Heroku H12 Request Timeout Error on Express route executing Python script - node.js

Question

I'm running into this major issue on my Heroku app: I have an Express route set up
as follows:
app.get('/recommendations/:data', (req, res) => {
let dataSplit = req.params.data.split("");
let dataInt = [];
dataSplit.forEach(char => dataInt.push(parseInt(char)));
function sendPrediction(dataInf) {
const process = spawn('python', ["./predict.py", dataInt]);
process.stdout.on('data', (data) => {
let jSonned = JSON.stringify(data.toString('utf-8'));
let chopped = jSonned.slice(1, jSonned.length - 3);
res.send(chopped.split(','));
});
}
sendPrediction(dataInt);
});
When I make the GET request to this route from the frontend component, it times out with an H12 error after 30 seconds as per Heroku's default behavior. Since this route works fine on my localhost, it leads me to believe that there is something wrong with the Python dependencies, as I can replicate the same behavior on localhost by deactivating my virtualenv. I know this isn't much to go by, but if anyone has seen something like this and can help I'd greatly appreciate it.
How can I figure out where the issue is?
Here is the error from Heroku logs:
2018-04-30T17:17:27.175201+00:00 heroku[router]: at=error code=H12
desc="Request timeout" method=GET path="/recommendations/41231" host=what-
doxd.herokuapp.com request_id=2249d6ac-a389-40c1-8f64-8947796494a7
fwd="12.23.56.98" dyno=web.1 connect=1ms service=30001ms status=503 bytes=0
protocol=https

Answer 1

Issue resolved: I needed to specify Python 2 and not let heroku default to python 3

Answer 2

First of all, you should attach your heroku logs to the question, so that we can see what Heroku is complaining about.
In absence of Heroku logs, I am guessing that you might not have a python buildpack defined for your app. Since your web dyno is built with node/express, you presumably already have a node.js buildpack (which Heroku may have added to your app automatically for you, if you didn't add it explicitly).
In that case, you need to add a python buildpack. Otherwise, python is not included in your app's slug.
See here for instructions on adding another buildpack to your app.