Why is my Flask app being detected as node.js on Heroku

Question

I recently made some changes to the structure of my Flask app hosted on heroku and now heroku has decided to detect it as a Node.js app intead of a Python app. My application uses both python (Flask) for the backend api and javascript for the front end.

The changes I made included integrating npm and bower into my application to streamline the javascript development of the app.

Answer 1

The problem was introduced when I added a package.json to my root directory when I started using npm. It seems that the build detection script runs the nodejs detection first (here) which leads to this code: if [ -f $1/package.json ]; then echo "Node.js" && exit 0 executing and Heroku thinks it's a nodejs app and exits before the python detection has a chance to run.

To solve this I had to manually tell Heroku that I wanted a python build using this command

heroku config:set BUILDPACK_URL=https://github.com/heroku/heroku-buildpack-python.

Answer 2

The package.json file is causing Heroku to detect it as a node.js app. To prevent this, add the file name to a .slugignore file:

echo 'package.json' >> .slugignore
git add .slugignore

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.slugignore is like .gitignore. It resides in the root directory of your repository and should contain a list of filenames and wildcard patterns. The matching files remain in your git repository, but are deleted from the slug after you push to Heroku. The deletion occurs before the buildpacks run, so the node.js buildpack won't find package.json and the app won't be misidentified as a node.js app.