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

[Paul Moore]

On 15 February 2015 at 18:25, Petr Viktorin <encukou at gmail.com> wrote:
> 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".

Windows doesn't have "python2" or "python3" commands, just "python".
To choose which version you use, set PATH or use an explicit path to
the executable.

The launcher offers shebang handling for scripts, and special-cases
the following values for Unix compatibility:

    #!/usr/bin/env python
    #!/usr/bin/python
    #!/usr/local/bin/python

They default to the "default Python" which is normally the latest
Python 2 version available, but which can be configured by the user.
They also support adding an explicit version (python2, python3,
python2.7, python3.4, ...)

As far as I know, this is *identical* behaviour to Unix - even to the
incredibly annoying (to me) choice of Python 2 as a default. So I
reconfigure the default in my personal settings to Python 3. Unix
users can do this too (although it may involve a symlink in a ~/bin
directory rather than an ini file change).

I see no reason for the zipapp module to buck the trend that the
default Python is "the latest 2.X version unless the user configures
it differently themselves". Hence the default of "#!/usr/bin/env
python". Certainly that means that users who use the default have to
be prepared for the fact that they are subject to user configuration
changes, and yes that includes running under either of Python 2 or 3.
If you don't like that, force a particular version with -p, and be
prepared for the *other* issues this may cause (user doesn't have that
version, for example).

The alternative, I guess, is to have *no* default and write no shebang
unless -p is specified. That matches how .py files work (you have to
type the shebang line in if you want it) and works the same way (on
Unix, a file without a shebang must be run with an explicit "python"
command, and on Windows it uses the default Python via the file type
association). Honestly, I'd be happy with this - the Windows
experience is identical, and it saves choosing something inappropriate
("in the face of ambiguity, refuse the temptation to guess" :-))

So, the options I see:

1. Stick with /usr/bin/env python
2. No shebang unless -p is specified
3. Unix users come up with a solution which is the same as the above
for Windows users, but which suits them better.

I don't see changing the Windows behaviour as an option.

If nobody comes up with anything for (3) I'll take the consensus
between (1) or (2). If there's no consensus, I'll pick something.
Probably 1, as why change if nobody has a better idea?

One final thought. This issue is no different from shebang lines in
.py scripts - the only real difference is that it's easy to see and
edit the shebang in a .py script. It may be that what's really needed
here is an option for the user to display and update the shebang line
in a pyz archive. I'll look at adding that to the PEP, regardless of
the conclusion to this debate.

Paul