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The DOM (document object model) is a tree like data structure representing the web page displayed by the browser. PyScript interacts with the DOM to change the user interface and react to things happening in the browser.

There are currently two ways to interact with the DOM:

  1. Through the foreign function interface (FFI) to interact with objects found in the browser's globalThis or document objects.
  2. Through the pydom module that acts as a Pythonic wrapper around the FFI and comes as standard with PyScript.


The foreign function interface (FFI) gives Python access to all the standard web capabilities and features, such as the browser's built-in web APIs.

This is available via the pyscript.window module which is a proxy for the main thread's globalThis object, or pyscript.document which is a proxy for the website's document object in JavaScript:

Accessing the window and document objects in Python
from pyscript import window, document

my_element = document.querySelector("#my-id")
my_element.innerText = window.location.hostname

The FFI creates proxy objects in Python linked to actual objects in JavaScript.

The proxy objects in your Python code look and behave like Python objects but have related JavaScript objects associated with them. It means the API defined in JavaScript remains the same in Python, so any browser based JavaScript APIs or third party JavaScript libraries that expose objects in the web page's globalThis, will have exactly the same API in Python as in JavaScript.

The FFI automatically transforms Python and JavaScript objects into the equivalent in the other language. For example, Python's boolean True and False will become JavaScript's true and false, while a JavaScript array of strings and integers, ["hello", 1, 2, 3] becomes a Python list of the equivalent values: ["hello", 1, 2, 3].


Instantiating classes into objects is an interesting special case that the FFI expects you to handle.

If you wish to instantiate a JavaScript class in your Python code, you need to call the class's new method:

from pyscript import window

my_obj ="some value")

The underlying reason for this is simply JavaScript and Python do instantiation very differently. By explicitly calling the JavaScript class's new method PyScript both signals and honours this difference.

More technical information about instantiating JavaScript classes can be found in the FAQ

Should you require lower level API access to FFI features, you can find such builtin functions under the pyscript.ffi namespace in both Pyodide and MicroPython. The available functions are described in our section on the builtin API.

Advanced users may wish to explore the technical details of the FFI.



The pyscript.web module is currently a work in progress.

We welcome feedback and suggestions.

The pyscript.web module is an idiomatically Pythonic API for interacting with the DOM. It wraps the FFI in a way that is more familiar to Python developers and works natively with the Python language. Technical documentation for this module can be found in the API section.

There are three core concepts to remember:

  • Find elements on the page via CSS selectors. The find API uses exactly the same queries as those used by native browser methods like qurerySelector or querySelectorAll.
  • Use classes in the pyscript.web.elements namespace to create and organise new elements on the web page.
  • Collections of elements allow you to access and change attributes en-mass. Such collections are returned from find queries and are also used for the children of an element.

You have several options for accessing the content of the page (i.e. the DOM), and these can be found in the pyscript.web.dom object. The head and body attributes reference the page's head and body. Furthermore, the find method can be used to return collections of elements matching your CSS query. Finally, all elements have a find method that searches within their children for elements matching your CSS query.

from pyscript.web import dom

# Print all the child elements of the document's head.
# Find all the paragraphs in the DOM.
paragraphs = dom.find("p")

The object returned from a query, or used as a reference to an element's children is iterable:

from pyscript.web import dom

# Get all the paragraphs in the DOM.
paragraphs = dom.find("p")

# Print the inner html of each paragraph.
for p in paragraphs:

Alternatively, it is also indexable / sliceable:

from pyscript.web import dom

# Get an ElementCollection of all the paragraphs in the DOM
paragraphs = dom.find("p")

# Only the final two paragraphs.
for p in paragraphs[-2:]:

It also makes available the following read and writable attributes related to all contained elements:

  • classes - the list of classes associated with the elements.
  • innerHTML - the innerHTML of each element.
  • style - a dictionary like object for interacting with CSS style rules.
  • value - the value attribute associated with each element.

For example, to continue the example above, paragraphs.innerHTML will return a list of all the values of the innerHTML attribute on each contained element. Alternatively, set an attribute for all elements contained in the collection like this:["background-color"] = "blue".

It's possible to create new elements to add to the DOM:

from pyscript.web import dom
from pyscript.web.elements import *

        div("Hello!", classes="a-css-class", id="hello"),
            option("apple", value=1),
            option("pear", value=2),
            option("orange", value=3),
            button(span("Hello! "), span("World!"), id="my-button"),
            button("Click me!"),
            classes=["css-class1", "css-class2"],
            style={"background-color": "red"}
                        span("Hello! "),
            classes=["css-class1", "css-class2"]

This example demonstrates a declaritive way to add elements to the body of the DOM. Notice how the first (unnamed) arguments to an element are its children. The named arguments (such as id, classes and style) refer to attributes of the underlying HTML element. If you'd rather be explicit about the children of an element, you can always pass in a list of such elements as the named children argument (you see this in the final div in the example above).

Of course, you can achieve similar results in an imperative style of programming:

from pyscript.web import dom
from pyscript.web.elements import *

my_div = div()["background-color"] = "red"

my_p = p()
my_p.content = "This is a paragraph."


# etc...

It's also important to note that the pyscript.when decorator understands element references from pyscript.web:

from pyscript import when
from pyscript.web import dom

btn = dom.find("#my-button")

@when("click", btn)
def my_button_click_handler(event):
    print("The button has been clicked!")

Should you wish direct access to the proxy object representing the underlying HTML element, each Python element has a _dom_element property for this purpose.

