Hi Mathieu,

[...]

> I think the issue here lies in the sitecustomize.py file introduced with
> cb72f9a773e0931ee3758c851d96007ded034e4c.
>
> python_root = os.path.realpath(sys.executable).split('/bin/')[0]
>
>
> When using the Python C library, sys.executable is not the expected
> python binary but the test binary.
>
> So when we try to find the associated libraries in the Python search
> path, this way:
>
> index = sys_path_absolute.index(python_site)
> sys.path = sys.path[:index] + matching_sites + sys.path[index:]
>
>
> we get:
>
> ValueError: '/home/mathieu/glade-3.38.2/build/lib/python3.9/site-packages' is not in list
>
>
> Running the test with the PYTHONPATH set to:
>
> PYTHONPATH=/home/mathieu/glade-3.38.2/build/lib/python3.9/site-packages:$PYTHONPATH 
>
> is a way to work around this issue.
>
> The fix here would be to replace sys.executable in the sitecustomize.py
> with something more adequate, but I have no idea what could it be.

From the doc [0], sys.prefix seems to be more correct.  This had been
suggested by Hartmut some time ago and I had this patch ready recently.
Perhaps it already addresses the issue at hand?

You'll find it attached.  It hasn't gotten much testing yet.

[0]  https://docs.python.org/3/library/sys.html#sys.prefix