Advanced Callbacks

Catching errors with PreventUpdate

In certain situations, you don’t want to update the callback output. You can
achieve this by
raising a PreventUpdate exception in
the callback function.

import dash
import dash_html_components as html
from dash.dependencies import Input, Output
from dash.exceptions import PreventUpdate

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

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

app.layout = html.Div([
    html.Button('Click here to see the content', id='show-secret'),
    html.Div(id='body-div')
])

@app.callback(
    Output(component_id='body-div', component_property='children'),
    Input(component_id='show-secret', component_property='n_clicks')
)
def update_output(n_clicks):
    if n_clicks is None:
        raise PreventUpdate
    else:
        return "Elephants are the only animal that can't jump"

if __name__ == '__main__':
    app.run_server(debug=True)

Displaying errors with dash.no_update

This example illustrates how you can show an error while keeping the previous
input, using dash.no_update
to update only some of the callback outputs.

import dash
import dash_core_components as dcc
import dash_html_components as html
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([
    html.P('Enter a composite number to see its prime factors'),
    dcc.Input(id='num', type='number', debounce=True, min=1, step=1),
    html.P(id='err', style={'color': 'red'}),
    html.P(id='out')
])

@app.callback(
    Output('out', 'children'),
    Output('err', 'children'),
    Input('num', 'value')
)
def show_factors(num):
    if num is None:
        # PreventUpdate prevents ALL outputs updating
        raise dash.exceptions.PreventUpdate

    factors = prime_factors(num)
    if len(factors) == 1:
        # dash.no_update prevents any single output updating
        # (note: it's OK to use for a single-output callback too)
        return dash.no_update, '{} is prime!'.format(num)

    return '{} is {}'.format(num, ' * '.join(str(n) for n in factors)), ''

def prime_factors(num):
    n, i, out = num, 2, []
    while i * i <= n:
        if n % i == 0:
            n = int(n / i)
            out.append(i)
        else:
            i += 1 if i == 2 else 2
    out.append(n)
    return out

if __name__ == '__main__':
    app.run_server(debug=True)

Enter a composite number to see its prime factors

Determining which Input has fired with dash.callback_context

In addition to event properties like n_clicks
that change whenever an event happens (in this case a click), there is
a global variable dash.callback_context,
available only inside a callback.
It has properties:
- triggered: list of changed properties. This will be empty on initial
load, unless an Input prop got its value from another initial callback.
After a user action it is a length-1 list, unless two properties of a
single component update simultaneously, such as a value and a timestamp
or event counter.
- inputs and states: allow you to access the callback params
by id and prop instead of through the function args. These have the form
of dictionaries { 'component_id.prop_name': value }
- outputs_list, inputs_list, and states_list: lists of inputs,
outputs, and state items arranged as you’ll find them in the callback
arguments and return value. This is mostly useful for
pattern-matching callbacks.
- response: The HTTP response object being constructed, useful for
changing cookies.
- record_timing: a method to report granular timing information, to be
seen in the Dev Tools

Here’s an example of how this can be done:

import json

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

app = dash.Dash(__name__)

app.layout = html.Div([
    html.Button('Button 1', id='btn-1'),
    html.Button('Button 2', id='btn-2'),
    html.Button('Button 3', id='btn-3'),
    html.Div(id='container')
])


@app.callback(Output('container', 'children'),
              Input('btn-1', 'n_clicks'),
              Input('btn-2', 'n_clicks'),
              Input('btn-3', 'n_clicks'))
def display(btn1, btn2, btn3):
    ctx = dash.callback_context

    if not ctx.triggered:
        button_id = 'No clicks yet'
    else:
        button_id = ctx.triggered[0]['prop_id'].split('.')[0]

    ctx_msg = json.dumps({
        'states': ctx.states,
        'triggered': ctx.triggered,
        'inputs': ctx.inputs
    }, indent=2)

    return html.Div([
        html.Table([
            html.Tr([html.Th('Button 1'),
                     html.Th('Button 2'),
                     html.Th('Button 3'),
                     html.Th('Most Recent Click')]),
            html.Tr([html.Td(btn1 or 0),
                     html.Td(btn2 or 0),
                     html.Td(btn3 or 0),
                     html.Td(button_id)])
        ]),
        html.Pre(ctx_msg)
    ])


if __name__ == '__main__':
    app.run_server(debug=True)

Legacy behaviour: using timestamps

Prior to dash v1.0, you needed to compare timestamp properties like
n_clicks_timestamp to find the most recent click. While existing uses of
*_timestamp continue to work for now, this approach is deprecated, and
may be removed in a future update. The one exception is
modified_timestamp from
dcc.Store,
which is safe to use, it is NOT deprecated.


