What is PyScript?
PyScript is an open source platform for running Python in modern web browsers.
PyScript brings together two of the most vibrant technical ecosystems on the planet: the web and Python. It lets you build rich, interactive web applications using Python, without the need for a backend server or entanglements with browser-based JavaScript (although PyScript also works well in these two contexts).
Write your application logic in Python, use Python libraries for data processing or visualisation, and deploy your work with just a URL. Your users run everything locally in their browser, making PyScript applications fast, secure, and easy to share.
How it works
PyScript is built on WebAssembly (abbreviated to WASM) - an instruction set for a virtual machine with an open specification and near-native performance. Modern browsers all support WebAssembly, making it a universal platform for running code beyond JavaScript.
PyScript takes versions of the Python interpreter compiled to WebAssembly and makes them easy to use inside the browser. You write Python code, and PyScript handles all the complexity of loading the interpreter, managing the environment, and bridging between Python and the browser's JavaScript APIs.
Two Python interpreters
PyScript supports two Python interpreters, letting you choose the right tool for your application:
Pyodide is the full CPython interpreter compiled to WebAssembly. It's the standard Python you already know: the same interpreter that runs on your laptop, with the same standard library and the same behaviour. Because it's genuine CPython, Pyodide gives you access to Python's vast ecosystem of packages from PyPI. Want to use NumPy, Pandas, Matplotlib, Scikit-learn, or thousands of other libraries? Pyodide makes it possible.
MicroPython is a lean, efficient reimplementation of Python 3 that includes a comprehensive subset of the standard library. At just 170KB, MicroPython loads almost instantly, making it ideal for mobile devices, slow connections, or any time you want your app to start quickly. Despite its small size, MicroPython is surprisingly capable, exposing Python's full expressiveness to the browser.
Both interpreters implement almost identical foreign function interfaces (FFI) to bridge Python and JavaScript, so your code works consistently regardless of which interpreter you choose (and PyScript provides abstractions around the differences between their FFIs).
Tip
When to use which interpreter:
Choose Pyodide when you need access to Python's extensive computing stack (NumPy, Pandas, Matplotlib, etc.), when you're working with complex Python packages, or when you need full CPython compatibility.
Choose MicroPython for mobile applications, when fast startup time matters, or when you're building lightweight applications that don't need heavy libraries.
The foreign function interface
The FFI (foreign function interface) is how Python and JavaScript communicate in PyScript. It automatically translates between Python and JavaScript objects, letting you use browser APIs directly from your Python code.
This bridge is bidirectional: Python can call JavaScript functions and access JavaScript objects, while JavaScript can call Python functions and access Python objects. The FFI handles all the type conversions automatically, so you can focus on writing your application logic.
Want to manipulate the DOM? Access the browser's document object.
Need to use a JavaScript library? Import it and call its functions from
Python. The FFI makes it seamless.
Put simply, PyScript helps Python and JavaScript to be friends that complement and amplify each others strengths.
Learn more about the FFI in the high level DOM interaction guide or dive deep into the FFI's technical details.
Key capabilities
PyScript provides a rich set of features that make building web applications with Python both powerful and enjoyable:
Full web platform access
PyScript gives you complete access to the DOM and all the web APIs implemented by your browser. Through the FFI, Python works seamlessly with everything the browser offers, including any third-party JavaScript libraries included in your page.
The pyscript.web module provides a Pythonic interface to the DOM,
making it feel natural to work with web page elements from Python. Find
elements with CSS selectors, manipulate content and attributes, handle
events - all with idiomatic Python code.
Python's vast ecosystem
Because Pyodide is genuine CPython compiled to WebAssembly, you have access to Python's deep and diverse ecosystem of libraries, frameworks, and modules. Whether you're doing data science, building visualisations, processing text, or working with APIs, there's probably a Python library to help.
Got a favourite library in Python? Now you can use it in the browser and share your work with just a URL. No server required, no complex deployment - just Python running where your users are.
