Switch to Python 3

[ginggs]

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[natoscott]

@ginggs I think this will cause build failures for all platforms not using python3.

Since this script works with both python2 and python3, it should be fine to leave it as-is - even according to Debian policy:
https://debian-python.readthedocs.io/en/latest/debian-policy.html

Interpreter name

Python scripts depending on the default Python version (see Main packages) or not depending on a specific Python version should use python (without a version) as the interpreter name.

Is it possible the policy changed since addition of this patch to the deb build? FWIW, on Fedora and RHEL8 the direction changed course in the last year or so such that /usr/bin/python is now a symlink to python3 - I wonder if the same has happened with Debian too now?

[ginggs]

@natoscott that policy does look rather old. The Interpreter Name section from the Python policy [1] on debian.org states:

Python scripts should not specify python as the interpreter name even if they do not require any particular version of Python as the script would stop working upon removal of the Python 2 stack.

There was also a recent announcement [2] on the debian-python mailing list describing the situation in Ubuntu 20.04 LTS and the upcoming Debian Bullseye.

I believe there's no python symlink to python3 by default in FreeBSD either. At least, there was not in the FreeBSD 12.1 VM I set up yesterday.

Anyway, this particular script is not shipped in the htop package, and is only used during the build, so it's no big deal for us to patch it in Debian/Ubuntu, or use shebangfix in FreeBSD [3].

[1] https://www.debian.org/doc/packaging-manuals/python-policy/python.html#interpreter_name
[2] https://lists.debian.org/debian-python/2020/07/msg00042.html
[3] https://svnweb.freebsd.org/ports/head/sysutils/htop/

[ginggs]

It seems this was already 'fixed' via 40ac7a8

[natoscott]

Wow, what a mess the python2 -to python3 transition has been for everyone. I wonder if things will be done differently with python4 when the time comes for the next major version bump.

I see in your linked email [2] above, it states "So yes, in the long term, Debian should have a python command again."

It seems this was already 'fixed' via 40ac7a8

Hmm, somehow I missed that amongst all the whitespace changes - I've reverted it for now while we figure out a plan that works for all platforms htop should build on.

One approach could be to use a configure.ac check to detect a lack of any /usr/bin/python binary/symlink, have no shebang line in scripts/MakeHeader.py, and tweak the Makefile to pass the script to whichever python binary configure.ac manages to find amongst a list of known locations like python, python2, python3?

AC_CHECK_PROGS(PYTHON, [python python3 python2])
AC_SUBST(PYTHON)

Or similarly, we could have configure generate scripts/MakeHeader.py from a scripts/MakeHeader.py.in (like we do for the man page) with a shebang line that will work for the build host?

[natoscott]

[...]
AC_CHECK_PROGS(PYTHON, [python python3 python2])
AC_SUBST(PYTHON)

Or similarly, we could have configure generate scripts/MakeHeader.py from a scripts/MakeHeader.py.in (like we do for the man page) with a shebang line that will work for the build host?

I've implemented this and pushed it now - but would be happy to see any other, betterer, solutions anyone comes up with.