AnyWidgetComponent

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import panel as pn

pn.extension()

Panel’s AnyWidgetComponent class simplifies the creation of custom Panel components using the AnyWidget Front End Module (AFM) specification maintained by AnyWidget.

This allows the Panel, Jupyter and other communities to collaborate and share JavaScript code for widgets.

import param
import panel as pn
from panel.custom import AnyWidgetComponent

pn.extension()

class CounterWidget(AnyWidgetComponent):
    _esm = """
    function render({ model, el }) {
      let count = () => model.get("value");
      let btn = document.createElement("button");
      btn.innerHTML = `count is ${count()}`;
      btn.addEventListener("click", () => {
        model.set("value", count() + 1);
        model.save_changes();
      });
      model.on("change:value", () => {
        btn.innerHTML = `count is ${count()}`;
      });
      el.appendChild(btn);
    }
    export default { render };
    """
    value = param.Integer()

CounterWidget().servable()

Note

Panel’s AnyWidgetComponent supports using the AnyWidget API on the JavaScript side and the param parameters API on the Python side.

If you are looking to create custom components using Python and Panel component only, check out Viewer.

API

AnyWidgetComponent Attributes

• _esm (str | PurePath): This attribute accepts either a string or a path that points to an ECMAScript module. The ECMAScript module should export a default object or function that returns an object. The object should contain a render function and optionally an initialize function. In a development environment such as a notebook or when using --dev flag, the module will automatically reload upon saving changes.

• _importmap (dict | None): This optional dictionary defines an import map, allowing you to customize how module specifiers are resolved.

• _stylesheets (list[str] | list[PurePath]): This optional attribute accepts a list of CSS strings or paths to CSS files. It supports automatic reloading in development environments. It works similarly to the AnyWidget _css attribute.

You may specify a path to a file as a string instead of a PurePath. The path should be specified relative to the file it is referenced in.

render Function

The _esm default object must contain a render function. It accepts the following parameters:

• model: Represents the parameters of the component and provides methods to .get values, .set values, and .save_changes. In addition to the AnyWidgets methods, Panel uniquely provides the get_child method to enable rendering of child models.

• el: The parent HTML element to which HTML elements are appended.

For more details, see AnyWidget.

Usage

Styling with CSS

Include CSS within the _stylesheets attribute to style the component. The CSS is injected directly into the component’s HTML.

import param
import panel as pn

from panel.custom import AnyWidgetComponent

pn.extension()

class CounterWidget(AnyWidgetComponent):
    _esm = """
    function render({ model, el }) {
      let count = () => model.get("value");
      let btn = document.createElement("button");
      btn.innerHTML = `count is ${count()}`;
      btn.addEventListener("click", () => {
        model.set("value", count() + 1);
        model.save_changes();
      });
      model.on("change:value", () => {
        btn.innerHTML = `count is ${count()}`;
      });
      el.appendChild(btn);
    }
    export default { render };
    """
    _stylesheets = [
        """
        button { color: white; font-size: 1.75rem; background-color: #ea580c; padding: 0.5rem 1rem; border: none; border-radius: 0.25rem; }
        button:hover { background-color: #9a3412; }
        """
    ]
    value = param.Integer()

CounterWidget().servable()

Note

The AnyWidget will automatically add the CSS class counter-widget to the el.

The AnyWidgetComponent does not add this class, but you can do it yourself via el.classList.add("counter-widget");.

Dependency Imports

JavaScript dependencies can be directly imported via URLs, such as those from esm.sh.

import panel as pn

from panel.custom import AnyWidgetComponent


class ConfettiButton(AnyWidgetComponent):

    _esm = """
    import confetti from "https://esm.sh/canvas-confetti@1.6.0";

    function render({ el }) {
      let btn = document.createElement("button");
      btn.innerHTML = "Click Me";
      btn.addEventListener("click", () => {
        confetti();
      });
      el.appendChild(btn);
    }
    export default { render }
    """

ConfettiButton().servable()

Use the _importmap attribute for more concise module references.

import panel as pn

from panel.custom import AnyWidgetComponent


class ConfettiButton(AnyWidgetComponent):

    _importmap = {
        "imports": {
            "canvas-confetti": "https://esm.sh/canvas-confetti@1.6.0",
        }
    }

    _esm = """
    import confetti from "canvas-confetti";

    function render({ el }) {
      let btn = document.createElement("button");
      btn.innerHTML = "Click Me";
      btn.addEventListener("click", () => {
        confetti();
      });
      el.appendChild(btn);
    }
    export default { render }
    """

ConfettiButton().servable()

See the import map documentation for more information about the import map format.

