On 11/22/18 1:34 PM, Nick Coghlan wrote:
On Mon, 19 Nov 2018 at 21:21, Petr Viktorin <encu...@gmail.com> wrote:
(Wow, am I glad to be legally allowed to talk about this now!)
It does make it much easier than having to dance around what the plans
for RHEL 8 actually were (especially since they weren't the same as
Fedora's plans, so the usual escape hatch didn't work).
That leaves advantages of "py" to:
- "py" is shorter (and many people already use it as a custom shortcut!)
- "py" is consistent with Windows (which is would be great for
classes/workshops, provided it doesn't make installation harder)
- plus something about distributions like Anaconda, which I don't know
much about
The main advantage I see to "py" is that it's deliberately ambiguous
about the exact target version (and can be made to favour Python 3
from the start), whereas "python" has had a much longer history of
referring to some variant of Python 2.x.
It's also the case that if cross-distro Py 2/3 source-compatible
scripts start saying "/usr/bin/py", then it's straightforward to add a
symlink to older systems that resolves to the same target as
/usr/bin/python, even without adding the launcher.
I'd update the PEP to allow flat-out leaving out "/usr/bin/python". That
way, users will get the "command not found" error message, which I
believe to be the most straightforward, easy to spot, easy to fix and
easy to workaround way to get out of this mess.
My initial reaction was "Doesn't it already say that?", but the answer
is no, it doesn't [...]
At the moment, there's no clear answer as to what to put into the
shebang lines of Python scripts that are cross-distro compatible, and
also source-compatible with Python 2 & Python 3:
- "/usr/bin/python" (and variants like "/usr/bin/env python") will
fail on RHEL/CentOS 8 and any system without Python 2 installed
- "/usr/bin/python3" (and variants) will fail on RHEL/CentOS 7 (unless
py3 has been installed from EPEL)
- "/usr/bin/python2" (and variants) will fail in the same places
"/usr/bin/python" fails, and more besides (since not all distros with
Python 2.x have a python2 symlink)
[...] if we were to amend the PEP to say that "/usr/bin/py" should exist,
and refer to either:
- the Python Launcher for Unix; or
- some version of Python 3.x (or any later Python version); or
- Python 2.7 (if there isn't a more recent version of Python installed)
Then we could also give script shebang line authors clearer guidance
on what to use:
- /usr/bin/python2 for scripts that definitely don't work on Python 3.x
- /usr/bin/python3 for scripts that definitely don't work on Python 2.x
- /usr/bin/python for single source scripts that seek broad
compatibility with older systems running Python 2.x
- /usr/bin/py for single source scripts that seek forward
compatibility with newer systems (and also with older systems that
have been configured to support it)
But, by the time py is part of major distros, it'll be just as valid to
just recommend /usr/bin/python3 here. If you need to support older
systems that you can configure -- well, install Python 3 on them!
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