Writing your own components

One of the really cool things about Dash is that
it is built on top of React.js, a JavaScript library for building web components.

The React community is huge. Thousands of components have been built and released with open source licenses. For example, here are just some of the
slider components and
table components that have been published by the React community, any of which could be adapted into a Dash component.

Creating a Component

To create a Dash component, fork our sample component repository and follow the instructions in the README.md:

If you are just getting started with React.js as a Python programmer, please check out our essay React for Python Devs

How Are Components Converted From React.js to Python?

Dash provides a framework that converts React components
(written in JavaScript) into Python classes that are
compatible with the Dash ecosystem.

On a high level, this is how that works:
- Components in Dash are serialized as JSON.
To write a Dash-compatible component, all of the props
shared between the Python code and the React code―numbers, strings, booleans,
as well as arrays or objects containing numbers, strings, or booleans―must be serializable as JSON.
For example, JavaScript functions are not valid input arguments.
In fact, if you try to add a function as a prop to your Dash component, you
will see that the generated Python code for your component will not include
that prop as part of your component’s accepted list of props.
(It’s not going to be listed in the Keyword arguments enumeration or in the
self._prop_names array of the generated Python file for your component).
- By annotating components with React docstrings (not required but helpful
and encouraged), Dash extracts the information about the component’s name,
properties, and description through React Docgen.
This is exported as a JSON file (metadata.json).
- At build time, Dash reads this JSON file and dynamically creates Python classes
that subclass a core Dash component. These classes include argument validation,
Python docstrings, types, and a basic set of methods. These classes are
generated entirely automatically.
A JavaScript developer does not need to
write any Python in order to generate a component that can be used in the
Dash ecosystem.
- You will find all of the auto-generated files from the build process in the
folder named after your component. When you create your Python package, by default any
non-Python files won’t be included in the actual package. To include these files
in the package, you must list them explicitly in MANIFEST.in. That is, MANIFEST.in
needs to contain each JavaScript, JSON, and CSS file that you have included in
your my_dash_component/ folder. In the dash-component-boilerplate repository,
you can see that all the JavaScript for your React component is included in the
build.js file.
- The Dash app crawls through the app’s layout property and checks which
component packages are included in the layout; then it extracts that
component’s necessary JavaScript or CSS bundles. Dash serves these bundles
to Dash’s front-end. These JavaScript bundles are used to render the components.
- Dash’s layout is serialized as JSON and served to Dash’s front-end. This
layout is recursively rendered with these JavaScript bundles and React.