How To Manage Python with Pyenv and Direnv

###Introduction

Whether you’re just getting started or you’re a seasoned Python developer, you may have found managing your Python environments to be tedious and painful. Managing Python versions, libraries, and various dependencies is like playing shuffleboard where the object is to not hit any other puck; if you do, the probability of a cascading effect of pucks flying everywhere you don’t want them to be will soon follow.

In this tutorial, you’ll install pyenv for managing python environments, install direnv to auto configure and source the virtualenv for projects, set a global Python version and a local Python version for projects, and configure a virtualenv and auto source the venv when moving into your project directories. By the end of this tutorial, you will be able to install any valid version of python, set up and configure virtual environments for each project, and bring sanity from chaos.

Prerequisites

We’re going to install both pyenv and direnv via homebrew to ease the install process. Before getting started, install homebrew if you don’t have it already.

Working with pyenv

To start, you’ll see how to work with pyenv.

Installing pyenv

To get things started, we’re going to install pyenv with homebrew and add the required pyenv init to our ~/.bashrc file.

brew install pyenv
echo 'eval "$(pyenv init -)"' >> ~/.bashrc # initialize pyenv on new shells
source ~/.bashrc # reinitialize bashrc to reflect changes in your current shell

Once it’s installed, take a moment to examine what our environment looks like. On a fresh system, you’ll see something similar to this:

which pyenv

Output
/usr/local/bin/pyenv

pyenv versions

Output
* system

which pip

Output
/usr/local/bin/pip

which python

Output
/usr/local/bin/python

This is a pretty standard snapshot. When you execute python version, you’ll notice * system, as the reference implies, this is your system’s python version. The asterisk denotes the current python binary sourced in your ${PATH}. A good rule of thumb is to leave the system binary alone. If you want to see the list of python versions that pyenv can install for you, use pyenv install --list.

pyenv install 2.7.15
pyenv install 3.7.0
pyenv versions

Output
* system (set by /Users/iamjohnnym/.pyenv/version)
  2.7.15
  3.7.0

Configuring the Base Requirements

Now, let’s upgrade pip since chances are, it installed an older version. The following command will loop through your installed versions and update pip to the latest.

for VERSION in $(pyenv versions --bare) ; do
  pyenv shell ${VERSION} ;
  pip install --upgrade pip ;
done

For the sake of our desired workflow, we want to install py2venv for our Python 2.x versions so we can mimic how python 3.x installs virtualenvs. python -m venv .venv.

for VERSION in $(pyenv versions --bare | egrep '^2.') ; do
  pyenv shell ${VERSION} ;
  pip install py2venv ;
done

Setting the Global Python Version

Even though we have pyenv installed, it will still default to system. To change this, set python 3.7.0 as our global version:

pyenv global 3.7.0
pyenv versions

Output
  system
  2.7.15
* 3.7.0 (set by /Users/iamjohnnym/.pyenv/version)

which python

Output
/Users/iamjohnnym/.pyenv/shims/python

We’ve got pyenv setup and functional. Let’s move on to direnv.

Working with direnv

direnv is a handy utility that allows you to create a file that’s placed in any directory that you want that functions like .bashrc. Whenever you enter the directory with this file, your shell will automatically execute it. The capabilities are endless but for the purpose of this post, we’re going to use it to configure a Python virtualenv based on the file .python-version and then activate it for us. The purpose is to create a seamless development flow. No need to worry about manually configuring or activating virtualenvs; let your computer do the work for you.

Installing direnv

Installation is straightforward with homebrew:

brew install direnv

direnv is now installed, and ready for exploitation. Let’s try a sample project.

Creating a New Project

Let’s start up a project. We’re going to create a new directory, set a local python version, set up our .envrc file, and activate it.

mkdir -p ~/python-projects/pyenv-tutorial
cd $_ # if you're unaware, $_ will execute the last argument of your command
pwd

Output
/Users/iamjohnnym/ python-projects/pyenv-tutorial

pyenv local 3.7.0
cat .python-version

Output
3.7.0

Configuring the Environment File

At this point, we’re ready to create our .envrc file. With your favorite editor, create that file and add the following contents:

# check if python version is set in current dir
if [ -f ".python-version" ] ; then
    if [ ! -d ".venv" ] ; then
        echo "Installing virtualenv for $(python -V)"
        # if we didn't install `py2venv` for python 2.x, we would need to use
        # `virtualenv`, which you would have to install separately.
        python -m venv .venv
    fi
    echo "Activating $(python -V) virtualenv"
    source .venv/bin/activate
fi
# announce python version and show the path of the current python in ${PATH}
echo "Virtualenv has been activated for $(python -V)"
echo "$(which python)"

Save the file. If you did this via a shell editor, such as vim. You’ll see the following message, direnv: error .envrc is blocked. Run direnv allow to approve its content. Don’t be alarmed as this is a security feature to prevent auto-execution of the file. Whenever this file is changed, it requires manual approval before it will auto-execute again. To activate it, simply type direnv allow from the project dir.

direnv allow

Output
direnv: loading .envrc
Installing virtualenv for Python 3.7.0
Activating Python 3.7.0 virtualenv
Virtualenv has been activated for Python 3.7.0
/Users/iamjohnnym/.personal/tutorials/pyenv-direnv/.venv/bin/python
direnv: export +VIRTUAL_ENV ~PATH

Conclusion

You now have the tools you need to manage different python versions and project dependencies!