How to compile SASS/SCSS files when deploying a Django app to Heroku

I’m using django-sass-processor to automatically compile the SASS files in a Django 2.1 project. Everything runs fine locally, but on Heroku the pages appear unstyled. The problem is that Heroku only runs ./manage.py collectstatic when building a Django app. This simply copies the static files to the ./staticfiles folder. As the SCSS files weren’t compiled yet, only the *.scss files end up in that folder.

The solution is simple: customizing how Heroku builds our Django app.

Heroku’s Python buildpack offers two hooks, pre_compile and post_compile. To use them, we just need to add bin/pre_compile or bin/post_compile in our app’s repository. For compiling SCSS, we want to use post_compile, as it will only run after all pip packages are installed.

Create a file in bin/post_compile in the root of your repository with the contents:

#!/usr/bin/env bash

cd "$1" || exit 1
echo "-----> Compiling SCSS"
python manage.py compilescss --traceback
echo "-----> Collecting static files"
python manage.py collectstatic --noinput --traceback

The first command, cd "$1", changes the current directory to the build directory passed in by Heroku. Then we just run the compilescss and compilestatic as needed.

If you commit this file and deploy to Heroku, you should already see the SCSS compilation working. The only remaining step is a small optimization. We want to avoid Heroku running compilestatic, as we’re running it ourselves. Simply run:

heroku config:set DISABLE_COLLECTSTATIC=1 --app <YOUR_APP_NAME>

That’s it! Everything should be running fine, and if you need to add other build steps (e.g. minifying, compressing images, etc.), just add them to the bin/post_compile file.

You can see how I’ve done in my Django project on https://github.com/vitorbaptista/lainonima.