dash                 2.14.1
dash-core-components 2.0.0
dash-html-components 2.0.0
dash-table           5.0.0
dash-testing-stub    0.0.2

import dash
from dash import html

import threading
import time

def some_function_on_a_different_thread():
    try:
        print("starting long work")
        time.sleep(5)
        print("ending long work")
    except SystemExit:
        print("Thread received SystemExit.")


def test_some_function(dash_duo):
    app = dash.Dash(__name__)
    app.layout = html.Div(id="nully-wrapper", children=0)

    dash_duo.start_server(app)
    time.sleep(1)

    # This function could interact with the webpage, or do some other important work
    # in the background.
    my_thread = threading.Thread(target=some_function_on_a_different_thread)
    my_thread.start()

    dash_duo.server.stop()