External Files

You can load JavaScript and CSS from files by providing the paths to these files.

Create the file counter_button.py.

from pathlib import Path

import param
import panel as pn

from panel.custom import AnyWidgetComponent

pn.extension()

class CounterButton(AnyWidgetComponent):

    value = param.Integer()

    _esm = Path("counter_button.js")
    _stylesheets = [Path("counter_button.css")]

CounterButton().servable()

Now create the file counter_button.js.

function render({ model, el }) {
    let value = () => model.get("value");
    let btn = document.createElement("button");
    btn.innerHTML = `count is ${value()}`;
    btn.addEventListener("click", () => {
      model.set('value', value() + 1);
      model.save_changes();
    });
    model.on("change:value", () => {
        btn.innerHTML = `count is ${value()}`;
    });
    el.appendChild(btn);
}
export default { render }

Now create the file counter_button.css.

button {
    background: #0072B5;
    color: white;
    border: none;
    padding: 10px;
    border-radius: 4px;
}
button:hover {
    background: #4099da;
}

Serve the app with panel serve counter_button.py --autoreload.

You can now edit the JavaScript or CSS file, and the changes will be automatically reloaded.

• Try changing the innerHTML from count is ${value()} to COUNT IS ${value()} and observe the update. Note that you must update innerHTML in two places.

• Try changing the background color from #0072B5 to #008080.

Displaying A Single Child

You can display Python objects by defining a Child parameter. Please note that this feature is currently not supported by AnyWidget.

Lets start with the simplest example:

import panel as pn

from panel.custom import AnyWidgetComponent, Child

pn.extension()


class Example(AnyWidgetComponent):

    child = Child()

    _esm = """
    function render({ model, el }) {
      const button = document.createElement("button");
      button.append(model.get_child("child"))
      el.appendChild(button);
    }

    export default { render };
    """


Example(child=pn.panel("A **Markdown** pane!")).servable()

If you provide a non-Viewable child it will automatically be converted to a Viewable by pn.panel:

Example(child="A **Markdown** pane!").servable()

If you want to allow a certain type of Panel components only you can specify the specific type in the class_ argument.

import panel as pn

from panel.custom import AnyWidgetComponent, Child


class Example(AnyWidgetComponent):

    child = Child(class_=pn.pane.Markdown)

    _esm = """
    function render({ model, el }) {
      const button = document.createElement("button");
      button.append(model.get_child("child"))
      el.appendChild(button);
    }

    export default { render };
    """


Example(child=pn.panel("A **Markdown** pane!")).servable()

The class_ argument also supports a tuple of types:

    child = Child(class_=(pn.pane.Markdown, pn.pane.HTML))

Displaying a List of Children

You can also display a List of Viewable objects using the Children parameter type:

import panel as pn

from panel.custom import AnyWidgetComponent, Children

pn.extension()


class Example(AnyWidgetComponent):

    objects = Children()

    _esm = """
    function render({ model, el }) {
      const div = document.createElement('div')
      div.append(...model.get_child("objects"))
      el.appendChild(div);
    }

    export default { render };
    """


Example(
    objects=[pn.panel("A **Markdown** pane!"), pn.widgets.Button(name="Click me!"), {"text": "I'm shown as a JSON Pane"}]
).servable()

You can change the item_type to a specific subtype of Viewable or a tuple of Viewable subtypes.

React

You can use React with AnyWidget as shown below.

import panel as pn
import param

from panel.custom import AnyWidgetComponent


class CounterButton(AnyWidgetComponent):

    value = param.Integer()

    _importmap = {
        "imports": {
            "@anywidget/react": "https://esm.sh/@anywidget/react",
            "react": "https://esm.sh/react",
        }
    }

    _esm = """
    import * as React from "react"; /* mandatory import */
    import { createRender, useModelState } from "@anywidget/react";

    const render = createRender(() => {
      const [value, setValue] = useModelState("value");
      return (
        <button onClick={() => setValue(value + 1)}>
          count is {value}
        </button>
      );
    });
    export default { render }
    """

CounterButton().servable()

Note

You will notice that Panel’s AnyWidgetComponent can be used with React and JSX without any build tools. Instead of build tools, Panel uses Sucrase to transpile the JSX code to JavaScript on the client side.

References

Tutorials

• Build Custom Components

How-To Guides

• Convert AnyWidget widgets

Reference Guides

• AnyWidgetComponent

• JSComponent

• ReactComponent

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