Extended callback syntax

The DjangoDash class allows callbacks to request extra arguments when registered.

To do this, simply add to your callback function the extra arguments you would like to receive after the usual parameters for your Input and State. This will cause these callbacks registered with this application to receive extra parameters in addition to their usual callback parameters.

If you specify a kwargs in your callback, it will receive all possible extra parameters (see below for a list). If you specify explicitly extra parameters from the list below, only these will be passed to your callback.

For example, the plotly_apps.py example contains this dash application:

import dash
from dash import dcc, html

from django_plotly_dash import DjangoDash

a2 = DjangoDash("Ex2")

a2.layout = html.Div([
    dcc.RadioItems(id="dropdown-one",options=[{'label':i,'value':j} for i,j in [
    ("O2","Oxygen"),("N2","Nitrogen"),("CO2","Carbon Dioxide")]
    ],value="Oxygen"),
    html.Div(id="output-one")
    ])

@a2.callback(
    dash.dependencies.Output('output-one','children'),
    [dash.dependencies.Input('dropdown-one','value')]
    )
def callback_c(*args,**kwargs):
    da = kwargs['dash_app']
    return "Args are [%s] and kwargs are %s" %(",".join(args), kwargs)

The additional arguments, which are reported as the kwargs content in this example, include

callback_context:  The Dash callback context. See the `documentation <https://dash.plotly.com/advanced-callbacks`_ on the content of this variable. This variable is provided as an argument to the callback as well as the dash.callback_context global variable. dash_app:For stateful applications, the DashApp model instance dash_app_id:The application identifier. For stateless applications, this is the (slugified) name given to the DjangoDash constructor. For stateful applications, it is the (slugified) unique identifier for the associated model instance. request:The Django request object. session_state:A dictionary of information, unique to this user session. Any changes made to its content during the callback are persisted as part of the Django session framework. user:The Django User instance.

Possible alternatives to kwargs

@a2.callback(
    dash.dependencies.Output('output-one','children'),
    [dash.dependencies.Input('dropdown-one','value')]
    )
def callback_c(*args, dash_app):
    return "Args are [%s] and the extra parameter dash_app is %s" %(",".join(args), dash_app)

@a2.callback(
    dash.dependencies.Output('output-one','children'),
    [dash.dependencies.Input('dropdown-one','value')]
    )
def callback_c(*args, dash_app, **kwargs):
    return "Args are [%s], the extra parameter dash_app is %s and kwargs are %s" %(",".join(args), dash_app, kwargs)

The DashApp model instance can also be configured to persist itself on any change. This is discussed in the Django models and application state section.

The callback_context argument is provided in addition to the dash.callback_context global variable. As a rule, the use of global variables should generally be avoided. The context provided by django-plotly-dash is not the same as the one provided by the underlying Dash library, although its property values are the same and code that uses the content of this variable should work unchanged. The use of this global variable in any asychronous or multithreaded application is not supported, and the use of the callback argument is strongly recommended for all use cases.

Using session state

The walkthrough of the session state example details how the XXX demo interacts with a Django session.

Unless an explicit pipe is created, changes to the session state and other server-side objects are not automatically propagated to an application. Something in the front-end UI has to invoke a callback; at this point the latest version of these objects will be provided to the callback. The same considerations as in other Dash live updates apply.

The live updating section discusses how django-plotly-dash provides an explicit pipe that directly enables the updating of applications.