direnv for local Ruby and Python (inc Pipenv) development

— 5 minute read

There are lots of ways to get isolated local development environments for your projects. Many people like to use Docker. If you want to go for an elegant option there's nix.

I wanted something lightweight, portable and with automatic switching of environments. I'm usually working with Python and already use pyenv for managing Python versions and have recently started using Pipenv to manage virtual environments and packages.

Since I'm learning Ruby again, it was time to get a similar Ruby environment set up where I could easily get a specific Ruby version available for each project and install dependencies in an isolated way.

Installing ruby-install and chruby is the first step in the process.

Enter direnv

The next step is to get the appropriate Python and Ruby environments activated when we enter project directories. Instead of using a different language specific tool for each language we can use direnv to automate switching language versions and activating project isolated dependency installs. (It's also useful for setting project specific environment variables and other handy tasks - see the direnv docs.)

Install direnv making sure to follow the instructions to hook it into your shell. Now create ~/.direnvrc with the following contents (with thanks to this post for the Ruby section):

# Python
use_python() {
  local python_root=$PYENV_ROOT/versions/$1
  load_prefix "$python_root"
  if [[ -x "$python_root/bin/python" ]]; then
    layout python "$python_root/bin/python"
  else
    echo "Error: $python_root/bin/python can't be executed."
    exit
  fi
}

# Ruby
use_ruby() {
  # enable the chruby command in an environment
  source /usr/local/opt/chruby/share/chruby/chruby.sh

  # desired Ruby version as first parameter
  local ver=$1

  # if version not given as parameter and there is a .ruby-version file, get
  # version from the file
  if [[ -z $ver ]] && [[ -f .ruby-version ]]; then
    ver=$(cat .ruby-version)
  fi

  # if the version still isn't set, error cause we don't know what to do
  if [[ -z $ver ]]; then
    echo Unknown ruby version
    exit 1
  fi

  # switch to the desired ruby version
  chruby $ver

  # Sets the GEM_HOME environment variable to `$PWD/.direnv/ruby/RUBY_VERSION`.
  # This forces the installation of any gems into the project’s sub-folder. If
  # you’re using bundler it will create wrapper programs that can be invoked
  # directly instead of using the `bundle exec` prefix.
  layout_ruby
}

Using direnv in projects

Now in any project directory you can add a .envrc file with the language and version we want, e.g. use ruby 2.5.1 or use python 3.6.5. (Make sure you've installed the versions you want with pyenv or ruby-install first.)

After saving the file, you'll be prompted to do a direnv allow. After that, each time you enter and leave the project directory, the relevant language version will be enabled and the environment will be configured to install and use dependencies from a .direnv directory in the project root. Now you can pip install ... or gem install bundler to start installing your dependencies.

Using direnv with pipenv

Direnv has a lightly documented Pipenv layout that was introduced recently. To start a new Python project that you want to use Pipenv with, create a directory for it and create a Pipfile with at least the following minimal content to specify the Python version you want:

[requires]
python_version = "3.6.5"

Then create a .envrc file with this in it: layout_pipenv.

Pipenv will now create the virtualenv like it normally does and direnv will handle automatically activating and deactivating it.

Update .gitignore

add .direnv and .enrvc to you global gitignore file to make sure they don't get added to repositories:

# direnv env #
##############
.env
.envrc
.direnv

Extend and embrace

Since direnv is installed globally you can extend the ideas above to other languages and tools you work with, such as automatic setting of GOPATH. See the direnv wiki for some ideas.