How to select random port number in Flask?

Most operating systems these days have a command called netstat that will give you a list of all ports that are currently in use, as well as which applications are running on them if you ask nicely enough. 🙂

It should be pretty easy to parse with os.popen or the subprocess module.

Aside from that, you could just track which ports you’re using as you start each server…

Also, if you happen to be doing this from inside an http request, you could look at the SERVER_PORT cgi variable from the wsgi environment.

Binding to port 0 is correct. That will make the operating system choose an available port between 1024 and 65535 for you.

To retrieve the chosen port after binding, use your_socket.getsockname()[1].

So all you need to find out is how to access the listening socket Flask uses.

socket.getsockname() docs: http://docs.python.org/library/socket.html#socket.socket.getsockname

As pointed by @VinaySajip Flask use standard server socket but it never assign the instance to any variable, just construct it and call serve_forever() in line.

Anyway instead try to extract socket from flask’s app as said @ThiefMaster, we can intercept bind_socket() call and read the address without take care of concurrent socket creation. Here is my solution:

from flask import Flask, request
import socketserver

app = Flask("")

original_socket_bind = SocketServer.TCPServer.server_bind
def socket_bind_wrapper(self):
    ret = original_socket_bind(self)
    print("Socket running at {}:{}".format(*self.socket.getsockname()))
    # Recover original implementation
    socketserver.TCPServer.server_bind = original_socket_bind
    return ret

@app.route("/")
def hello():
    return 'Hello, world! running on {}'.format(request.host)

socketserver.TCPServer.server_bind = socket_bind_wrapper   #Hook the wrapper
app.run(port=0, debug=True)

I agree with the accepted answer that

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, I just discovered that Werkzeug exposes an option to pass a file descriptor for use as it’s socket when running werkzeug.serving.BaseWSGIServer (which is what flask’s run() function ends up doing). Using this functionality it is possible to select a random available port and then tell Werkzeug to use it.

Here is the startup of my application:

import logging
import pprint
import sys
from flask import Flask

applog = logging.getLogger(__name__)
applog.setLevel(logging.INFO)
app = Flask(__name__)

if __name__ == '__main__':

    app.config.from_object('myapp.default_settings')
    try:
        app.config.from_envvar('MYAPP_SETTINGS')
    except RuntimeError:
        applog.warning("MYAPP_SETTINGS environment variable not set or invalid. Using default settings.")

    # SERVER_NAME should be in the form of localhost:5000, 127.0.0.1:0, etc...
    # So, split on : and take the second element as the port
    # If the port is 0, we need to pick a random port and then tell the server to use that socket
    if app.config['SERVER_NAME'] and int(app.config['SERVER_NAME'].split(':')[1]) == 0:
        import socket, os

        # Chose a random available port by binding to port 0
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.bind((app.config['SERVER_NAME'].split(':')[0], 0))
        sock.listen()

        # Tells the underlying WERKZEUG server to use the socket we just created
        os.environ['WERKZEUG_SERVER_FD'] = str(sock.fileno())

        # Update the configuration so it matches with the port we just chose
        # (sock.getsockname will return the actual port being used, not 0)
        app.config['SERVER_NAME'] = '%s:%d' % (sock.getsockname())

    # Optionally write the current port to a file on the filesystem
    if app.config['CREATE_PORT_FILE']:
        with open(app.config['PORT_FILE'], 'w') as port_file:
            port_file.write(app.config['SERVER_NAME'].split(':')[1])

    applog.info("Running app on %s" % (app.config['SERVER_NAME']))
    applog.info("Active Settings:%s" % pprint.pformat(app.config, compact=True))
    app.run()

Contents of myapp.default_settings:

SERVER_NAME = '127.0.0.1:0'
PORT_FILE = 'current_port'
CREATE_PORT_FILE = True

The important bit here is setting os.environ['WERKZEUG_SERVER_FD']. Werkzeug looks at this environment variable during the run_simple function and, if it is defined, passes it as the fd parameter to the make_server function. This eventually is used as the socket for communication.

While I can’t guarantee the stability of this approach (I don’t know how well supported the WERKZEUG_SERVER_FD environment variable is) I prefer it to the suggested solutions so far because:

1. I don’t have to loop through a range of ports catching exceptions, I just get the first available port straight from the OS.
2. There is no chance my chosen random port gets taken between the time I bind a random port and when I run the application because the port I am binding is the port my application ends up using.

The code above also logs the port being used and optionally writes the current port being used to a file specified by a configuration option.