python3 spkg-configure: Only search for "python3", implement "configure --with-python=/PATH/TO/PYTHON"

[dimpase]

We make the configure search for a suitable python3 more straightforward.

Instead of searching for various names such as python3.8, python3.7 etc., we only look for python3.

As is standard practice, users will set their PATH so that the python3 that is accessible from the PATH is the desired Python version.

As an additional mechanism, we add configure --with-python=/PATH/TO/PYTHON. In contrast to the (undocumented) configure PYTHON3=/PATH/TO/PYTHON, it runs the usual tests whether this python is actually suitable.

Depends on #30576
Depends on #29500

CC: @mkoeppe @embray @orlitzky

Component: build: configure

Author: Matthias Koeppe, Dima Pasechnik

Branch/Commit: f19d9a4

Reviewer: Dima Pasechnik, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30546

[dimpase]

comment:1

perhaps copypasted comments in .m4 file may be trimmed.

[dimpase]

Upstream: Reported upstream. No feedback yet.

[mkoeppe]

comment:3

I don't know if this is a defect. If you have a directory containing python3 in the front of your path, shouldn't that be the python we are using?

[dimpase]

comment:5

Why is that the case? It's perfectly fine to have different fully-featured python3.x installed.
It works on Homebrew to use its python3.8 in Sage, while python3 still is python3.7
It works on Gentoo, which may have python3.6-9 installed all at the same time.
(Python plays an important role on Linux, one can't easily bump the version)

E.g. that's what I have on a Gentoo box, without doing anything special:

# python3 <tab>
python3            python3.7-config   python3.7m-config  python3.8-config   python3.9-config   
python3.7          python3.7m         python3.8          python3.9          python3-config 

On Ubuntu 18.04 one can install python3.8 (the system python is 3.6, and I'm not sure if it works well in Sage)

[mkoeppe]

comment:7

Replying to @dimpase:

Why is that the case?

Because the user seems to have expressed a preference for the python3 provided by the directory in the front of the PATH.

[dimpase]

comment:8

Replying to @mkoeppe:

Replying to @dimpase:

Why is that the case?

Because the user seems to have expressed a preference for the python3 provided by the directory in the front of the PATH.

for this, we should revive --with-python= option, not this kinds of silly hacks.

Besides, as I said, it's not always safe to prepend stuff to your PATH.

[mkoeppe]

comment:9

Putting the bin directory of a Python installation in the front of the PATH is the standard way of activating a venv, for example.

[mkoeppe]

Replying to @dimpase:

As modifying PATH to put the best python 1st might be error-prone (think about Conda, Homebrew, etc)

I would think this needs some elaboration.