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Introduction to dynamic elements

The previous section on static elements exposes how element libraries and elements are defined. A major limitation with static elements is that you cannot change their property values at runtime. In Taipy GUI, you can bind Python variables or expressions to properties so that the user interface is immediately updated should the application modify the value of a variable.

Custom elements that allow for variable binding are called dynamic elements in Taipy GUI.
As we have seen in the previous section, every custom element must declare the properties that it relies on. Doing so means that every property must indicate the type it supports. This is handled by the PropertyType class.
Every custom element that wants to use a property with a dynamic type must be defined as a dynamic element.
Although a custom element might not have any dynamic property (that is, a property with a type that is dynamic), it can be implemented as a dynamic element nevertheless to leverage the expressivity of how these elements are implemented.

Dynamic elements use TypeScript and JavaScript code to dynamically generate HTML code to produce the pages that can be displayed in a browser. Taipy GUI relies on the React JavaScript library to simplify the development of graphical components.

Minimal knowledge

The following sections contain code samples written in TypeScript that leverage the React library. This manual is in no way a beginner's guide on either technology.

To create your custom dynamic elements, you need to know about React functional components and hooks and how to create simple components.
You also need to know the basics about packaging React components using webpack that bundles the JavaScript modules into a JavaScript library that element libraries can load to implement the front-end part of the custom elements.

Declaring a dynamic element

The declaration of a dynamic element looks very similar to the declaration of a static element.
The fundamental change is that if the render_xhtml argument of the Element constructor is not set or is not a function then the element is considered dynamic. That is, implemented using a React component. You can specify the name of the component using the react_component argument. If you don't, Taipy GUI will use a capitalized camel case transformation of the element name as the target React component name.

Here is how a custom element library containing a dynamic element would be declared:

from taipy.gui.extension import ElementLibrary, Element, ElementProperty

class CustomLibrary(ElementLibrary):

    def __init__(self) -> None:
        self.elements = {
              "<element1 name>": Element("<default property name>", {
                  "<property1 name>": ElementProperty(<property1 type>, ...),
              # This is optional and can be omitted if the component name can be inferred
              # from the element name
              react_component="<element1 component name>"),

    def get_name(self) -> str:
        return "<library name>"

    def get_elements(self) -> dict:
        return self.elements

    def get_scripts(self) -> list[str]:
        return ["<javascript module pathname>"]

There are two significant differences to spot compared to a custom element library that has only static elements:

  • A dynamic element is created using the react_component parameter, indicating that it is implemented using a React component. The value of this parameter must be the name of the React component, as exported by the JavaScript module where this component is defined;
  • The method get_scripts() must be overloaded to return the path to the JavaScript bundle file that contains the definition for the React components of the dynamic elements. The code above assumes that the custom element library needs only one file.

Implementing a dynamic element

The front-end side of dynamic elements is implemented as standard React components.
We recommend defining each component in its own source file for a clear code organization. In this manual, we will exclusively use TypeScript to enjoy better type checking and make the code more readable. The custom element library template repository provides the skeleton of an extension library project structure.

Here are the key things to know when creating a React component that implements the front-end for a custom visual element:

  • The component name should be one used when the element is declared, using the react_component argument of the Element constructor.
    If this parameter is not used, Taipy GUI uses a camel case transformation of the element name.
  • All component source files are bundled in a JavaScript library. We are using webpack for this purpose.
  • Components must be exported by the JavaScript library with the same name as the one used in the Element constructor.
  • Property names must be valid Python identifiers.
  • The camel case transformation of the property names, with a lowercase initial letter, are used to name the keys of the props argument for the React component.
    We recommend that we declare a TypeScript interface that provides better typing for this argument: each property of the interface should have the transformed name of the element property.
    Note that you can overwrite the name of the property in the React component by using the js_name argument of the ElementProperty constructor.
  • Dynamic properties use two keys in the functional component props argument:

    • The (transformed) property name as described above;
    • A similar camel case-transformed version of the property name as if it had been prefixed by 'default_'.
      This slot in the props is used to render the component before the Taipy GUI back-end has submitted any update (which is then handled by the other slot).

To illustrate this, let's assume we want to create a dynamic custom element called sizable_label. This element has two dynamic properties:

  • label, the default property, stores a string that the element should display;
  • size is a numeric property that stores how large the label should be rendered.

The element declaration would look like this in the definition of the dictionary that holds all the element declarations:

"sizable_label": Element("label", {
                   "label": ElementProperty(PropertyType.dynamic_string),
                   "size": ElementProperty(PropertyType.dynamic_number)

The default name for the React component would be "SizableLabel", and its properties will be named after the element's property names.
Here is how the definition for the React component will look like (typically, this code will be stored in a file called "SizableLabel.tsx", in the directory "<project dir>/<package dir>/front-end/src"):

interface SizableLabelProps {
    label?: string;
    defaultLabel: string;
    size?: number;
    defaultSize: number;

const SizableLabel = (props: SizableLabelProps) => {

When the component is entirely defined, it must be exported by the JavaScript library.
This is done by adding the export directive in the file "<project dir>/<package dir>/front-end/src/index.ts".

