Getting started

Try it online

Try Pyodide in a REPL directly in your browser (no installation needed).

Setup

To include Pyodide in your project you can use the following CDN URL:

You can also download a release from Github releases or build Pyodide yourself. See Serving Pyodide packages for more details.

The pyodide.js file defines a single async function called loadPyodide which sets up the Python environment. When the loadPyodide function finishes, Pyodide installs global namespace called pyodide.

async function main() {
  await loadPyodide({ indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/" });
  // Pyodide is now ready to use...
  console.log(pyodide.runPython(`
    import sys
    sys.version
  `));
});
main();

Running Python code

Python code is run using the pyodide.runPython function. It takes as input a string of Python code. If the code ends in an expression, it returns the result of the expression, translated to Javascript objects (see Type translations). For example the following code will return the version string as a Javascript string:

pyodide.runPython(`
  import sys
  sys.version
`);

After importing Pyodide, only packages from the standard library are available. See Loading packages for information about loading additional packages.

Complete example

Create and save a test index.html page with the following contents:

<!DOCTYPE html>
<html>
  <head>
      <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
  </head>
  <body>
    Pyodide test page <br>
    Open your browser console to see Pyodide output
    <script type="text/javascript">
      async function main(){
        await loadPyodide({
          indexURL : "https://cdn.jsdelivr.net/pyodide/dev/full/"
        });
        console.log(pyodide.runPython(`
            import sys
            sys.version
        `));
        console.log(pyodide.runPython("print(1 + 2)"));
      }
      main();
    </script>
  </body>
</html>

Alternative Example

<!DOCTYPE html>
<html>
<head>
    <script src="https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js"></script>
</head>

<body>
  <p>You can execute any Python code. Just enter something in the box below and click the button.</p>
  <input id='code' value='sum([1, 2, 3, 4, 5])'>
  <button onclick='evaluatePython()'>Run</button>
  <br>
  <br>
  <div>
    Output:
  </div>
  <textarea id='output' style='width: 100%;' rows='6' disabled></textarea>

  <script>
    const output = document.getElementById("output");
    const code = document.getElementById("code");

    function addToOutput(s) {
      output.value += '>>>' + code.value + '\n' + s + '\n';
    }

    output.value = 'Initializing...\n';
    // init Pyodide
    async function main(){
      await loadPyodide({ indexURL : 'https://cdn.jsdelivr.net/pyodide/dev/full/' });
      output.value += 'Ready!\n';
    }
    let pyodideReadyPromise = main();

    async function evaluatePython() {
      await pyodideReadyPromise;
      try {
        let output = pyodide.runPython(code.value);
        addToOutput(output);
      } catch(err) {
        addToOutput(err);
      }
    }
  </script>
</body>

</html>

Accessing Python scope from Javascript

You can also access from Javascript all functions and variables defined in Python by using the pyodide.globals object.

For example, if you run the code x = numpy.ones([3,3]) in Python, you can access the variable x from Javascript in your browser’s developer console as pyodide.globals.get("x"). The same goes for functions and imports. See Type translations for more details.

You can try it yourself in the browser console:

pyodide.runPython(`
  import numpy
  x=numpy.ones((3, 4))
`);
pyodide.globals.get('x').toJs();
// >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]

// create the same 3x4 ndarray from js
x = pyodide.globals.get('numpy').ones(new Int32Array([3, 4])).toJs();
// x >>> [ Float64Array(4), Float64Array(4), Float64Array(4) ]

Since you have full access to Python global scope, you can also re-assign new values or even Javascript functions to variables, and create new ones from Javascript:

// re-assign a new value to an existing variable
pyodide.globals.set("x", 'x will be now string');

// create a new js function that will be available from Python
// this will show a browser alert if the function is called from Python
pyodide.globals.set("alert", alert);

// this new function will also be available in Python and will return the squared value.
pyodide.globals.set("square", x => x*x);

// You can test your new Python function in the console by running
pyodide.runPython("square(3)");

Feel free to play around with the code using the browser console and the above example.

Accessing Javascript scope from Python

The Javascript scope can be accessed from Python using the js module (see Importing Javascript objects into Python). This module represents the global object window that allows us to directly manipulate the DOM and access global variables and functions from Python.

import js

div = js.document.createElement("div")
div.innerHTML = "<h1>This element was created from Python</h1>"
js.document.body.prepend(div)

See Serving Pyodide packages to distribute Pyodide files locally.