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Cannot run Dash in a thread in Debug mode #952
Description
I would like to build an application using some kind of model-view-controller pattern with two main threads:
- One thread running the Dash application (View)
- Another one performing real time control of a machine through a serial link (Model & Controller)
Here is a minimal working example of a typical code to perform such a task:
import dash
from dash.dependencies import Input, Output
import dash_html_components as html
import dash_core_components as dcc
import threading
import time
counter = 0
class DashThread(threading.Thread):
def __init__(self, name):
threading.Thread.__init__(self)
self.name = name
def run(self):
global counter
app = dash.Dash(__name__)
app.layout = html.Div([
dcc.Interval(id='my_interval', disabled=False, n_intervals=0),
html.Div("Counter :", style={"display": "inline-block"}),
html.Div(children=None, id="cnt_val", style={"display": "inline-block", "margin-left": "15px"}),
])
@app.callback(Output('cnt_val', 'children'), [Input('my_interval', 'n_intervals')])
def update(n_intervals):
return counter
app.run_server(dev_tools_silence_routes_logging=True, debug=True)
class CountingThread(threading.Thread):
def __init__(self, name):
threading.Thread.__init__(self)
self.name = name
def run(self):
global counter
while True:
counter += 1
print(counter)
time.sleep(1)
a = DashThread("The Dash Application")
b = CountingThread("An Independent Thread")
b.start()
a.start()
a.join()
b.join()This scripts creates two Thread objects:
DashThreadcontaining a Dash application which periodically updates the display of the global variablecounter.CountingThread, which periodically increases the global variablecounter
However, the code crashes with the error:
Traceback (most recent call last):
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/threading.py", line 932, in _bootstrap_inner
self.run()
File "/Users/XXX/OneDrive - Biosency/Python/05_a_atmos/src/test_c.py", line 31, in run
app.run_server(dev_tools_silence_routes_logging=True, debug=True)
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/site-packages/dash/dash.py", line 1718, in run_server
self.server.run(host=host, port=port, debug=debug, **flask_run_options)
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/site-packages/flask/app.py", line 990, in run
run_simple(host, port, self, **options)
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/site-packages/werkzeug/serving.py", line 1050, in run_simple
run_with_reloader(inner, extra_files, reloader_interval, reloader_type)
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/site-packages/werkzeug/_reloader.py", line 330, in run_with_reloader
signal.signal(signal.SIGTERM, lambda *args: sys.exit(0))
File "/Users/XXX/anaconda3/envs/multiproc_env/lib/python3.8/signal.py", line 47, in signal
handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main threadThis seems to be related to the fact that Flask does not like to be launched in another thread that the main one, especially with the debug=True option, as documented here and there.
I tried to adapt this answer yielding the following code:
[...]
import time
counter = 0
app = dash.Dash(__name__)
class DashThread(threading.Thread):
[...]
def run(self):
global counter
global app
app.layout = html.Div([
[...]without success. The issue disappears, however, if I remove the debug=True argument, leading:
[...]
def update(n_intervals):
return counter
app.run_server(dev_tools_silence_routes_logging=True) # , debug=True) <- Commented out
class CountingThread(threading.Thread):
[...]it works fine. Except of course, I am no longer able to use the debugging functionalities such as reload on save, etc.
Could this issue be solved by changing the way Flaskis called inside the Dash libraries?
Reproduced with Python 3.8, dash 1.20.0, flask 1.1.2.
Activity
berthoud commentedon Dec 29, 2023
Before finding this I asked the same question on stackexchange at https://stackoverflow.com/questions/77724451/how-to-run-a-dash-app-with-threading-and-with-debug-true maybe someone can help us there.
sanskarbiswal commentedon Mar 22, 2024
Try using the following
app.run_server(debug=True, use_reloader=False)berthoud commentedon Mar 23, 2024
This works - thanks Sanskarbiswal