Trying to run Flask app gives "Address already in use"

It means there's another service's using that port (8080 in this case). Maybe because you forgot close another running Flask app and it's using 8080 port.

However, you could change the port you're using, for example change it to 4444 like this:

if __name__=="__main__":
    app.run(host=os.getenv('IP', '0.0.0.0'), 
            port=int(os.getenv('PORT', 4444)))

But anyways, I think you'd like to know which program is using that part if it's not your program. You could use nmap or netcat GNU program to check it.

Here's the netcat way (from here):

$ sudo netstat -nlp | grep 8080
tcp  0  0  0.0.0.0:8080  0.0.0.0:*  LISTEN  125004/nginx

When you got it, I'd suggest stop it manually (for example if it's nginx or other HTTP servers, then stop it via service command or systemctl if you're using systemd Linux)

You can also kill it via command kill:

kill <pid>

You can also kill it via killall or pkill, it use a process name instead of it's pid:

killall/pkill <process name>

Try to kill all the other processes which are running on your server using this command

sudo fuser -k xxxx/tcp

replace xxxx with your port name

You can get the pid of all running processes having python keyword using the command:

ps -fA | grep python

After getting the pid's use kill command as follows:

kill -9 pid

After running above two commands now run the flask app, it will work fine