Installing Python3.6 alongside Python3.7 on Mac

Try using brew for example if already using Python 3:

$ brew unlink python

Then install python 3.6.5:

$ brew install --ignore-dependencies https://raw.githubusercontent.com/Homebrew/homebrew-core/f2a764ef944b1080be64bd88dca9a1d80130c558/Formula/python.rb

To get back to python 3.7.4_1 use:

$ brew switch python 3.7.4_1

And if need 3.6 again switch with:

$ brew switch python 3.6.5_1