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You can run Python in a browser page by loading a browser-compatible runtime such as Pyodide, waiting for it to initialize, and calling Python code from JavaScript. The HTML still runs in the browser as usual; JavaScript provides the bridge to Python. For a small test, Pyodide can evaluate 1 + 2 directly in the page.
Run Python from an HTML page with Pyodide
Pyodide runs Python in the browser using WebAssembly. The page loads its JavaScript interface, initializes the Python runtime asynchronously, then passes Python source to pyodide.runPython(). The official quickstart demonstrates this basic flow; see the Pyodide usage guide and Pyodide quickstart.
Minimal complete HTML example
Save this as index.html. The distribution URL is versioned so the runtime does not silently change when you deploy the page.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Python in the browser</title>
</head>
<body>
<h1>Python in the browser</h1>
<button id="run" disabled>Loading Python…</button>
<pre id="output">Starting runtime…</pre>
<script src="https://cdn.jsdelivr.net/pyodide/v0.27.7/full/pyodide.js"></script>
<script>
const button = document.querySelector("#run");
const output = document.querySelector("#output");
let pyodide;
async function startPython() {
try {
pyodide = await loadPyodide();
button.disabled = false;
button.textContent = "Run Python";
output.textContent = "Python is ready.";
} catch (error) {
output.textContent = `Could not start Python: ${error.message}`;
console.error(error);
}
}
button.addEventListener("click", () => {
try {
const result = pyodide.runPython(`
message = "Hello from Python"
2 + 3
`);
output.textContent = `Python returned: ${result}`;
} catch (error) {
output.textContent = `Python error: ${error.message}`;
console.error(error);
}
});
startPython();
</script>
</body>
</html>The example uses a versioned Pyodide CDN distribution. Check the current stable guide for its currently documented version and browser support before deploying; the version number in this example is a pinned example, not a claim that it is the latest release. A deployed page should keep its runtime URL pinned and deliberately upgrade it after checking the release and package compatibility.
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Serve the page over HTTP
For a page without local-file access needs, opening the HTML file may appear to work, but a local web server is a better development setup—especially when the Python code needs to load data files. From the directory containing index.html, run:
python -m http.server 8000Then open http://localhost:8000/ in your browser. The server command uses your installed desktop Python only to serve the files; the Python code in the page runs in the browser runtime.
Pass values between JavaScript and Python
runPython() returns the result of the final Python expression as a JavaScript value when the value can be converted. To send JavaScript data into Python, use the Pyodide Python globals interface. This is useful for form inputs or values calculated by page code.
const userName = document.querySelector("#name").value;
pyodide.globals.set("user_name", userName);
const greeting = pyodide.runPython('"Hello, " + user_name');
document.querySelector("#result").textContent = greeting;Python variables and functions in the global Python scope can also be accessed through pyodide.globals. Keep the boundary explicit: ordinary browser UI work—reading an input, updating an element, wiring an event—can remain JavaScript, while the computation or Python-specific logic runs in Python.
When embedding a multi-line Python string in JavaScript, use a template literal and avoid inserting untrusted text into source code. For user-provided values, pass the value through pyodide.globals.set() instead of concatenating it into executable Python source.
Load packages and files
Packages beyond the standard library
Immediately after importing Pyodide, the quickstart says only standard-library packages are available. Additional packages may need to be loaded using Pyodide’s package-loading guidance, and compatibility depends on the package and its dependencies. Do not assume that every package installed in desktop Python works unchanged in a browser runtime. Check the current usage documentation for the supported package-loading approach and package availability.
Local and external data files
Browser security prevents ordinary JavaScript from freely reading local files through a file:/// URL. If your Python program needs a project data file during development, serve the page and file over HTTP, as in the local-server step above. The Pyodide FAQ discusses external files and local-file restrictions. Its notes about File System API support are not a universal workaround across browsers; in particular, the documented availability differs for Firefox and Safari.
For a file selected by the visitor, use a browser file input and pass the selected content into your application rather than trying to read arbitrary disk paths. A web page does not get unrestricted access to a visitor’s filesystem.
