Changing Default Python 3 in Terminal for Mac OS

Sometimes Python gets upgraded before you're ready. For example, I just used homebrew to upgrade Python. It installed version 3.7 and not 3.6.6 as I intended. This meant that typing $ python3 -V returned 3.7.0. In the long run, this is great. But when it comes to creating a virtual environment -- slightly frustrating.

Let's fix that so we can change $ python3 to use the version we want at any time.

** Using homebrew ** I have the following versions of python installed:

Python 3.6.2
Python 3.6.5
Python 3.7.0

You can find this by

$ brew install python
$ ls /usr/local/Cellar/python

After you pick the version you want to represent python3 in the following:

$ python3 -V

You can then change your Python Version by opening up your bash_profile

$ nano ~/.bash_profile

Add the following line replacing 3.7.0 with any other version you have installed

export PYTHONPATH="/usr/local/Cellar/python/3.7.0/bin/python3:$PYTHONPATH"

Save and close.

Something I couldn't figure out is how to install specific versions of Python using homebrew. I have the ones installed above because I installed it awhile ago. If you find out how, please let me know in the comments otherwise we can just install directly from Python.org like...

Python.org install

Well, assuming you install the pkg version of Python (like Python 3.6.6) your install would be located in /Library/Frameworks/Python.framework/Versions/3.6

$ nano ~/.bash_profile

Add the following line replacing 3.6 with any version you have installed

export PYTHONPATH="/Library/Frameworks/Python.framework/Versions/3.6:$PYTHONPATH"

Save and close.

Test Command After saving your ~/.bash_profile you have to open a new Terminal window for the changes to take place.

$ python3 -V

You should now see the version you need. Also, depending on what you have installed, you can try

$ python3.7 -V
$ python3.6 -V
$ python3.5 -V

To find where those versions are installed you can run

$ which python3.5

Use that location to replace the PYTHONPATH whenever you may need.