serve.py

import time
import subprocess
import sys

from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler


class ScriptReloader(FileSystemEventHandler):
    def on_modified(self, event):
        subprocess.run([sys.executable, "app.py"])


observer = Observer()
event_handler = ScriptReloader()
observer.schedule(event_handler, "app.py", recursive=True)
observer.start()

try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    observer.stop()
observer.join()