python_venv() {
  MYVENV=./venv
  # when you cd into a folder that contains $MYVENV
  [[ -d $MYVENV ]] && source $MYVENV/bin/activate > /dev/null 2>&1
  # when you cd into a folder that doesn't
  [[ ! -d $MYVENV ]] && deactivate > /dev/null 2>&1
}
autoload -U add-zsh-hook
add-zsh-hook chpwd python_venv