[Python-Dev] PEP 441 - Improving Python ZIP Application Support

[Petr Viktorin]

On Sun, Feb 15, 2015 at 5:43 PM, Paul Moore <p.f.moore at gmail.com> wrote:
> On 15 February 2015 at 16:15, Petr Viktorin <encukou at gmail.com> wrote:
>> On Sun, Feb 15, 2015 at 2:21 PM, Paul Moore <p.f.moore at gmail.com> wrote:
>>> So the usage would be something like
>>>
>>>     python -m zipapp [options] dir_to_zip
>>>
>>> Options:
>>>     -p <interpreter>    The interpreter to use on the shebang line
>>> (defaulting to /usr/bin/env python)
>>
>> On many systems this default would mean Python 2. Even if the official
>> recommendation changes for 3.5, the status quo might linger for a few
>> years.
>> On the other hand, the number of distros that don't ship Python 3 is
>> small, and the reason they're slow-moving tends to be stability and/or
>> compliance, so they're not the target audience for zipapp. And the
>> python3 symlink is not going away any time soon.
>> So I'd suggest `/usr/bin/env python3` for the default.
>
> But that will fail for users who only have Python 2 installed. And it
> won't pick up an activated virtualenv (assuming virtualenvs on Unix
> don't include a python3 command, which I believe is true - it
> certainly won't on Windows).

Yes, it would fail with only Python 2 installed. I don't think that's a problem.

On POSIXy systems the "python3" symlink is created in all venvs. I
thought (perhaps naïvely) that Windows doesn't do shebangs natively,
so there's some Python-specific mechanism around them, which should
handle "python3".

If the app was single-source compatible with both 2 and 3, then
`/usr/bin/env python` would be a good choice. But I don't think apps
(as opposed to libraries) have much incentive to keep the backwards
compatibility.

> As a Windows user, I believe that the command "python" should run the
> version of Python that you choose to be your default. I know it's not
> quite that simple on Unix, and the system "python" command is
> typically Python 2, with "python3" for the system Python 3 version,
> but I also know that tools like pyenv work like that. Equally, I've
> seen lots of scripts with "#!/usr/bin/env python" as the shebang line,
> and none with "#!/usr/bin/env python3". That may just be my limited
> experience with Unix though.

On Linux, what version "python" is depends on the distro. Currently
the recommendation from Python upstream is for it to be Python 2,
though, and I know of just one distro that does not do this (Arch,
because it switched before the recommendation was put in place).

If you're in a venv, then of "python" is that venv's Python, of
course. But the default mode of operation for Python-unaware users is
to not be in a venv.

> I don't really want "#!/usr/bin/env python3" as the default shebang on
> Windows (at a minimum, it would do the wrong thing for my current
> environment). I'm open to advice from the Unix users as to how to set
> things up better on Unix, but there's a whole new set of problems that
> will arise as soon as we have different defaults on Unix and Windows,
> so IMO there would need to be pretty significant benefits to justify
> doing that.

I'm not familiar with how the shebang works on Windows, but if
"/usr/bin/env python3" breaks things, I find it highly unfortunate.

> And of course it *is* only a default - users can set whatever they
> want. (But getting defaults right is important, so that's not to
> dismiss the point).

I'm afraid the point is whether it's even possible to have a shebang
that does the right thing on both platforms.