Dash callbacks not working, if dash app is created and called from flask

Dash Python
1

Dash app fails, when it is created and called through the flask sever. Dash 'Callback’s are not working and gives Post Request 400. Any help appreciated.

A sample code to reproduce the problem is below. I use latest Dash 0.22.0, Dash HTML Components 0.11.0, and Dash Core Components 0.24.1, and Flask 1.0.2:

# file 'simple_flask.py'

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_wtf.csrf import CSRFProtect


# Initialise flask App 
app = Flask(__name__, instance_relative_config=True)
#app.config.from_object('config.Config')

db = SQLAlchemy()         # SQLAlchemy
csrf_protect = CSRFProtect()

def set_app(app):

    # Setup WTForms CSRFProtect
    app.secret_key = 'My super secret key'
    csrf_protect.init_app(app)

    # Setup Flask-SQLAlchemy
    db.init_app(app) 
     
    # run in app context
    with app.test_request_context():
        db.create_all()

        from simple_dash import create_dash
        create_dash(app)


if __name__ == '__main__':
    set_app(app)
    app.run(host='127.0.0.1', port=5002, debug=True) 


## file 'simple_dash.py'

import dash
import dash_html_components as html
import dash_core_components as dcc

def create_dash(app):
    dash_app = dash.Dash(__name__, server=app, 
                        #static_folder='/static',
                        #url_base_pathname='/app/oga/', 
                        # csrf_protect=False
                        )
    dash_app.layout = html.Div([
        dcc.Dropdown(
            id='my-dropdown',
            options=[
                {'label': 'New York City', 'value': 'NYC'},
                {'label': 'Montreal', 'value': 'MTL'},
                {'label': 'San Francisco', 'value': 'SF'}
            ],
            value='NYC'
        ),
        html.Div(id='output-container')
    ])


    @dash_app.callback(
        dash.dependencies.Output('output-container', 'children'),
        [dash.dependencies.Input('my-dropdown', 'value')])
    def update_output(value):
        return 'You have selected "{}"'.format(value)

The missing reference after loading the page and any subsequent change in drop-down is: “POST /_dash-update-component HTTP/1.1” 400

2

Not sure this is the case, but I’ve managed to solve some missing requests by changing the host from 127.0.0.1 (local) to 0.0.0.0, in other terms:
if name == ‘main’:
set_app(app)
app.run(host=‘0.0.0.0’, port=5002, debug=True)

3

I changed the host to 0.0.0.0 from 127.0.0.1 and it didn’t help. I still get one missing POST request per dropdown callback, i.e. when the app loads and when I change the dropdown.

4

With direction from @T4rk1n, it was a conflict between CSRFProtect flask extension and Dash. I removed CSRFProtect for now.