uv, direnv, and simple .envrc files

Direnv Libraries

First, it turns out you can pretty easily define custom direnv functions like the built-in ones (layout python, etc...). You do this by adding functions to ~/.config/direnv/direnvrc or in ~/.config/direnv/lib/ as shell scripts. I use this extensively to make my .envrc files easier to maintain and smaller. Now that I'm using uv here is my default for python:

function use_standard-python() {
    source_up_if_exists

    dotenv_if_exists

    source_env_if_exists .envrc.local

    use venv

    uv sync
}

What does that even mean?

Let me explain each of these commands and why they are there:

• source_up_if_exists -- this direnv stdlib function is here because I often group my projects into directories with common configuration. For example, when working on Chicon 8, I had a top level .envrc that set up the AWS configuration to support deploying Wellington and the Chicon 8 website. This searches up til it finds a .envrc in a higher directory, and uses that. source_up is the noisier, less-adaptable sibling.

• dotenv_if_exists -- this loads .env from the current working directory. 12-factor apps often have environment-driven configuration, and docker compose uses them relatively seamlessly as well. Doing this makes it easier to run commands from my shell that behave like my development environment.

• source_env_if_exists .envrc.local -- sometimes you need more complex functionality in a project than just environment variables. Having this here lets me use .envrc.local for that. This comes after .env because sometimes you want to change those values.

• use venv -- this is a function that activates the project .venv (creating it if needed); I'm old and set in my ways, and I prefer . .venv/bin/activate.fish in my shell to the more newfangled "prefix it with a runner" mode.

• uv sync -- this is a super fast, "install my development and main dependencies" command. This was way, way too slow with pip, pip-tools, poetry, pdm, or hatch, but with uv, I don't mind having this in my .envrc

function use_venv() {
    uv venv
    source .venv/bin/activate
}

Using it in a sentence

With this set up in direnv's configuration, all I need in my .envrc file is this:

use standard-python

I've been using this pattern for a while now; it lets me upgrade how I do default Python setups, with project specific settings, easily.