Start a flask application in separate thread

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).

Updated answer for Python 3 that's a bit simpler:

from flask import Flask                                                         
import threading

data = 'foo'
host_name = "0.0.0.0"
port = 23336
app = Flask(__name__)

@app.route("/")
def main():
    return data

if __name__ == "__main__":
    threading.Thread(target=lambda: app.run(host=host_name, port=port, debug=True, use_reloader=False)).start()