The DOM

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:

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

FFI

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].

Info

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 = window.MyJavaScriptClass.new("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.

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 builtin helpers.

PyDom

The Standard Web APIs are massive and not always very user-friendly. PyDom is a Python module that exposes the power of the web with an easy and idiomatic Pythonic interface on top.

While the FFI interface described above focuses on giving full access to the entire Standard Web APIs, pydom focuses on providing a small, intuitive and yet powerful API that prioritises common use cases fist. For this reason, its first layer is simple and intuitive (but limited to the most common use cases), but pydom also provides a secondary layer that can be used to directly use full FFI interface of a specific element.

It does not aim to replace the regular Web [Javascript] API nor to be as wide and offer feature parity. On the contrary, it's intentionally small and focused on the most popular use cases while still providing [a backdoor] access to the full JS API.

Pydom draws inspiration from popular Python APIs/Libraries known to be friendly and easy to learn, and other successful projects related the web as well (for instance, JQuery was a good source of inspiration).

Warning

PyDom is currently a work in progress.

We welcome feedback and suggestions.

Core Concepts

Pydom builds on topic of very few and simple core concepts:

Element: any component that is part of a web page. This is a rough abstraction of an HTMLElement. In general, pydom elements always map to an underlying HTML Element in a web page
ElementCollection: a collection of one or more Elements. It is a rough abstraction of a HTMLCollection.
Querying: a method to query elements on a page based on a selector. Pydom supports standard HTML DOM query selectors to locate DOM elements as other native JavaScript methods like querySelector or querySelectorAll.

Following, we'll look into each one of these aspects a bit more in detail.

Element

pydom Element is simply just an abstraction of a tranditional Element in a web page. Every Element always maps to an underlying JavaScript Element in a web page. These 2 elements are always in sync and any change of state in one is reflected in the other.

Creating a new element

New elements can be created by using the pydom.create method and passing the type of element being created. Here's an example of what it looks like:

(To execute and explore the following code, click on the "load" button. The result will be conveniently displayed in the box on the below of the code example)

from pyweb import pydom

# Creating an element directly from pydom creates an unbounded element.
new_div = pydom.create("div")

# Creating an element from another element automatically creates that element
# as a child of the original element
new_p = new_div.create("p", classes=["code-description"], html="Ciao PyScripters!")

# elements can be appended to any other element on the page
pydom['#element-creation-example'][0].append(new_div)

Result will go here

For more details about pydom.create please refer to its reference documentation.

Setting the content of an element

The Element interface offers 2 main ways to set an element content: the html and the content attributes:

content: sets the innerHTML field via the PyScript display function. This takes care of properly rendering the object being passed based on the object mimetype. So, for instance, if the objects is an image, it'll be properly rendered on the element
html: directly sets the innerHTML field of the underlying element without attemnpting any conversion.

In general, we suggest using content directly as it'll take care of most use cases without requiring any extra logic from the user.

Changing the element style

Elements have a style attribute that can be used to change the element style rules. The style attribute can be used as a dictionary and, to set a style rule for the element, simply set the correct key on the .style attribute. For instance, the following code changes the background color of the element just created in the example above:

new_p.style["background-color"] = "yellow"

to remove a specific style key, simply use the pop method as you'd to to remove a key from a dictionary:

new_p.style.pop("background-color")

In addition to the dictionary interface to explicitly set CSS rules, the style attribute also offers a convenient visible property that can be use show/hide an element.

new_p.style.visible = False

Other useful aspects of the Element API

append: method to append a new child to the element.
children: list of the children of the element.
value: allows to set the value attribute of an element.
clone: method that creates a clone of the element. NODE: The clone elements will not be attached to any element.
show_me: method to scroll the page to where the element is placed.

ElementCollection

Element Collections represent a collection of elements typically returned from a query. For instance:

paragraphs = pydom['p']

In the example above, paragraphs is an ElementCollection that maps to all p elements in the page.

As any collections, ElementCollection can be used to iterate over a collection of elements or to pick specific elements or slices of elements in the collection. For instance:

for element in paragraphs: 
  display(element.html)

# let's now display only the last 2 elements
for element in paragraphs[-2:]:
  display(element.html)

Interacting with an ElementCollection

Besides from allowing operations as an iterable object, ElementCollection objects also offer a few convenient methods to directly interact with th elements in the collection. For instance, it's possible to ask for specific attributes of the elements in the collection directly:

display(paragraphs.html)

The example above displays a list with the value of the html attribute for all the elements in the paragraphs collection.

The same way we can read attributes, we can also set an attribute directly in the collection. For instance, you can directly set the html content of all the elements in the collection

# This will change the text of all H1 elements in the page
pydom['h1'].html = "That's cool :)"

or perhaps change their style

paragraphs.style['background-color'] = 'lightyellow'

ElementCollection currently support the following attributes:

style: just like in Element, this proxy attribute can be used to change the style of the elements in a collection by setting the proper CSS rules, using style with the same API as a dictionary.
html: allows to change the html attribute on all the elements of a collection.
value: allows to change the value attribute on all the elements of a collection.

Working with JavaScript

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

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).
Using the Universal Module Definition (UMD), an out-of-date and non-standard way to create JavaScript modules.
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.

[js_modules.main]
"https://cdn.jsdelivr.net/npm/[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 = 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="https://cdn.jsdelivr.net/npm/[email protected]/index.js"></script>

<!--
Vanilla JS just to check the expected object is in the global context of the
web page.
-->
<script>
    console.log(html);
</script>

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;
document.body.append(html.escape("<>"))

You can find an example of this technique here:

https://pyscript.com/@agiammarchi/floral-glade/v1

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 https://esm.run/your-module-name, a service that provides an out of the box way to consume the module in the correct and standard manner:

Use esm.run to automatically convert a non-standard UMD module

<!doctype html>
<script type="module">
    // this utility escapes and unescape HTML chars
    import { escape, unescape } from "https://esm.run/html-escaper";
    // esm.run returns a module       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    console.log(escape("<>"));
    // log: "&lt;&gt;"
</script>

If a similar test works for the module you want to use, use the esm.run 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 esm.run. 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="https://cdn.jsdelivr.net/npm/[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.

[js_modules.main]
# will simulate a standard JS module
"./wrapper.js" = "html_escaper"

main.py - 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;
document.body.append(html.escape("<>"))

You can see this approach in action here:

https://pyscript.com/@agiammarchi/floral-glade/v2

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:

https://pyscript.com/@agiammarchi/floral-glade/v3

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.

[js_modules.main]
"code.js" = "code"

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

Reference the expected configuration file.

<script type="py" src="./main.py" 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")
print(greeting)
result = code.fibonacci(12)
print(result)

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

https://pyscript.com/@ntoll/howto-javascript/latest