT the default behaviour of the Windows installer does not add
>> Python to PATH. Correct me if I'm wrong as I haven't used it myself
>> for some time but when I tell my students to "install Python 3.5" and
>> then to type "python" in the terminal a lot of the Windows users come
>> back to me with PATH problems (specifically that python was not added
>> to PATH).
>
> I would have thought that is relatively easy to resolve. A pain to
> have to do in a classroom environment, certainly, but manageable.

It's easy enough to resolve if they bring their computer directly to
me. It's less easy to resolve by email. It just becomes something many
students waste time trying to fix when they're not yet ready to
meaningfully learn from the experience.

> The
> reasons Python is not added to PATH are something that has been
> debated over and over again on various lists. As far as I know,
> neither option is ideal, but "not on PATH by default" is typically
> seen as the least likely to fail in confusing ways.

Why would we not want python on PATH? If it's because of having
multiple installed Pythons then maybe it's possible to improve the
installer so that it gives the user a selection of which is on PATH
and adds it by default if it isn't already there. A simple rule: "if
Python is not on PATH then the installer assumes that the user wants
to add it" would suffice.

> You could (relatively easily) write a small Windows Scripting Host
> script to locate the version of Python you wanted people to use via
> the registry, and add it to the front of PATH. That would need
> elevation to work, of course, but would give a known-good starting
> point.

I would probably have more success telling them to tick the "add to
PATH" box on the installer :)

(A proportion would get it wrong whatever I say though)

> On Unix, tell people just "run python3".

It's not always a question of telling people what command to run. The
particular scenario that just came up is that of a C program with a
Makefile and a Python script that tests the output of the C program.
So I give the students a skeleton project that looks like

proj-1.0.zip/
    proj-1.0/
        projmain.c
        Makefile
        tests/
            runtests.py

The idea is that they should fill in the C code to complete a program.
The runtests.py script runs their program testing input/output
combinations and scores how much they have completed.

Now in Makefile I have

test: proj.exe
    python tests/runtests.py

So the students can run "make test" to rebuild their project and run
the tests. The question is how to set this up when "python" is not
guaranteed to be on PATH and could be many different versions of
Python. ATM I have to exhaustively test all combinations of Python 2.7
and 3.5 and Windows, OSX and Linux and then I still hit up against
problems because the way in which they've installed Python is not the
same as the machines I tested on.

Testing the output of a subprocess in 2/3 compatible code is painful
because it comes out as bytes in 3 and str in 2. Really the only
necessary incompatibility that I should need to deal with in this
scenario is Windows line endings. All the others like which version of
python will run, whether or not python is on PATH and the 2/3
incompatibilities are all down to Python rather than the inherent
variabilities of the OSes.

--
Oscar
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