Need to check if a package is supported by PyScript? Use our PyScript Packages website to check and/or report the status of any third party packages.
AI and data science built in
Python is famous for its extraordinary usefulness in artificial intelligence and data science. The Pyodide interpreter comes with many of the libraries needed for this sort of work already included: NumPy, Pandas, Matplotlib, Scikit-learn, and more.
Build interactive data visualisations, create machine learning demonstrations, or develop educational tools that let users experiment with algorithms - all running locally in the browser with no backend required!
Mobile-friendly MicroPython
Thanks to MicroPython's tiny size and fast startup, PyScript provides a compelling story for Python on mobile devices. Your application loads quickly on first visit and almost instantly on subsequent visits (thanks to browser caching).
This makes PyScript practical for mobile web applications, progressive web apps, or any scenario where fast initial load matters.
Background processing with workers
Expensive computation can block the main thread, making your application appear frozen. PyScript supports web workers, letting you run Python code in background threads that don't interfere with the user interface.
Think of workers as independent subprocesses in your web page. They're perfect for data processing, simulations, or any CPU-intensive work. Learn more in the workers guide.
Device capabilities
PyScript gives your Python code access to modern browser capabilities: capture photos and video with the camera, record media, read and write files, store data locally, and integrate with all the other available web APIs.
Build applications that feel native, with access to device hardware and local storage, all from Python.
Extensible plugin system
PyScript has a small, efficient core called PolyScript. Most of PyScript's functionality is actually implemented through PolyScript's plugin system.
This architecture ensures a clear separation of concerns: PolyScript focuses on being small, efficient, and powerful, whilst the PyScript plugins build Pythonically upon these solid foundations.
The plugin system also means developers independent of the PyScript core team can create and contribute plugins whose functionality reflects the unique and diverse needs of PyScript's users.
Our aim: digital empowerment
At the core of PyScript is a philosophy of digital empowerment.
The web is the world's most ubiquitous computing platform, mature and familiar to billions of people. Python is one of the world's most popular programming languages: it's easy to teach and learn, used across countless domains (data science, education, games, embedded systems, artificial intelligence, finance, physics, film production - to name but a few), and the Python ecosystem contains a vast number of popular and powerful libraries.
PyScript brings together the ubiquity, familiarity, and accessibility of the web with the power, depth, and expressiveness of Python.
This means PyScript isn't just for programming experts but, as we like to say, for the 99% of the rest of the planet who use computers.
By making Python accessible in the browser, PyScript lowers barriers to creating and sharing software. You don't need to understand server deployment, database management, or complex build processes. You don't need to learn a new language or framework. You just need Python and a web browser.
This democratisation of web development means more people can create tools to solve their own problems, share their knowledge with others, and contribute to the digital world we all inhabit.
What you can build
PyScript opens up new possibilities for Python developers and new capabilities for web applications:
Educational tools and interactive tutorials - Create lessons where students can experiment with code directly in their browser, see visualisations update in real-time, and learn by doing.
Data analysis dashboards - Build interactive visualisations of your data using Pandas and Matplotlib, then share them with colleagues who can explore the data themselves without installing anything.
Scientific simulations - Develop models and simulations that run entirely in the browser, letting others experiment with parameters and see results instantly.
Creative coding projects - Make generative art, music visualisations, or interactive games using Python libraries you already know.
Rapid prototypes - Test ideas quickly without setting up backend infrastructure. Share prototypes with a simple URL.
Browser-based tools - Create utilities that process files, transform data, or automate tasks - all running locally for privacy and speed.
The only limit is your imagination (and perhaps your users' patience whilst the interpreter loads, but that's getting faster all the time).
Next steps
Ready to start building with PyScript?
Explore interacting with the web page to learn how a PyScript application interacts with the browser, or jump to Configuration to understand how to set up your Python environment.
Want to see PyScript in action first? Check out the example applications to see what's possible and learn from working code.