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Choose between Pyodide, PyScript, and Brython
These projects offer different integration styles; the available documentation does not establish that they have identical compatibility or performance. Choose based on your interface preference, package needs, and how Python must interact with the DOM.
| Option | Good fit | What to verify |
|---|---|---|
| Pyodide | You want a direct JavaScript API to initialize a browser Python runtime and explicitly call Python. | Package availability, runtime distribution, browser support, and whether long computation needs a worker. Official guide |
| PyScript | You want an HTML-oriented browser application platform. Its project describes using technologies including Pyodide and MicroPython. | Use the current documentation for the exact syntax and supported features of the version you adopt. PyScript project |
| Brython | You want a Python 3 browser implementation with interfaces for DOM elements and events. | Follow its project documentation for serving files and loading modules; the cited file/HTTP guide is specifically under the 3.10 documentation path. Brython project · File and HTTP documentation |
Compare the actual packages and dependencies your application uses, how code accesses browser APIs, and how you plan to deliver and pin the runtime. The cited project pages do not provide a basis for ranking these options by speed.
Keep the browser responsive
Pyodide’s stable usage guide warns that WebAssembly running on the main browser thread can make the UI unresponsive during long computations. A short calculation may be acceptable on the main thread; heavier work should be moved off it. The guide identifies a Web Worker as one solution. Worker-based designs require you to communicate inputs and results between the page and worker, so test the data types and package behavior your particular application needs.
Initialization is asynchronous, too. Disable controls that depend on Python until await loadPyodide() completes, and show a visible loading or error state instead of letting an early click call an uninitialized runtime.
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The Pyodide stable guide recommends recent browsers and lists tested minimums of Firefox 112, Chrome 112, and Safari 16.4. These are the versions listed by that documentation, not a guarantee for every device or later browser release. Check the guide again when shipping, and test on the browsers your audience uses.
For deployment, use the versioned Pyodide distribution rather than a development CDN URL; the development documentation warns against using that URL for deployed applications. The usage guide distinguishes a distribution that includes packages from an NPM mirror containing only the runtime. Choose the delivery route that matches your package and build requirements, and make sure your deployment serves all required runtime assets.
Troubleshooting common problems
loadPyodide is not defined
The Pyodide script may not have loaded, or your initialization code may run before the script is available. Confirm the pyodide.js script URL is correct and versioned, check the browser console and network panel for a failed request, and ensure the initialization script follows the runtime script in the HTML.
Python runs before initialization finishes
loadPyodide() is asynchronous. Await it before calling runPython(), and keep dependent buttons disabled until initialization succeeds. Catch initialization failures and present an error message so the page does not fail silently.
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Pyodide starts with standard-library packages. Check whether the package is available for the browser runtime and follow its package-loading instructions before importing it. A package that works on desktop Python may rely on native components or dependencies that are not available in the browser.
Python cannot open a local file
A page opened from file:/// is subject to browser security restrictions. Run a local HTTP server and access the page through http://localhost; for visitor-selected files, use a browser file input rather than requesting unrestricted filesystem access. See the Pyodide FAQ.
The page freezes during a calculation
Long-running work on the main thread blocks rendering and input handling. Move the work to a Web Worker and use the worker pattern described in the Pyodide usage guide. If a task is expected to be short, also avoid running it repeatedly in a tight event loop.
The deployed page behaves differently from development
Check that the deployed page uses the intended pinned runtime and has access to all needed runtime assets. Verify that the browser is one of the currently supported targets and that any required package is loaded. Avoid development CDN URLs in production, and inspect the browser console and network requests for missing files or initialization errors.
Or skip the browser setup
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts cookie and consent banners before capture and removes more than 60 known consent platforms, newsletter popups, and chat widgets; each step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing status. Its MCP server provides take_screenshot, get_page_info, and capture_pdf for Claude, Cursor, and other MCP clients. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000 shots.
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Frequently Asked Questions
Can I run Python in a browser without installing Python on the visitor’s computer?
Yes. A browser runtime such as Pyodide runs Python in the page; visitors do not need a desktop Python installation.
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Can browser Python access a visitor’s files or run every desktop package?
No. Browser security restricts local filesystem access, and package compatibility depends on the browser runtime and its available dependencies.
Does running Python in HTML require a server?
A simple page may load, but serving it over HTTP is the sensible development setup when files or other web resources are involved.
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