[QGIS-Developer] Changes in Python versions on QGIS 3.18

[Sebastian M. Ernst]

Hi Nyall,

>> Until recently [2], such a constraint did not even exist [3] (other than
>> a rather broad "Python 3.X"). Now, you're technically sort of supporting
>> 3.7 to 3.9, although it has not been documented anywhere as far as I
>> know other than in PRs and issues etc. - please correct me if I am wrong.
> 
> The discussion happened in public here:
> https://github.com/qgis/QGIS-Enhancement-Proposals/issues/197

I am sorry if I did not make myself clear: I am aware that it happened
in public, yes. The documentation of the new constraint can therefore be
found - in public - in a Github issue (in a repository other than the
main QGIS one). Did it ever make its way into the documentation for
plugin developers or ("building QGIS from source" developer
documentation)? In other words, as a (plugin) developer, I currently
have to search and monitor several github repos and at least this
mailing list constantly if I want to know about this kind of stuff.
Actually, I have yet to find the place where updates of critical OSGeo4W
as well as QGIS for Windows installer components are potentially
discussed or announced - in this context, Python 3.9 was also a big
surprise for me.

Besides, in the past, I have basically manually tested my way through a
bunch of relevant permutations of {OS | OS version | QGIS version |
Python version} before I knew what was compatible and what was not.
There is no "support matrix" or similar. This is the kind of work and
responsibility that you're currently leaving to the plugin developers IF
they want to make sure that their stuff actually works across a broad
range of QGIS deployments.

> The rationale is that users on older, slower moving distros can easily
> stick to the stable 3.16 LTR without issues. I.e. older, stable distro
> = older, stable QGIS release.

Makes sense, agreed.

Sebastian