Improving performance with memoization

Memoization allows you to bypass long computations by storing the
results of function calls.

To better understand how memoization works, let’s start with a simple
example.

import time
import functools32

@functools32.lru_cache(maxsize=32)
def slow_function(input):
    time.sleep(10)
    return 'Input was {}'.format(input)

Calling slow_function('test') the first time will take 10 seconds.
Calling it a second time with the same argument will take almost no time
since the previously computed result was saved in memory and reused.

The Performance section of the Dash docs delves a
little deeper into leveraging multiple processes and threads in
conjunction with memoization to further improve performance.

When Are Callbacks Executed?

This section describes the circumstances under which the dash-renderer
front-end client can make a request to the Dash back-end server (or the
clientside callback code) to execute a callback function.

When A Dash App First Loads

All of the callbacks in a Dash app are executed with the initial value
of their inputs when the app is first loaded. This is known as the
“initial call” of the callback. To learn how to suppress this behavior,
see the documentation for the
prevent_initial_call
attribute of Dash callbacks.

It is important to note that when a Dash app is initially loaded in a
web browser by the dash-renderer front-end client, its entire callback
chain is introspected recursively.

This allows the dash-renderer to predict the order in which callbacks
will need to be executed, as callbacks are blocked when their inputs are
outputs of other callbacks which have not yet fired. In order to unblock
the execution of these callbacks, first callbacks whose inputs are
immediately available must be executed. This process helps the
dash-renderer to minimize the time and effort it uses, and avoid
unnecessarily redrawing the page, by making sure it only requests that
a callback is executed when all of the callback’s inputs have reached
their final values.

Examine the following Dash app:

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

app = dash.Dash()
app.layout = html.Div(
    [
        html.Button("execute callback", id="button_1"),
        html.Div(children="callback not executed", id="first_output_1"),
        html.Div(children="callback not executed", id="second_output_1"),
    ]
)


@app.callback(
    Output("first_output_1", "children"),
    Output("second_output_1", "children"),
    Input("button_1", "n_clicks")
)
def change_text(n_clicks):
    return ["n_clicks is " + str(n_clicks), "n_clicks is " + str(n_clicks)]

if __name__ == '__main__':
    app.run_server(debug=True)
callback not executed
callback not executed

Notice that when this app is finished being loaded by a web browser and
ready for user interaction, the html.Div components do not say
“callback not executed” as declared in the app’s layout, but rather
“n_clicks is None” as the result of the
change_text() callback being
executed. This is because the “initial call” of the callback occurred
with n_clicks having the value of None.

As A Direct Result of User Interaction

Most frequently, callbacks are executed as a direct result of user
interaction, such as clicking a button or selecting an item in a
dropdown menu. When such interactions occur, Dash components communicate
their new values to the dash-renderer front-end client, which then
requests that the Dash server execute any callback function that has the
newly changed value as input.

If a Dash app has multiple callbacks, the dash-renderer requests
callbacks to be executed based on whether or not they can be immediately
executed with the newly changed inputs. If several inputs change
simultaneously, then requests are made to execute them all.

Whether or not these requests are executed in a synchronous or
asynchronous manner depends on the specific setup of the Dash back-end
server. If it is running in a multi-threaded environment, then all of
the callbacks can be executed simultaneously, and they will return
values based on their speed of execution. In a single-threaded
environment however, callbacks will be executed one at a time in the
order they are received by the server.

In the example application above, clicking the button results in the
callback being executed.

As An Indirect Result of User Interaction

When a user interacts with a component, the resulting callback might
have outputs that are themselves the input of other callbacks. The
dash-renderer will block the execution of such a callback until the
callback whose output is its input has been executed.

