Override builtin cd to source closest virtual environment

cd

# Source closest virtual environment, ``venv``, if exists in any parent
# directory.
cd () {
    local venv_dir="venv" prevpwd="$PWD"

    builtin cd "$@"

    if [ "$prevpwd" != "$PWD" ]; then
        local base_dir="$PWD"

        # Always try to deactivate for nested virtual environments.
        deactivate &>/dev/null

        # Find the closest parent directory with ``venv``.
        while [[ ! -d "$base_dir/$venv_dir" ]] && [[ "$base_dir" != '/' ]]; do
            base_dir="$(dirname "$base_dir")"
        done

        # Source virtual environment if it exists.
        if [[ -f "$base_dir/$venv_dir/bin/activate" ]]; then
            source "$base_dir/$venv_dir/bin/activate"
        fi
    fi
}