Two apps using same port do not raise any error

Dash Python
1

Hi all,

Please see the following two simple apps using the same port:

app1.py:

#!/usr/bin/env python

import dash
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([

        'Group outliers by: ',
        html.Div([
            dcc.Dropdown(
                id='group-by',
                options=[{'label': 1, 'value': 'hi'}, {'label': 2, 'value': 'hello'}],
                value='hello'
                ),
            ],
        ),
        'Sample HTML'

])

app.run_server(debug=False, port= 8050, host= 'localhost')

http://localhost:8050 reflects app1. While app1 is running, I launch another app using the same port:

app2.py:

#!/usr/bin/env python

import dash
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([

        'The dropdown menu is vanished',
        html.Br(),
        'Sample HTML'

])

app.run_server(debug=False, port= 8050, host= 'localhost')

When refreshed, http://localhost:8050 is not overwritten to reflect app2. This behavior would have been fine if we could get an error.

The question-- is it possible to raise an error from dash if the port is already in use? A way around would be to check the port at the beginning of each app:

import socket
s= socket.socket
s.connect(('localhost',8050))
# if not is use, would give `ConnectionRefusedError`
# otherwise none

Thanks!

2

I get an error if the port is in use (Ubuntu 18.04/Python 3.7). What OS/Python are you on?

1 Like
3

It looks like the error comes up on Linux but not on Windows:

 * Serving Flask app "app2" (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
Traceback (most recent call last):
  File "./app2.py", line 20, in <module>
    app.run_server(debug=False, port= 8050, host= 'localhost')
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/dash/dash.py", line 1574, in run_server
    self.server.run(host=host, port=port, debug=debug, **flask_run_options)
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/flask/app.py", line 990, in run
    run_simple(host, port, self, **options)
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/werkzeug/serving.py", line 1052, in run_simple
    inner()
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/werkzeug/serving.py", line 1005, in inner
    fd=fd,
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/werkzeug/serving.py", line 848, in make_server
    host, port, app, request_handler, passthrough_errors, ssl_context, fd=fd
  File "/home/avatar/miniconda3/lib/python3.7/site-packages/werkzeug/serving.py", line 740, in __init__
    HTTPServer.__init__(self, server_address, handler)
  File "/home/avatar/miniconda3/lib/python3.7/socketserver.py", line 452, in __init__
    self.server_bind()
  File "/home/avatar/miniconda3/lib/python3.7/http/server.py", line 137, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/home/avatar/miniconda3/lib/python3.7/socketserver.py", line 466, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

Does it make sense? :o

4

I just tried on a Windows laptop, and here i don’t get the error. So it seems to be OS related indeed…

1 Like