IMHO, the best way to use two different Python versions on macOS is via homebrew. After installing homebrew on macOS, run the commands below on your terminal.

brew install python@2
brew install python

Now you can run Python 2.7 by invoking python2 or Python 3 by invoking python3. In addition to this, you can use virtualenv or pyenv to manage different versions of python environments.

I have never personally used miniconda but from the documentation, it looks like it is similar to using pip and virtualenv in combination.

The simplest way would be to add an alias to python3 to always point to the native python installed. Add this line to the .bash_profile file in your $HOME directory at the last,

alias python="python3"

Doing so makes the changes to be reflected on every interactive shell opened.

Yes, we support this but not via brew link. Add $(brew --prefix python@3.10)/libexec/bin to the front of your PATH and the default python3 will change.

See brew caveats python@3.10 for more info.

pyenv is the thing you want. It works very very well:

pyenv lets you easily switch between multiple versions of Python. It's simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well. This project was forked from rbenv and ruby-build, and modified for Python.

https://github.com/pyenv/pyenv

Install it via Homebrew:

$ brew update
$ brew install pyenv

It handles the download, compilation, and installation of various pythons for you, e.g.:

$ pyenv install 3.7.2

It can show you which versions you've installed, and which is active:

$ pyenv versions
  system
  3.6.7
* 3.7.2

When you're in a new project directory, just tell pyenv which python version to use there:

$ pyenv local 3.6.7  # Because tensorflow isn't compat. with 3.7 :-(

You can set a 'default' version everywhere else:

$ pyenv global 3.7.2

UPDATE, AUG 2024: I've gradually switched over to asdf because I work with a lot of different languages. The asdf project also publishes new version recipes very quickly. The asdf CLI interface is a little clunkier (verbose) because it can handle all languages. But pyenv never let me down for all the years I used it. If I was only doing Python development I might still be using it.

Changing the default python executable's version system-wide could break some applications that depend on python2.

However, you can alias the commands in most shells, Since the default shells in macOS (bash in 10.14 and below; zsh in 10.15) share a similar syntax. You could put alias python='python3' in your ~/.profile, and then source ~/.profile in your ~/.bash_profile and/or your~/.zsh_profile with a line like:

[ -e ~/.profile ] && . ~/.profile

This way, your alias will work across shells.

With this, python command now invokes python3. If you want to invoke the "original" python (that refers to python2) on occasion, you can use command python, which will leaving the alias untouched, and works in all shells.

If you launch interpreters more often (I do), you can always create more aliases to add as well, i.e.:

alias 2='python2'
alias 3='python3'

Tip: For scripts, instead of using a shebang like:

#!/usr/bin/env python

use:

#!/usr/bin/env python3

This way, the system will use python3 for running python executables.