Once again, the technical details of these classes are described in the built-in API documentation.

Working with JavaScript

There are three ways in which JavaScript can get into a web page.

  1. As a global reference attached to the window object in the web page because the code was referenced as the source of a script tag in your HTML (the very old school way to do this).
  2. Using the Universal Module Definition (UMD), an out-of-date and non-standard way to create JavaScript modules.
  3. As a standard JavaScript Module which is the modern, standards compliant way to define and use a JavaScript module. If possible, this is the way you should do things.

Sadly, this being the messy world of the web, methods 1 and 2 are still quite common, and so you need to know about them so you're able to discern and work around them. There's nothing WE can do about this situation, but we can suggest "best practice" ways to work around each situation.

Remember, as mentioned elsewhere in our documentation, the standard way to get JavaScript modules into your PyScript Python context is to link a source standard JavaScript module to a destination name:

Reference a JavaScript module in the configuration.
"[email protected]/dist/leaflet-src.esm.js" = "leaflet"

Then, reference the module via the destination name in your Python code, by importing it from the pyscript.js_modules namespace:

Import the JavaScript module into Python
from pyscript.js_modules import leaflet as L

map ="map")

# etc....

We'll deal with each of the potential JavaScript related situations in turn:

JavaScript as a global reference

In this situation, you have some JavaScript code that just globally defines "stuff" in the context of your web page via a script tag. Your HTML will contain something like this:

JavaScript as a global reference
<!doctype html>
This JS utility escapes and unescapes HTML chars. It adds an "html" object to
the global context.
<script src="[email protected]/index.js"></script>

Vanilla JS just to check the expected object is in the global context of the
web page.

When you find yourself in this situation, simply use the window object in your Python code (found in the pyscript namespace) to interact with the resulting JavaScript objects:

Python interaction with the JavaScript global reference
from pyscript import window, document

# The window object is the global context of your web page.
html = window.html

# Just use the object "as usual"...
# e.g. show escaped HTML in the body: &lt;&gt;

You can find an example of this technique here:

JavaScript as a non-standard UMD module

Sadly, these sorts of non-standard JavaScript modules are still quite prevalent. But the good news is there are strategies you can use to help you get them to work properly.

The non-standard UMD approach tries to check for export and module fields in the JavaScript module and, if it doesn’t find them, falls back to treating the module in the same way as a global reference described above.

If you find you have a UMD JavaScript module, there are services online to automagically convert it to the modern and standards compliant way to d o JavaScript modules. A common (and usually reliable) service is provided by, a service that provides an out of the box way to consume the module in the correct and standard manner:

Use to automatically convert a non-standard UMD module
<!doctype html>
<script type="module">
    // this utility escapes and unescape HTML chars
    import { escape, unescape } from "";
    // returns a module       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    // log: "&lt;&gt;"

If a similar test works for the module you want to use, use the CDN service within the py or mpy configuration file as explained at the start of this section on JavaScript (i.e. you'll use it via the pyscript.js_modules namespace).

If this doesn't work, assume the module is not updated nor migrated to a state that can be automatically translated by services like You could try an alternative (more modern) JavaScript module to achieve you ends or (if it really must be this module), you can wrap it in a new JavaScript module that conforms to the modern standards.

The following four files demonstrate this approach:

index.html - still grab the script so it appears as a global reference.
<!doctype html>
<!-- land the utility still globally as generic script -->
<script src="[email protected]/index.js"></script>
wrapper.js - this grabs the JavaScript functionality from the global context and wraps it (exports it) in the modern standards compliant manner.
// get all utilities needed from the global.
const { escape, unescape } = globalThis.html;

// export utilities like a standards compliant module would do.
export { escape, unescape };
pyscript.toml - configure your JS modules as before, but use your wrapper instead of the original module.
# will simulate a standard JS module
"./wrapper.js" = "html_escaper" - just import the module as usual and make use of it.
from pyscript import document

# import the module either via
from pyscript.js_modules import html_escaper
# or via
from pyscript.js_modules.html_escaper import escape, unescape

# show on body: &lt;&gt;

You can see this approach in action here:

A standard JavaScript module

This is both the easiest and best way to import any standard JS module into Python.

You don't need to reference the script in your HTML, just define how the source JavaScript module maps into the pyscript.js_modules namespace in your configuration file, as explained above.

That's it!

Here is an example project that uses this approach:

My own JavaScript code

If you have your own JavaScript work, just remember to write it as a standard JavaScript module. Put simply, ensure you export the things you need to. For instance, in the following fragment of JavaScript, the two functions are exported from the module:

code.js - containing two functions exported as capabilities of the module.
Some simple JavaScript functions for example purposes.

export function hello(name) {
    return "Hello " + name;

export function fibonacci(n) {
    if (n == 1) return 0;
    if (n == 2) return 1;
    return fibonacci(n - 1) + fibonacci(n - 2);

Next, just reference this module in the usual way in your TOML or JSON configuration file:

pyscript.toml - references the code.js module so it will appear as the code module in the pyscript.js_modules namespace.
"code.js" = "code"

In your HTML, reference your Python script with this configuration file:

Reference the expected configuration file.
<script type="py" src="./" config="./pyscript.toml" terminal></script>

Finally, just use your JavaScript module’s exported functions inside PyScript:

Just call your bespoke JavaScript code from Python.
from pyscript.js_modules import code

# Just use the JS code from Python "as usual".
greeting = code.hello("Chris")
result = code.fibonacci(12)

You can see this in action in the following example project: