Until recently I relied on OS X native python for anything 2.7.x and Homebrew’s python3 for anything 3.x.x. As already implied by the lots of ‘x’-es: Far from ideal. But it worked. Until I accidentally ran a brew upgrade which put my python 3 version to a bleeding edge 3.6.0. Newer than what’s provisioned on our stack and besides that I ran into some errors when installing requirements in a new virtualenv that I suspected to be related to the python version bump.

So… how to downgrade? After some googling I quickly found this gist, where it’s advised in the comments to use pyenv. Combined with pyenv-virtualenv it was easy to get back to the python version of before.

Installation

Installation is straightforward:

brew update
brew install pyenv
brew install pyenv-virtualenv

Furthermore, add this to your ~/.bashrc or ~/.bash_profile (whatever fits your setup best) to have a virtualenv automatically be enabled for a certain folder:

eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"

Adding python versions

Straightforward:

pyenv install 3.5.3

Creating a virtualenv

By default virtualenvs created using pyenv-virtualenv are located in ~/.pyenv/versions/. The syntax has changed as well, compared to ‘vanilla’ virtualenv. As virtualenvs are now in a central location, my advise would be to simply name them similar to the project they belong to. So if you’re working on a project called ‘api’, create a virtualenv in that folder using:

pyenv virtualenv 3.5.3 api

Making auto-activate work

To have the previously added bash profile commands work, the folder’s python version needs to be set not to the python version but to the virtualenv name:

pyenv local api

This wasn’t immediately clear from the readme on GitHub but makes sense when you look how the ~/.pyenv folder is organized:

(api) [api:]$ cat .python-version
api
(api) [api-2:]$ ls -1 ~/.pyenv/versions/
3.5.3
api
(api-2) [api-2:]$ ls -l ~/.pyenv/versions/
# edited for readability
3.5.3
api-2 -> /Users/tibobeijen/.pyenv/versions/3.5.3/envs/api-2

Finding your virtualenv

Finding your virtualenv in PyCharm or Intellij is a bit trickier than before. As the virtualenv now resides in a hidden folder you need to have those visible by default in open dialogs (I believed I already tweaked that, I might be wrong or Sierra has messed up). Anyway, as pointed out on StackExchange, do so by:

defaults write -g AppleShowAllFiles -bool true

When Xcode doesn’t play nice

As a sidenote, if you run into all kinds of compile errors, it doesn’t hurt to check the state of your xcode installation. Things not working after a xcode update has bitten me more than once. To illustrate: The xcode upgrade after upgrading to Sierra caused my command line tools to disappear.

xcode-select --install
xcodebuild -license

That’s it. Hope the above helps setting pyenv up quickly.