More flexibility for Python interpreter creation

[tobiasherp]

Currently the installer is not very flexible when it comes to the creation of a Python interpreter:

• It doesn't allow the location to be specified but always creates that Python-x.z directory in the created Plone directory.
• It only allows to specify a Python 2.7 (2.7.18, currently) or Python 3 (3.8.5, currently) to be built.

Since we create a virtualenv anyway, we should be able to create the Python build in some more generic place, e.g. the /opt/ directory.

Furthermore, the buildable Python 2 and 3 versions are hardcoded (using PYTHON_MD5 and PYTHON3_MD5 variables, among others, which is not very maintainable). IMO, the installer should be able to build

• the newest 3.6 version
• the newest 3.7 version
• the newest 3.8 version
• any version it ever supported to be explicitly specified (up to now, there haven't been any, but it would be very easy to support e.g. 3.6.9 which was built in the latest release).

My changed main_install_script.sh so far

• supports a --python-prefix option, e.g. to create the interpreter in the /opt/ directory
• supports the specification of the demanded Python version:
  • a x.y.z specification is accepted if the MD5 hash is known (in the PYTHON_MD5[x.y.z] array variable)
  • a x.y specification is fed into the BEST_PYTHON variable to find out about the newest x.y.z release.

E.g., with the options --python-prefix=/opt/ --build-python=3.7 (run as root), it will create a Python 3.7.9 interpreter (BEST_PYTHON[3.7]) in the /opt/Python-3.7 directory (WANT_PYTHON automatically adjusted accordingly).

There are no double sets of PYTHON_… / PYTHON3_… variables anymore. We could easily support new Python releases by adding a PYTHON_MD5[3.x.y] line and update a BEST_PYTHON[3.x] entry while still supporting the build of the previously-best Python release.

I'd like to add a python-only installation method as well. One might want to create a Python in /opt/, requiring root permissions, but a non-root Plone installation.

[tkimnguyen]

A couple of thoughts:

• it is not necessarily the case that Plone will have been tested with the latest Python
• allowing Python to be installed in a different location adds complication

What I've liked about the unified installer is that it makes decisions that are consistent and it is conservative. It is not "latest Plone" and it does not do everything that anyone could possibly want. For those who know enough and who demand the ability to install latest Python and place it in different locations, etc., they can start with what the unified installer does and can either make local changes after the fact or can fork the unified installer and do what they need in their own fork. It's not just because I'm lazy but I do not think the unified installer maintainer(s) should be asked to test even more options.

[tkimnguyen]

However, if there is a way to ensure that only Python versions that Plone has been successfully tested with are easily added to the installer, that would be excellent.

Also, if there is a way to set up automated testing here so that we can know for sure that, wherever Python gets installed, things are known to work, that too would be excellent.

So really the next step is to set up automated testing.

[tkimnguyen]

Specifically referencing Steve's comment about testing: https://community.plone.org/t/unified-installer-python-version/12849/16?u=tkimnguyen