Venving like a pro

TL;DR Add the code at the bottom of this page to your .zshrc or .bashrc and from now on you'll automagically set up, activate and deactivate your venvs on the fly when switching directories.

Correctly setting up, activating and deactivating Python's virtual environments ('venv' for short) can be a hassle. When you forget to switch venvs on time you'll see unexpected results and mess up your pip installs. The script below smoothens the venving experience by implementing the following steps:

Hook in on directory changes Every time you enter a directory, the script checks if everything is okay venv-wise. It overrides the default cd command to do some venv-related checks after each execution of cd. This is done by creating the cd() function, which first executes the regular cd command, and then calls the custom check_venv function to see if any venv work is needed in the new dir.

Deactivate out-of-scope venvs First the check_venv function checks if a venv is active while the new directory is not part of the directory tree of that active venv. In this case the deactivate command is executed to leave the old venv.

Activate venv when present When no venv is active, and the new directory already contains a venv/ directory; simply activate it.

Set up venv when needed In case the landing directory contains a requirements.txt file, but no venv/ directory, it is clearly time to create a new venv. The script prompts the user what to do.

• Nothing for now: no venv is created, and the script will repeat the question on the next visit.
• Never create a venv here: a .novenvplease file is placed, and the next time the directory is visited, no questions are asked.
• Create a venv: nice!

Creating the venv A common place to specify which Python version should be used in a project is the .python-version file in the root of a project. If that file exists and the version of Python is installed, it will be used to create the venv. As a result that version becomes the default Python version in the venv, which is exactly what we want. The fallback is just plain python3, which should (almost) always be available.

With the resulting Python version, the venv is created, activated, pip updated to the latest version and all dependencies from requirements.txt are installed.

Closing remarks

• The script was inspired by this stackoverflow answer. Thank you user MS_!
• As the script has become quite large, you may want to put it in a separate file and include that, instead of cluttering your .bashrc/.zshrc.
• Depending on your taste, you may want to let the script fail when the correct Python version is not installed instead of just using any other version.
• Known limitation: if you directly jump into a subdirectory of a Python project, no auto-venving occurs.

function check_venv() { 
  if [[ ! -z "$VIRTUAL_ENV" ]] ; then 
    # a venv is active, check if the new dir starts with the root 
    # of that venv, deactivate otherwise 
    basedir="$(dirname "$VIRTUAL_ENV")" 
    if [[ "$PWD"/ != "$basedir"* ]] ; then 
      deactivate 
    fi 
  fi 
 
  if [[ -z "$VIRTUAL_ENV" ]] ; then 
    # no venv active, check if we should activate one 
    if [[ -d ./venv ]] ; then 
      source ./venv/bin/activate 
    elif [[ -f requirements.txt && ! -f .novenvplease ]] ; then 
      echo "This dir has a requirements.txt, but no venv/ yet. What do you want to do?" 
      echo "1) Create the venv" 
      echo "2) Not now" 
      echo "3) Never for this directory" 
      echo -n "Enter your choice (1, 2, or 3): " 
      read choice 
      case $choice in 
        1) 
          echo "Creating venv..." 
          PYTHON="python3" 
          if [[ -f .python-version ]] ; then 
            PYTHON_VERSION=$(head -1 .python-version) 
            if ! type "python${PYTHON_VERSION}" &> /dev/null; then 
              echo "Python version ${PYTHON_VERSION} not found, trying with 'python3' instead." 
            else 
              echo "Using python version ${PYTHON_VERSION} from .python-version" 
              PYTHON="python${PYTHON_VERSION}" 
            fi 
          fi 
          $PYTHON -m venv venv 
          source venv/bin/activate 
          $PYTHON -m pip install --upgrade pip 
          $PYTHON -m pip install -r requirements.txt 
          ;; 
        3) 
          echo "Marking dir as no-venving by creating file '.novenvplease'." 
          touch .novenvplease 
          ;; 
      esac 
    fi 
  fi 
} 
 
function cd() { 
  builtin cd "$@" 
  check_venv 
} 
 
# check if the dir the shell started in is a venv 
check_venv 

• Mypy on steroids
  Configuring mypy as a daemon for snappy type checking.