You can use BackgroundScheduler()
from APScheduler package (v3.5.3):
import time
import atexit
from apscheduler.schedulers.background import BackgroundScheduler
def print_date_time():
print(time.strftime("%A, %d. %B %Y %I:%M:%S %p"))
scheduler = BackgroundScheduler()
scheduler.add_job(func=print_date_time, trigger="interval", seconds=3)
scheduler.start()
# Shut down the scheduler when exiting the app
atexit.register(lambda: scheduler.shutdown())
Note that two of these schedulers will be launched when Flask is in debug mode. For more information, check out this question.