[FIXED] Start a flask application in separate thread

Issue

I'm currently developing a Python application on which I want to see real-time statistics. I wanted to use Flask in order to make it easy to use and to understand.

The issue is that my Flask server should start at the very beginning of my Python application and stop at the very end. It should look like this:

def main():
    """ My main application """
    from watcher.flask import app
    # watcher.flask define an app as in the Quickstart flask documentation.
    # See: http://flask.pocoo.org/docs/0.10/quickstart/#quickstart

    app.run() # Starting the flask application

    do_my_stuff()

    app.stop() # Undefined, for the idea 

Because I need my application context (for the statistics), I can't use a multiprocessing.Process. Then I was trying to use a threading.Thread, but it looks like Werkzeug doesn't like it:

 * Running on http://0.0.0.0:10079/
Exception in thread Flask Server:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
  File ".../develop-eggs/watcher.flask/src/watcher/flask/__init__.py", line 14, in _run
    app.run(host=HOSTNAME, port=PORT, debug=DEBUG)
  File ".../eggs/Flask-0.10.1-py2.7.egg/flask/app.py", line 772, in run
    run_simple(host, port, self, **options)
  File ".../eggs/Werkzeug-0.7-py2.7.egg/werkzeug/serving.py", line 609, in run_simple
    run_with_reloader(inner, extra_files, reloader_interval)
  File ".../eggs/Werkzeug-0.7-py2.7.egg/werkzeug/serving.py", line 524, in run_with_reloader
    signal.signal(signal.SIGTERM, lambda *args: sys.exit(0))
ValueError: signal only works in main thread

How can I do that without running Flask in the main thread?

Solution

You're running Flask in debug mode, which enables the reloader (reloads the Flask server when your code changes).

Flask can run just fine in a separate thread, but the reloader expects to run in the main thread.

---

To solve your issue, you should either disable debug (app.debug = False), or disable the reloader (app.use_reloader=False).

Those can also be passed as arguments to app.run: app.run(debug=True, use_reloader=False).

Answered By - Thomas Orozco