Just tried "python" and "python3" on various Linux distros, which output respectively:

On an Ubuntu 20.04 desktop VM:

  python  => Python 2.7.18 (default, Jul  1 2022, 12:27:04)
  python3 => Python 3.8.10 (default, May 26 2023, 14:05:08)

On an Ubuntu 19.04 server:

  python  => -bash: python: command not found
  python3 => Python 3.7.5 (default, Apr 19 2020, 20:18:17)

On an Ubuntu 20.10 server:

  python  => -bash: python: command not found
  python3 => Python 3.8.10 (default, Jun  2 2021, 10:49:15)

I no longer have access to some RHEL7 and RHEL8 machines used for work recently, but if I recall correctly they do this by default:

Red Hat Enterprise Linux 7:

  python  => Some version of Python 2
  python3 => Some version of Python 3

Red Hat Enterprise Linux 8:

  python  => -bash: python: command not found # (use "python2" for Python 2)
  python3 => Some version of Python 3

You can change the default behaviour of unversioned "python" to version 2 or 3 on all the above systems, I think, so if you're running a Linux distro when "python" gets you Python 3, that configuration might have been done already.

MacOS 10.15 (Catalina) does something interesting:

  python  => WARNING: Python 2.7 is not recommended.
             This version is included in macOS for compatibility with legacy software.
             Future versions of macOS will not include Python 2.7.
             Instead, it is recommended that you transition to using 'python3' from within Terminal.
             Python 2.7.16 (default, Jun  5 2020, 22:59:21)
  python3 => Python 3.8.2 (default, Jul 14 2020, 05:39:05)

To be fair, few of these would qualify as "modern". Ubuntu 19.04 and 20.10, macOS 10.15 are all out of support, and RHEL 7 is almost ten years old and nearing the end of its support.

> I just type “python” and Python 3 happens.

That was the old way. Python now recommends against installing Python3 in a way that does that, and most modern *nix don't.

13.5.2 has /usr/bin/python3 (it’s 3.9.6) but not python2 or just python. Not sure when they changed, and YMMV with Homebrew.

I suspect my confusion stemmed from mostly invoking `ipython` which doesn't include the 3 suffix (ok, part of the confusion may've been pub-related too :D).