MacOS: How to downgrade homebrew Python?

You can switch versions with brew switch. For instance I just downgraded Python 3.7.0 to 3.6.5 like this:

brew switch python 3.6.5

Unfortunately, the brew versions command has been deprecated, and it's currently pretty complicated to locate the available versions. I'd love to hear a simple solution to this. Meanwhile, if you know the version you want to switch to, try the above command.

I agree with the answers here that virtualenvs are a good idea, but having the version of Python you need in homebrew is also a good idea. The way my virtualenvs were created, bin/python was a symlink to /usr/local/bin/python, so things broke when Python was updated via homebrew.

First, it's generally considered bad practice to rely on system python for user land code if you can avoid it. You need to assume that system utilities require a specific version of system python, and your user land code may then be locked to that python version forever, which is not wise (unless you're writing system utilities, in which case just use /bin/python, but then you wouldn't be asking this question...).

Secondly, I am unclear why you need 2.7.10 instead of 2.7.13. All pythons with the same minor revision number (2.7) should always be compatible. If you needed 2.6, that would be a different story since that's a change in minor version. Code written for 2.7.x should all be compatible.

However, assuming your use case really does require using a specific Python version - getting to an actual solution now - be sure sure you really upgraded system python to begin with. If you enter the command: which python, do you get /usr/bin/python (system) or /usr/local/bin/python (brew installed user-land python). For example, /usr/bin/python -V gives me 2.7.10 even though python -V gives me 2.7.13 (via brew).

It's possible that you installed the latest python 2.7.x via brew which puts /usr/local/bin/python as a symlink in your $PATH, or you perhaps have a python alias pointing somewhere you don't want. Verify your $PATH order.

You can reset your homebrew python by removing it (brew uninstall python), or by changing the symlink (ln -s -f /usr/bin/python /usr/local/bin/python). However, using virtualenv removes the need for much of these sorts of gymnastics.

If you want to monkey with prior versions of Python installed via homebrew, this answer should help: How to install older formula using Brew?

One final option: if you absolutely must have a specific python version, pyenv can help.

brew install pyenv
pyenv install 2.7.10
pyenv global 2.7.10