Debugging a Flask app running in Gunicorn

For Heroku users, there is a simpler solution than creating a bin/web script like suggested by Nick.

Instead of foreman start, just use foreman run python app.py if you want to debug your application in development.

The accepted solution doesn't work for me.

Gunicorn is a pre-forking environment and apparently the Flask debugger doesn't work in a forking environment.

Attention

Even though the interactive debugger does not work in forking environments (which makes it nearly impossible to use on production servers) [...]

Even if you set app.debug = True, you will still only get an empty page with the message Internal Server Error if you run with gunicorn testserver:app. The best you can do with gunicorn is to run it with gunicorn --debug testserver:app. That gives you the trace in addition to the Internal Server Error message. However, this is just the same text trace that you see in the terminal and not the Flask debugger.

Adding the if __name__ ... section to the testserver.py and running python testserver.py to start the server in development gets you the Flask debugger. In other words, don't use gunicorn in development if you want the Flask debugger.

app = Flask(__name__)
app.config['DEBUG'] = True

if __name__ == '__main__':
    app.run()

## Tip for Heroku users: Personally I still like to use `foreman start`, instead of `python testserver.py` since [it sets up all the env variables for me](https://devcenter.heroku.com/articles/config-vars#using-foreman). To get this to work:

Contents of Procfile

web: bin/web

Contents of bin/web, file is relative to project root

#!/bin/sh

if [ "$FLASK_ENV" == "development" ]; then
        python app.py
else
        gunicorn app:app -w 3
fi

In development, create a .env file relative to project root with the following contents (docs here)

FLASK_ENV=development
DEBUG=True

Also, don't forget to change the app.config['DEBUG']... line in testserver.py to something that won't run Flask in debug mode in production.

app.config['DEBUG'] = os.environ.get('DEBUG', False)

I had similiar problem when running flask under gunicorn I didn't see stacktraces in browser (had to look at logs every time). Setting DEBUG, FLASK_DEBUG, or anything mentioned on this page didn't work. Finally I did this:

app = Flask(__name__)
app.config.from_object(settings_map[environment])
if environment == 'development':
    from werkzeug.debug import DebuggedApplication
    app_runtime = DebuggedApplication(app, evalex=False)
else:
    app_runtime = app

Note evalex is disabled because interactive debbugging won't work with forking (gunicorn).

The Flask config is entirely separate from gunicorn's. Following the Flask documentation on config files, a good solution would be change my source to this:

app = Flask(__name__)
app.config.from_pyfile('config.py')

And in config.py:

DEBUG = True