I am running MacOS Big Sur (11.2.3).

With python2, I have Tkinter built-in.

With python3, it has to be installed manually and it's very simple, just run:

$ brew install python-tk

To run python2 in a terminal, execute python file.py.

To run python3 in a terminal, execute python3 file.py.

Based on the comments from above and the fact that Python must be linked to Tcl/Tk framework:

If you don't have Xcode command line tools, install those:

xcode-select --install

If you don't have Tcl/Tk brew installation (check brew list), install that:

brew install tcl-tk

Then, run "brew uninstall python" if that was not installed with option --with-tcl-tk (the current official option). Then install Python again, linking it to the brew installed Tcl/Tk:

brew install python --with-tcl-tk

12/18 Update: No longer possible for various reasons.

Below is now outdated. You'll have to install Python directly from python.org if you want to remove those warnings.

2018 Update

brew reinstall python --with-tcl-tk

Note: Homebrew now uses Python 3 by default - Homebrew Blog. Docs.

Testing

python should bring up system’s Python 2, python3 should bring up Python 3.

idle points to system Python/tcl-tk. It will show an out-dated tcl-tk error (unless you brew install python@2 --with-tcl-tk)

idle3 should bring up Python 3 with no warnings.

Caveat

--with-tcl-tk will install python directly from python.org, which you'll see when you run brew info python.

More info here.