A very handy function to auto-activate specific python virtual environments upon entering a directory containing a magic file.

zshrc

# Setup python virtual environment in a given directory subtree
function chpwd_auto_pyvenv() {
	local MAGIC=".zsh_pyvenv"

	# Check if we encountered a magic previously
	if [[ -z ${ZSH_PYVENV_PARENT+x} ]]; then 
		# Check if this directory contains a MAGIC
		if [[ -a "$PWD/$MAGIC" ]]; then
			echo "Activating virtual environment"
			# Activate the environment specified
			source "$(<$PWD/$MAGIC)/bin/activate"
			# Save the current directory
			export ZSH_PYVENV_PARENT=$PWD
		else
			# No MAGIC here...
		fi
	else
		# Check whether PWD is a subfolder of ZSH_PYVENV_PARENT
		case $PWD/ in
			$ZSH_PYVENV_PARENT/*)
				# Yes, we are in a subdir
				echo "Entering virtual environment subdir..."
				;;
			*)
				# No, we left the MAGIC tree
				echo "Leaving virtual environment"
				deactivate
				unset ZSH_PYVENV_PARENT
				;;
		esac
	fi
}

chpwd_functions=(${chpwd_functions[@]} "chpwd_auto_pyvenv")