In a custom Heroku Python buildpack, how can I set a config var?

Question

I'm trying to set a custom config var for my Python app to use. Specifically, the current SHA to use as a URL param in static files to force the CDN to re-prime on each deploy. I'm trying to do it with a custom buildpack, based on the normal Heroku Python one (https://github.com/heroku/heroku-buildpack-python).

Where I am right now, I've started modifying the compile script. So far, I have been able to get the value I need, but running up near the top, around line 30, before GIT_DIR is unset

export GIT_SHA=$(git log -1 --format="%h")

then later, around line 175, I think is where it sets the config vars for the app. I tried adding my own:

set-env GIT_SHA '$GIT_SHA'

To no avail

I've run heroku labs:enable user-env-compile which I think is a necessary step, but I can't for the life of me figure out how to get the buildpack to actually set the config var for my app to use.

EDIT Was able to solve this with Andrew's suggestion. I created a custom buildpack which calls a Python script that uses the Heroku python bindings to set the var, reading it from the environment variable set in the build pack.

Answer 1

If my understanding of your question is correct, you want to set an env variable at compile time, but read it during execution (whenever a static file URL is accessed in your app). Is that accurate?

Compilation is done on a totally different dyno than the application is served under, so executing set-env during compile time might change the environment of the compilation dyno but won't affect the environment of the production dynos, which are spun up later.

I don't think heroku labs:enable user-env-compile is relevant here because that lets you read from the config during compile time, but it does not allow you to write to it.

If you really want to use env variables, you could use the Heroku API's python bindings to dynamically modify the configuration of your app. You could also try to save a temporary file somewhere with the compiled output, and then read from that file in the part of your buildpack that starts up your dyno. Or it may be possible to fetch the SHA directly from a production dyno at start-up time, without involving the compilation dyno at all.

However, all of this is fairly irregular and there is probably a cleaner way to accomplish your goal of versioning static files on your CDN.