You can easily kill the Flask application and restart it, but sometimes
you will get an error that the address is already in use.
To recover from this problem, run the lsof program to see which program
that applies the 5000 port (Flask runs its server on http://127.0.0.1:5000,
which means that it uses the 5000 port). Find the PID of the program
that occupies the port and force abortion of that program:
Terminal> lsof -i :5000
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
python 48824 hpl 3u IPv4 1128848 0t0 TCP ...
Terminal> kill -9 48824
You are now ready to restart a Flask application.