gunicorn + Flask on Heroku, module name and app name are incorrect

Question

I am trying to run a Flask app on Heroku (with nginx + gunicorn). My Heroku procfile is:

web: bin/start-nginx gunicorn -c config/gunicorn.py myflaskmodule:app

I keep getting errors from gunicorn such as:

gunicorn.errors.HaltServer: <HaltServer 'App failed to load.' 4>

or, depending on the app name I have tried, the gunicorn error can be:

Application object must be callable.

The Flask app is set up within a directory named myflaskmodule, containing a static directory, templates directory, auth.py, db.py, btac.py, and __init__.py.

In btac.py a blueprint is defined as:

 bp = Blueprint('btac', __name__, url_prefix='/btac')

Then I define several flask functions in btac.py such as:

@bp.route('/', methods=['GET'])
def index():
  # Do some things...
  return render_template()

In __init.py__ I have one function:

def create_app(test_config=None):
  app = Flask(__name__, instance_relative_config=True)

  # lots of other unrelated content here...

  db.init_app(app)
  app.register_blueprint(auth.bp)
  app.register_blueprint(btac.bp)
  return app

In the terminal (or a bash script), after executing export FLASK_ENV=development and export FLASK_APP=myflaskmodule, the Flask app runs locally with flask run.

How do I define the last argument in the Heroku procfile? It should be module_name:app_name. I have tried myflaskmodule:app, myflaskmodule:myflaskmodule, myflaskmodule:bp, myflaskmodule:btac.

I am fairly confident the first part of the argument should be myflaskmodule (the name of the main Flask directory), but I can't seem to correctly define the name of the Flask app.

Flask==1.0.2, gunicorn==19.9.0

A. Pine A. Pine · Accepted Answer · 2018-09-28T00:33:43

If you put web: gunicorn "myflaskmodule:create_app()" in your Procfile that should work. In the code above you don't have an app variable to use with myflaskmodule:app, but you do have a function that returns a Flask app, so you can use that.

gunicorn + Flask on Heroku, module name and app name are incorrect

1 Answers