How to automatically activate virtualenvs when cd'ing into a directory

Question

I have a bunch of projects in my ~/Documents. I work almost exclusively in python, so these are basically all python projects. Each one, e.g. ~/Documents/

Answer 1

This is a zsh only solution.

This is an improvement over daveruinseverything's answer which is an improvement over MS_'s answer.

We are using precmd hook instead of overwriting cd.

We have added another extra feature. Suppose the directory structure is

├── .venv
│   ├── bin
│   │   └── activate
├── subdir
│   ├── subdir1
│   │   ├── subdir2
│   │   │   └── test2.txt
│   │   └── test1.txt
│   └── test.txt
└── testing.py

If you now open new terminal in subdir2, or directly cd to subdir2 from other place, it will activate the venv.

The solution is:

autoload -Uz add-zsh-hook
add-zsh-hook precmd automatically_activate_python_venv

function automatically_activate_python_env() {
  if [[ -z $VIRTUAL_ENV ]] ; then
    activate_venv
  else
    parentdir="$(dirname ${VIRTUAL_ENV})"
    if [[ "$PWD"/ != "$parentdir"/* ]] ; then
      deactivate
      activate_venv
    fi
  fi
}

function activate_venv() {  
  local d n
  d=$PWD
  
  until false 
  do 
  if [[ -f $d/.venv/bin/activate ]] ; then 
    source $d/.venv/bin/activate
    break
  fi
    d=${d%/*}
    # d="$(dirname "$d")"
    [[ $d = *\/* ]] || break
  done
}