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From: Stephen Sinclair <radarsat1@gmail.com>
Newsgroups: linux.debian.maint.python
Subject: help with FTBFS bug due to python3.8/3.9 confusion
Date: Mon, 26 Oct 2020 09:00:02 +0100
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Hello Debian Python members,

I am looking into a bug in a Debian Science package that I maintain
and need some information.  It is a C++ library with a complicated
CMake setup that also produces a Python binding via swig.

Recently a FTBFS bug was filed against the package because apparently
it does not select the right Python after the introduction of Python
3.9.

The bug report can be found here:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=972394

The recommendations are to change the build-dep on "python-dev" to
"python-all-dev", and I can confirm that this does allow the package
to build.  (Is this the right thing to do?)

However, autopkgtest tests still fail.  The commenter in that bug
report claims, "the lack of 3.8 modules in the package also explains
your problem."  However, I do not understand that claim -- wouldn't it
be counter-productive to specify which minor version of Python the
package depends on?  I would need to update this manually when the
default switches to Python 3.9.

Although the CMake build is quite complicated and therefore I can
expect such problems, but I am nonetheless confused why it would
select the wrong Python version, as I do specify the Python executable
via the command,

> -DPYTHON_EXECUTABLE=$(shell which python3)"

Is there a better way to find the path to the default python interpreter?

kind regards,
Steve