I'm a bit fuzzy on which distro does what, but I had thought we might
do pure python3 builds for the distros that have already made the leap
(e.g. Ubuntu bionic). That's kind of a hacky way to get some mixed
python2 vs. python3 testing (assuming we use a mixture of ubuntu and
centos in testing).
Am I being naive to think that we mainly just need to build the python
version that ships with a given distro? Are there people who would
want e.g. python 3 packages for CentOS 7?
I agree with you in the sense that, if we could somehow include, among
the distros that teuthology uses, a distro that does not ship Python 2
at all, then we would get some Python 3 testing out of that.
What makes a distro a "Python 3 distro" is not merely the presence of
Python 3, but rather the _absence_ of Python 2 (in the base system).
As long as Python 2 is present, there is always the possibility
(probability, even) that a given Python script will decide to use it.
There is lots of code out there (not just in Ceph) where Python is
called to run a script. Before Python 3 became a thing, calling Python
was just a matter of running /usr/bin/python and that's what we still
see lots of this code doing.
Now, one would think that with the advent of Python 3, distros would
just switch the symlink over. None of this code would need to be
changed, because we can assume that /usr/bin/python is:
(a) present, and
(b) a symlink pointing to the preferred Python version on the system.
These are reasonable assumptions, right? Wrong.
What is actually happening is that we can no longer assume that the
/usr/bin/python symlink is present. If it _is_ present, it continues to
point to /usr/bin/python2 OR it is at least _possible_ that it might be
pointing to /usr/bin/python2.
(For Fedora I found
http://fedora-python.readthedocs.io/en/latest/plans/default-python-module/
which says that, as of Fedora 28, "users can choose" what
/usr/bin/python point to. I have not looked at Ubuntu Bionic at all yet.
In openSUSE Leap 15.0, /usr/bin/python points to /usr/bin/python2 but
Python 3 can be assumed to be present.)
The upshot of this is that all distro-agnostic code (and we have very
little distro-gnostic code) needs to be changed from the straightforward
"use /usr/bin/python" to the more nuanced "check for the existence of
/usr/bin/python2 and /usr/bin/python3 and behave accordingly", where
"behave accordingly" probably now means "prefer python2 if it is
present", but in the future should change to "prefer python3 if it is
present".
Nathan
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