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
globalThisordocumentobjects. - Through the
pydommodule 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:
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:
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.
pyscript.web
Warning
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
findAPI uses exactly the same queries as those used by native browser methods likequrerySelectororquerySelectorAll. - Use classes in the
pyscript.web.elementsnamespace 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
findqueries 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.
print(dom.head.children)
# 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:
print(p.html)
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:]:
print(p.html)
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- thevalueattribute 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: paragraphs.style["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 *
dom.body.append(
div(
div("Hello!", classes="a-css-class", id="hello"),
select(
option("apple", value=1),
option("pear", value=2),
option("orange", value=3),
),
div(
button(span("Hello! "), span("World!"), id="my-button"),
br(),
button("Click me!"),
classes=["css-class1", "css-class2"],
style={"background-color": "red"}
),
div(
children=[
button(
children=[
span("Hello! "),
span("Again!")
],
id="another-button"
),
br(),
button("b"),
],
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()
my_div.style["background-color"] = "red"
my_div.classes.append("a-css-class")
my_p = p()
my_p.content = "This is a paragraph."
my_div.append(my_p)
# 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.
- As a global reference attached to the
windowobject in the web page because the code was referenced as the source of ascripttag 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:
[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:
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:
<!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:
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: <>
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:
<!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: "<>"
</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:
<!doctype html>
...
<!-- land the utility still globally as generic script -->
<script src="https://cdn.jsdelivr.net/npm/[email protected]/index.js"></script>
...
// get all utilities needed from the global.
const { escape, unescape } = globalThis.html;
// export utilities like a standards compliant module would do.
export { escape, unescape };
[js_modules.main]
# will simulate a standard JS module
"./wrapper.js" = "html_escaper"
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: <>
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:
/*
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:
[js_modules.main]
"code.js" = "code"
In your HTML, reference your Python script with this configuration file:
<script type="py" src="./main.py" config="./pyscript.toml" terminal></script>
Finally, just use your JavaScript module’s exported functions inside PyScript:
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: