How to get the current port number in Flask?

Question

Using Flask, how can I get the current port number that flask is connected to? I want to start a server on a random port using port 0 but I also need to know which port I am on.

Edit

I think I've found a work around for my issue, although it isn't an answer to the question. I can iterate through ports starting with 49152 and attempt to use that port through app.run(port=PORT). I can do this in a try catch block so that if I get an Address already in use error, I can try the next port.

Answer 1

You can't easily get at the server socket used by Flask, as it's hidden in the internals of the standard library (Flask uses Werkzeug, whose development server is based on the stdlib's BaseHTTPServer).

However, you can create an ephemeral port yourself and then close the socket that creates it, then use that port yourself. For example:

# hello.py
from flask import Flask, request
import socket

app = Flask(__name__)

@app.route('/')
def hello():
    return 'Hello, world! running on %s' % request.host

if __name__ == '__main__':
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.bind(('localhost', 0))
    port = sock.getsockname()[1]
    sock.close()
    app.run(port=port)

will give you the port number to use. An example run:

$ python hello.py 
* Running on http://127.0.0.1:34447/

and, on browsing to http://localhost:34447/, I see

Hello, world! running on localhost:34447

in my browser.

Of course, if something else uses that port between you closing the socket and then Flask opening the socket with that port, you'd get an "Address in use" error, but you may be able to use this technique in your environment.