> To debug that would not be trivial even for me though.
What's there to debug? If your Python-based software was properly installed, it should continue to use `/usr/bin/python` and work fine. If it has a `/usr/bin/env python` shebang, too bad, you need to either change that or re-satisfy dependencies. Which is simply part of the process of manually installing software.
> I wouldn't expect brew to automagically install Python for me in that case.
Your expectation is off then I'm afraid. Most package managers happen to install their own copy of things rather than pick up things you installed from other channels, especially when [other channels may have problems](https://github.com/Homebrew/brew-evolution/pull/12).
> Yes, let's just kick that can down the road.
What's being kicked down the road? Homebrew currently supports macOS up to 10.12, with `/usr/bin/python` being 2.7 and no `/usr/bin/python3`. Why do you expect current code to work with 10.16 or later?
> "just use pip" is not a suitable way to duck responsibility for people who don't know the differences between versions of Python and just want to use some software.
People who "just want to use ansible" (or whatever Python-based software from homebrew/core) should be served just fine. For people who declare themselves Python developers by importing ansible, or whatever other module or package, yes, they are actually responsible for knowing how to use pip and knowing the differences between versions of Python.
> if someone has already installed a bunch of python software, it doesn't carry over to the brew installed Python version
If you actually need to access modules in `/usr/lib/python2.7/site-packages` from brewed Python, just set `PYTHONPATH`. I'm sure you already know that, though.
> As for another reason why one might not want to automagically override an Apple provided Python installation, if someone has already installed a bunch of python software, it doesn't carry over to the brew installed Python version (if I remember correctly).
Unlike, say, Debian's default repository (with various `python-*` and `python3-` packages and the `dist-packages` structure which I always find annoying), homebrew/core is no pip replacement. If you need to import a Python module, use pip (preferably in a virtualenv). Our current approach to Python-based software in core (note that only standalone software is allowed) is to separate them as much as possible — each Python-based formula is in a separate virtualenv. `brew install` should not write to `/usr/local/lib/pythonx.y/site-packages/` in most cases.
Update: I realized I was tackling a different problem.
> In 2020...
That's too much of a stretch. Of course something will be figured out by then if necessary.