Advanced Callbacks | Dash for Python Documentation (2024)

To get the most out of this page, make sure you’ve read about Basic Callbacks in the Dash Fundamentals.

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.

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.

Updating Component Properties when a Callback is Running

New in Dash 2.16

You can use the running argument on a callback to update specific component-property pairs when the callback is running. For example, you could disable the button that triggered the callback while the callback is still running.

running accepts a list of three element tuples, where:

• The first element of each tuple must be an Output dependency object referencing a property of a component in the app layout.
• The second element is the value that the property should be set to while the callback is running.
• The third element is the value the property should be set to when the callback completes.

In the following example, our running argument sets the disabled property on the submit-button to True while the callback is running and sets it back to False once the callback completes.

There is a known issue where using running with a multi-pages app doesn’t work as expected when a user changes page when the callback is running.

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. Using dash.callback_context, you can determine which component/property pairs triggered a callback.

Below is a summary of properties of dash.callback_context outlining the basics of when to use them. For more detail and examples see Determining Which Callback Input Changed.

For more examples of minimal Dash apps that use dash.callback_context, go to the community-driven Example Index.

Properties for callback_context

In Dash 2.4 and later, dash.callback_context (or dash.ctx) has three additional properties to make it easier to work with.

• triggered_id: The id of the component that triggered the callback.
• triggered_prop_ids: A dictionary of the component ids and props that triggered the callback. Useful when multiple inputs can trigger the callback at the same time, or multiple properties of the same component can trigger the callback.
• args_grouping: A dictionary of the inputs used with flexible callback signatures. The keys are the variable names and the values are dictionaries containing:
  • “id”: the component ID. If it’s a pattern matching ID, it will be a dict.
  • “id_str”: for pattern matching IDs, it’s the stringified dict ID with no white spaces.
  • “property”: the component property used in the callback.
  • “value”: the value of the component property at the time the callback was fired.
  • “triggered”: a boolean indicating whether this input triggered the callback.

Dash 2.4 and earlier versions of Dash have the following properties

• triggered: list of all the Input props that changed and caused the callback to execute.
  It is empty when the callback is called on initial load, unless an Input prop got its
  value from another initial callback. Callbacks triggered by user actions typically have
  one item in triggered, unless the same action changes two props at once or the callback
  has several Input props that are all modified by another callback based on a single user action.

More about empty triggered lists: For backward compatibility purposes, an empty
triggered is not really empty. It’s
falsy so that you can use if triggered to detect the initial call, but it still has a placeholder
element so that ctx.triggered[0]["prop_id"].split(".") yields a blank ID and prop ["", ""]
instead of an error.

• 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:

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 timeimport functools32@functools32.lru_cache(maxsize=32)def slow_function(input): time.sleep(10) return f'Input was {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 circ*mstances 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:

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:

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 circ*mstance, 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.

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
into 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:

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.

Callbacks with No Outputs

New in 2.17

All previous examples have inputs that trigger callbacks as well as outputs that are updated. Callbacks also support having no outputs. This can be useful for cases where you want some code to run when an action triggers a callback, but you don’t want to update any component property. For example, you may want to fetch some data and save it, send emails, or interact with other external services outside of the Dash ecosystem.

In the following example, using the dcc.Upload component, we allow the user to upload a file to the server.

Our example has no return statements. Callbacks without outputs should not return a value, because there are no outputs to update. If you return a value from the callback when there is no output to update, you’ll see an error in Dash Dev Tools.

Setting Properties Directly

New in 2.17

You can update component-property pairs directly from a callback without them being callback outputs by using set_props. With this approach, conditionally updating different component-property pairs is simpler because you don’t need to add them all as outputs and use dash.no_update.

set_props takes the ID of the component to update as its first argument, followed by a dict of properties to update. Each dict key should be a property name, with the key’s value being the value to update the component property to.

Here, we have a callback that uses set_props to set the modal’s is_open property to False when the callback is triggered by a user selecting the modal_close button. Otherwise, it’s set to True and the modal content is displayed. We have no outputs on this callback, but set_props can also be combined with outputs.

This example is based on the Popup On Row Selection Example on the Dash AG Grid page, which instead uses outputs and dash.no_update.

On callbacks that don’t run in the background, like in the following example, updates made using set_props and via outputs all happen at the same time, when the callback finishes running. On background callbacks, set_props updates take place immediately. See the Background Callbacks page for an example.

In the example above, we use set_props to simplify the code. In other cases, however, using Outputs may lead to simpler code because the Outputs are defined in the callback signature, making it easier to tell which component properties a callback updates.

Limitations

• Component properties updated using set_props won’t appear in the callback graph for debugging.
• Component properties updated using set_props won’t appear as loading when they are wrapped with a dcc.Loading component.
• set_props doesn’t validate the id or property names provided, so no error will be displayed if they contain typos. This can make apps that use set_props harder to debug.
• Using set_props with chained callbacks may lead to unexpected results.

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