Flask: OSError: [Errno 98] Address already in use - but why? [duplicate]

Question

I have a very simple code here:

from flask import Flask

app = Flask(__name__)

if __name__ == '__main__':
    app.run(debug=True)

I have run it the first time and it worked, then ctrl+c didn't work so I just killed python terminal myself. Then I tried to run it again and I get an error:

 * Serving Flask app "untitled1" (lazy loading)
 * Environment: production
 WARNING: Do not use the development server in a production 
 environment.
 Use a production WSGI server instead.
 * Debug mode: on
 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3/dist- 
 packages/spyderlib/widgets/externalshell/sitecustomize.py", line 699, 
 in runfile
 execfile(filename, namespace)
 File "/usr/lib/python3/dist- 
 packages/spyderlib/widgets/externalshell/sitecustomize.py", line 88, 
in execfile
exec(compile(open(filename, 'rb').read(), filename, 'exec'), 
namespace)
File "/home/teoman/Desktop/flaskapp/untitled1.py", line 13, in 
<module>
app.run(debug=True)
File "/home/teoman/.local/lib/python3.5/site-packages/flask/app.py", 
line 943, in run
run_simple(host, port, self, **options)
File "/home/teoman/.local/lib/python3.5/site- 
packages/werkzeug/serving.py", line 795, in run_simple
s.bind(get_sockaddr(hostname, port, address_family))
OSError: [Errno 98] Address already in use

I have found similar questions asked but I couldn't understand it because of the solutions were also complicated, I'm very new to Flask app (like ten minutes), I use Linux Mint MATE.

NOTE: localhost:5000 works fine still I do not have any python command-lines open working Flask app.

Answer 1

It seems like some other application is using port 5000. You can use netstat to see which processes are using ports with the command below:

netstat -tulpn

You can also check common apps using port 5000 by visiting this page