running Thread object cause twice running

[SigmaStar]

following code cause MyThread object running twice.

import threading
import time
from flask import Flask

class MyThread(threading.Thread):
    def __init__(self):
        super().__init__()
        self.event = threading.Event()

    def run(self):
        print("Hello")


app = Flask(__name__)
t1 = MyThread()
t1.start()
app.run(host='0.0.0.0', port = 19001, debug = True)

set debug as False can avoid this problem but why is that? Is that a feature?

[SigmaStar]

my python version is 3.9.13 and Flask version is 1.1.2

[ThiefMaster]

This is because of the dev server's autoreloader which restarts the process (the original one monitors for changes, while the new one runs the actual app)

I'd also question why you need to run a background thread in a webapp process (and not use celery, cronjobs, etc. to offload long-running or periodic tasks) ;)