uWSGI cannot find "application" using Flask and Virtualenv

Question

Using uWSGI to serve a simple wsgi app, (a simple "Hello, World") my configuration works, but when I try to run a Flask app, I get this in uWSGI's error logs:

current working directory: /opt/python-env/coefficient/lib/python2.6/site-packages
writing pidfile to /var/run/uwsgi.pid
detected binary path: /opt/uwsgi/uwsgi
setuid() to 497
your memory page size is 4096 bytes
detected max file descriptor number: 1024
lock engine: pthread robust mutexes
uwsgi socket 0 bound to TCP address 127.0.0.1:3031 fd 3
Python version: 2.6.6 (r266:84292, Jun 18 2012, 14:18:47)  [GCC 4.4.6 20110731 (Red Hat 4.4.6-3)]
Set PythonHome to /opt/python-env/coefficient/
*** Python threads support is disabled. You can enable it with --enable-threads ***
Python main interpreter initialized at 0xbed3b0
your server socket listen backlog is limited to 100 connections
*** Operational MODE: single process ***
added /opt/python-env/coefficient/lib/python2.6/site-packages/ to pythonpath.
unable to find "application" callable in file /var/www/coefficient/flask.py
unable to load app 0 (mountpoint='') (callable not found or import error)
*** no app loaded. going in full dynamic mode ***
*** uWSGI is running in multiple interpreter mode ***`

Note in particular this part of the log:

unable to find "application" callable in file /var/www/coefficient/flask.py

unable to load app 0 (mountpoint='') (callable not found or import error)

******no app loaded. going in full dynamic mode******

This is my Flask app:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello, World, from Flask!"

Before I added my Virtualenv's pythonpath to my configuration file, I was getting an ImportError for Flask. I solved this though, I believe (I'm not receiving errors about it anymore) and here is my complete configuration file:

uwsgi:
  #socket: /tmp/uwsgi.sock 
  socket: 127.0.0.1:3031
  daemonize: /var/log/uwsgi.log
  pidfile: /var/run/uwsgi.pid
  master: true
  vacuum: true
  #wsgi-file: /var/www/coefficient/coefficient.py
  wsgi-file: /var/www/coefficient/flask.py
  processes: 1
  virtualenv: /opt/python-env/coefficient/
  pythonpath: /opt/python-env/coefficient/lib/python2.6/site-packages

This is how I start uWSGI, from an rc script:

/opt/uwsgi/uwsgi --yaml /etc/uwsgi/conf.yaml --uid uwsgi

And if I try to view the Flask program in a browser, I get this:

**uWSGI Error**

Python application not found

Any help is appreciated.

Answer 1

unable to find "application" callable in file /var/www/coefficient/flask.py

is the key :)

Your app is defining an 'app' callable, so you have to instruct uWSGI to search for it, instead of 'application'.

You can use the option

callable: app

and it will works (this is explained in official Flask docs)

Answer 2

Alternatively, you can add module = flaskapp:app to your ini.

Also, indeed, callable is addressed in uwsgi-docs more clearly:

Flask exports its WSGI function (the one we called “application” at the beginning of this quickstart) as “app”, so we need to instruct uWSGI to use it: uwsgi --wsgi-file myflaskapp.py --callable app