Take the following Dash app:

import dash
from dash.dependencies import Input, Output
import dash_html_components as html
from datetime import datetime
import time

app = dash.Dash()
app.layout = html.Div(
    [
        html.Button("execute fast callback", id="button_3"),
        html.Button("execute slow callback", id="button_4"),
        html.Div(children="callback not executed", id="first_output_3"),
        html.Div(children="callback not executed", id="second_output_3"),
        html.Div(children="callback not executed", id="third_output_3"),
    ]
)


@app.callback(
    Output("first_output_3", "children"),
    Input("button_3", "n_clicks"))
def first_callback(n):
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return "in the fast callback it is " + current_time


@app.callback(
    Output("second_output_3", "children"), Input("button_4", "n_clicks"))
def second_callback(n):
    time.sleep(5)
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return "in the slow callback it is " + current_time


@app.callback(
    Output("third_output_3", "children"),
    Input("first_output_3", "children"),
    Input("second_output_3", "children"))
def third_callback(n, m):
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return "in the third callback it is " + current_time


if __name__ == '__main__':
    app.run_server(debug=True)
callback not executed
callback not executed
callback not executed

The above Dash app demonstrates how callbacks chain together. Notice
that if you first click “execute slow callback” and then click “execute
fast callback”, the third callback is not executed until after the slow
callback finishes executing. This is because the third callback has the
second callback’s output as its input, which lets the dash-renderer
know that it should delay its execution until after the second callback
finishes.

When Dash Components Are Added To The Layout

It is possible for a callback to insert new Dash components into a Dash
app’s layout. If these new components are themselves the inputs to other
callback functions, then their appearance in the Dash app’s layout will
trigger those callback functions to be executed.

In this circumstance, it is possible that multiple requests are made to
execute the same callback function. This would occur if the callback in
question has already been requested and its output returned before the
new components which are also its inputs are added to the layout.

Prevent Callback Execution Upon Initial Component Render

You can use the prevent_initial_call
attribute to prevent callbacks
from firing when their inputs initially appear in the layout of your
Dash application.

This attribute applies when the layout of your Dash app is initially
loaded, and also when new components are introduced into the layout when
a callback has been triggered.

import dash
from dash.dependencies import Input, Output
import dash_html_components as html
from datetime import datetime
import time

app = dash.Dash()

app.layout = html.Div(
    [
        html.Button("execute callbacks", id="button_2"),
        html.Div(children="callback not executed", id="first_output_2"),
        html.Div(children="callback not executed", id="second_output_2"),
        html.Div(children="callback not executed", id="third_output_2"),
        html.Div(children="callback not executed", id="fourth_output_2"),
    ]
)


@app.callback(
    Output("first_output_2", "children"),
    Output("second_output_2", "children"),
    Input("button_2", "n_clicks"), prevent_initial_call=True)
def first_callback(n):
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return ["in the first callback it is " + current_time, "in the first callback it is " + current_time]


@app.callback(
    Output("third_output_2", "children"), Input("second_output_2", "children"), prevent_initial_call=True)
def second_callback(n):
    time.sleep(2)
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return "in the second callback it is " + current_time


@app.callback(
    Output("fourth_output_2", "children"),
    Input("first_output_2", "children"),
    Input("third_output_2", "children"), prevent_initial_call=True)
def third_output(n, m):
    time.sleep(2)
    now = datetime.now()
    current_time = now.strftime("%H:%M:%S")
    return "in the third callback it is " + current_time


if __name__ == '__main__':
    app.run_server(debug=True)
callback not executed
callback not executed
callback not executed
callback not executed

However, the above behavior only applies if both the callback output and
input are present in the app layout upon initial load of the application.

It is important to note that prevent_initial_call
will not prevent a callback from firing in the case where the callback’s input is inserted
nto the layout as the result of another callback after the app initially
loads unless the output is inserted alongside that input!

In other words, if the output of the callback is already present in the
app layout before its input is inserted into the layout,
prevent_initial_call
will not prevent its execution when the input is first inserted into the layout.

Consider the following example:

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

app = dash.Dash(__name__, suppress_callback_exceptions=True)
server = app.server
app.layout = html.Div([
    dcc.Location(id='url'),
    html.Div(id='layout-div'),
    html.Div(id='content')
])

@app.callback(Output('content', 'children'), Input('url', 'pathname'))
def display_page(pathname):
    return html.Div([
        dcc.Input(id='input', value='hello world'),
        html.Div(id='output')
    ])

@app.callback(Output('output', 'children'), Input('input', 'value'), prevent_initial_call=True)
def update_output(value):
    print('>>> update_output')
    return value

@app.callback(Output('layout-div', 'children'), Input('input', 'value'), prevent_initial_call=True)
def update_layout_div(value):
    print('>>> update_layout_div')
    return value

In this case, prevent_initial_call
will prevent the update_output()
callback from firing when its input is first inserted into the app
layout as a result of the display_page()
callback. This is because both the input and output of the callback are already
contained within the app layout when the callback executes.