For our example, this would be the content of this file:

import SizableLabel from "./SizableLabel";

export { SizableLabel };

Building the front-end module

To build the JavaScript module that Taipy GUI can load on the user's browser, there are three steps to take:

  • Convert the TypeScript code to JavaScript;
  • Resolve the dependency in the Taipy GUI JavaScript module;
  • Bundle all JavaScript code into the extension library JavaScript module.

All the steps are handled by the package.json file located at the root of the front-end NPM project (that is, in the <package dir>/front-end directory in our examples).
The commands declared in this file heavily rely on webpack to handle the build process: the "scripts" property of the package.json content defines the "build" entry as invoking "webpack".
The webpack.config.js configuration file should sit next to package.json. This file contains all the configuration parameters that we will discuss in the rest of this section.

Converting TypeScript

TypeScript is a language that is transformed into JavaScript before it can be packaged in a JavaScript bundle. This process is known as 'transpilation'.
Here is how the project must be set up in order to use TypeScript and support the React JSX syntax (used in .tsx source files):

  • To transpile TypeScript, you need to install the NPM modules "typescript" and "ts-loader":

    npm install --save-dev typescript ts-loader

    Note that in the package.json provided with Taipy GUI, these two modules already appear in the "devDependencies" property.

  • A file called tsconfig.json must exist next to the file webpack.config.js. This file contains the configuration to support JSX and convert TypeScript to vanilla JavaScript.
    Here is an excerpt of the tsconfig.json file we use in the dynamic library examples. We do not explain all the settings that appear in the file we provide and let the reader take a peek at the TSConfig documentation for a complete description of the tsconfig.json file content:

      "compilerOptions": {
        "target": "es5",
        "outDir": "./dist/",
        "module": "esnext",
        "moduleResolution": "node",
        "jsx": "react-jsx",
        "allowJs": true,
      "include": [ "src" ]
    Note that you must make sure that the directory where TypeScript files are located (the "include" property) and the output directory (the "outDir" property) are set according to your project structure.

  • webpack must be configured so it can transpile TypeScript code.
    The two essential settings in the webpack.config.js file are shown here:

    resolve: { extensions: [".ts", ".tsx"] },
    module: {
      rules: [
          test: /\.tsx?$/,
          use: "ts-loader",
          exclude: /node_modules/

The settings of the resolve and rules properties make webpack look for TypeScript files (.ts and .tsx) and process those files using the "ts-loader" TypeScript transpiler loader.

Resolve Taipy GUI dependency

As you can see in the implementation code for React components, we depend on a JavaScript library that Taipy GUI provides.
In the source code for the components, you will find directives similar to the following:

import { ... } from "taipy-gui";

The "taipy-gui" module is provided by Taipy GUI, where it has been installed.
You can query where Taipy GUI was installed by issuing the command:

pip show taipy-gui

To simplify the understanding of the rest of this section, let's assume that you store the directory path returned by this command in the environment variable 'TAIPY_GUI_DIR'.

To install the Taipy GUI JavaScript module, you need to run the following command:

npm i $TAIPY_GUI_DIR/taipy/gui/webapp
npm i %TAIPY_GUI_DIR%\taipy\gui\webapp
Note that in the package.json file that is installed in Taipy GUI (in $TAIPY_GUI_DIR/taipy/doc/extension/example_library/front-end), there is an entry called "install" in the "scripts" section that invokes an NPM script to perform this task.

This dependency must be explicitly declared for webpack to use as an external library.
In the webpack.config.js file you can find the following lines, that assumes that the environment variable "TAIPY_GUI_DIR" was set as indicated above:

  externals: {"taipy-gui": "TaipyGui"},

  plugins: [
    new webpack.DllReferencePlugin({
      manifest: path.resolve(
      name: "TaipyGuiDependencies"
These two settings ensure that the Taipy GUI JavaScript module is accessible for resolving the symbols it exposes and that this module is not bundled itself in the generated custom extension library JavaScript module.

Bundle the JavaScript code

webpack must be configured to indicate where the front-end JavsScript is produced.
This is done by setting the output property in webpack.config.js:

    output: {
      filename: "extensionLibrary.js",
      path: path.resolve(__dirname, "dist"),
      library: {
        // Camel case transformation of the library name
        name: "MyExtensionLibrary",
        type: "umd"
This indicates that all the source files should be bundled in a single module file called 'extensionLibrary.js', located in the 'dist' directory.

When the different configuration files are up-to-date, building the extension library bundle is just a matter of running:

npm run build

Debug mode

To debug your front-end code in the browser, you will want to use the command:

npm run build:dev
This preserves the symbols in the module file and generates a map file next to it, allowing for stepping in your TypeScript source code even after transpilation.

With the settings listed above, the result of this command should be a file called extensionLibrary.js under the dist directory.
This path, relative to the Python file where the element library is defined, must appear in the value returned by the method get_scripts() of the ElementLibrary subclass.

Property types

Elements properties are what control the rendering and the behavior of front-end components.
The Taipy GUI Extension API exposes functions that allow components to consume data coming from the application and send messages to the Python code that can then handle user interactions.

The API depends on the type of the properties you need to implement.
Here are the sections that address the different use cases that your custom element may need: