Flask application failing on Heroku: no module named app

Question

I have a Flask application that I have deployed to Heroku, but I get an error:

2018-08-27T12:39:32.197715+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=heroku-seb-test.herokuapp.com request_id=3e02d494-1a3f-4467-9eaa-aaa551b4ca03 fwd="91.143.113.54" dyno= connect= service= status=503 bytes= protocol=https

It appears that it is a mistake in my Flask app structure, because just above it throws a Python / Flask exception:

File "/app/.heroku/python/lib/python2.7/site- 
packages/gunicorn/util.py", line 357, in import_app 
__import__(module)
ImportError: No module named app

Here is my folder structure:

• Folio
  • routes.py
  • (Procfile and other dependencies)
  • static
    • css
    • js
    • images
    • lib
  • templates
    • index.html
  • venv

Here is my routes.py file:

from flask import Flask
from flask import render_template, request, flash, session, url_for, 
redirect

app = Flask(__name__)

@app.route('/')
def index():
return render_template('index.html')

app.run(debug=True)

Here is my Procfile:

web: gunicorn app:app

Answer 1

The Procfile tells Heroku how to run your application. gunicorn expects to be given an argument in the form of module.variable identifying the thing it should run.

Today you're telling it that your app is in a variable called app in a module called app:

web: gunicorn app:app

But your module isn't called app—it's called routes (the file is routes.py). Update your Procfile to point to the correct object:

web: gunicorn routes:app

Commit that change and push to Heroku to see its effect. (Or you can use heroku local to try it out locally.)