However, because the app layout contains only the output of the
callback, and not its input, prevent_initial_call
will not prevent the update_layout_div()
callback from firing. Since suppress_callback_exceptions=True is specified here,
Dash has to assume that the input is present in the app layout when the app is
initialized. From the perspective of the output element in this example,
the new input component is handled as if an existing input had been
provided a new value, rather than treating it as initially rendered.

Circular Callbacks

As of dash v1.19.0, you can create circular updates
within the same callback.

Circular callback chains that involve multiple callbacks are not supported.

Circular callbacks can be used to keep multiple inputs synchronized to
each other.

Synchronizing a Slider with a Text Input Example

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

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

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

app.layout = html.Div(
    [
        dcc.Slider(
            id="slider-circular", min=0, max=20, 
            marks={i: str(i) for i in range(21)}, 
            value=3
        ),
        dcc.Input(
            id="input-circular", type="number", min=0, max=20, value=3
        ),
    ]
)
@app.callback(
    Output("input-circular", "value"),
    Output("slider-circular", "value"),
    Input("input-circular", "value"),
    Input("slider-circular", "value"),
)
def callback(input_value, slider_value):
    ctx = dash.callback_context
    trigger_id = ctx.triggered[0]["prop_id"].split(".")[0]
    value = input_value if trigger_id == "input-circular" else slider_value
    return value, value

if __name__ == '__main__':
    app.run_server(debug=True)

Displaying Two Inputs with Different Units Example

import dash
from dash.dependencies import Input, Output, State
import dash_core_components as dcc
import dash_html_components as html
external_stylesheets = ["https://codepen.io/chriddyp/pen/bWLwgP.css"]

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

app.layout = html.Div([
    html.Div('Convert Temperature'),
    'Celsius',
    dcc.Input(
        id="celsius",
        value=0.0,
        type="number"
    ),
    ' = Fahrenheit',
    dcc.Input(
        id="fahrenheit",
        value=32.0,
        type="number",
    ),
])

@app.callback(
    Output("celsius", "value"),
    Output("fahrenheit", "value"),
    Input("celsius", "value"),
    Input("fahrenheit", "value"),
)
def sync_input(celsius, fahrenheit):
    ctx = dash.callback_context
    input_id = ctx.triggered[0]["prop_id"].split(".")[0]
    if input_id == "celsius":
        fahrenheit= None if celsius is None else (float(celsius) * 9/5) + 32
    else:
        celsius = None if fahrenheit is None else (float(fahrenheit) - 32) * 5/9
    return celsius, fahrenheit

if __name__ == "__main__":
    app.run_server(debug=True)
Convert Temperature
Celsius
= Fahrenheit

Synchronizing Two Checklists

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

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

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

options = [
    {"label": "New York City", "value": "NYC"},
    {"label": "Montréal", "value": "MTL"},
    {"label": "San Francisco", "value": "SF"},
]
all_cities = [option["value"] for option in options]

app.layout = html.Div(
    [
        dcc.Checklist(
            id="all-checklist",
            options=[{"label": "All", "value": "All"}],
            value=[],
            labelStyle={"display": "inline-block"},
        ),
        dcc.Checklist(
            id="city-checklist",
            options=options,
            value=[],
            labelStyle={"display": "inline-block"},
        ),
    ]
)
@app.callback(
    Output("city-checklist", "value"),
    Output("all-checklist", "value"),
    Input("city-checklist", "value"),
    Input("all-checklist", "value"),
)
def sync_checklists(cities_selected, all_selected):
    ctx = dash.callback_context
    input_id = ctx.triggered[0]["prop_id"].split(".")[0]
    if input_id == "city-checklist":
        all_selected = ["All"] if set(cities_selected) == set(all_cities) else []
    else:
        cities_selected = all_cities if all_selected else []
    return cities_selected, all_selected

if __name__ == "__main__":
    app.run_server(debug=True)