Direnv: Fix missing virtualenv shell prefix

Something minor I came across today.

I like to use a tool called direnv, I'm also currently using zsh as my shell of choice (with ohmyzsh.

Day to day I write a lot of Python, so I use direnv to automatically activate (or create) python virtual environments when I navigate into certain directories. See an example:

Current Behaviour

cd blog
direnv: loading ~/code/blog/.envrc
direnv: export +VIRTUAL_ENV ~PATH
➜  blog git:(main) ~ echo $VIRTUAL_ENV
/home/jackevans/code/blog/.direnv/python-3.11.1

At a glance it's difficult to tell whether my current shell currently has a $VIRTUAL_ENV activated or not.

Desired Behaviour

If I manually source a virtualenv, the behaviour is slightly different, instead my prompt will show a prefix (python-3.11.1)

➜  blog git:(main) ~ direnv deny
direnv: error /home/jackevans/code/blog/.envrc is blocked. Run `direnv allow` to approve its content

➜  blog git:(main) ~ source .direnv/python-3.11.1/bin/activate
(python-3.11.1) ➜  blog git:(main) ~ deactivate

The Fix

I came across this thread which pointed me to the official direnv documentation

The solution is to place the following snippet in your .zshrc

setopt PROMPT_SUBST

show_virtual_env() {
  if [[ -n "$VIRTUAL_ENV" && -n "$DIRENV_DIR" ]]; then
    echo "($(basename $VIRTUAL_ENV))"
  fi
}
PS1='$(show_virtual_env)'$PS1

Then enjoy your consistent virtualenv prompt:

➜  blog git:(main) source ~/.zshrc
(python-3.11.1) ➜  flitter git:(main)