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:

https://cdn.jsdelivr.net/pyodide/dev/full/pyodide.js

You can also download a release from GitHub releases or build Pyodide yourself. See Downloading and deploying Pyodide for more details.

The pyodide.js file defines a single async function called loadPyodide which sets up the Python environment and returns the Pyodide top level namespace.

async function main() {
  let pyodide = await loadPyodide();
  // 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(){
        let pyodide = await loadPyodide();
        console.log(pyodide.runPython(`
            import sys
            sys.version
        `));
        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() {
        let pyodide = await loadPyodide();
        output.value += "Ready!\n";
        return pyodide;
      }
      let pyodideReadyPromise = main();

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

Accessing Python scope from JavaScript#

All functions and variables defined in the Python global scope are accessible via the pyodide.globals object.

For example, if you run the code x = numpy.ones([3,3]) in Python global scope, you can access the global variable x from JavaScript in your browser’s developer console with 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. Go to

https://cdn.jsdelivr.net/pyodide/dev/full/console.html

and type the following into the browser console:

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

You can assign new values to Python global variables or create new ones from Javascript.

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

// add the js "alert" function to the Python global scope
// this will show a browser alert if called from Python
pyodide.globals.set("alert", alert);

// add a "square" function to Python global scope
pyodide.globals.set("square", x => x*x);

// Test the new "square" Python function
pyodide.runPython("square(3)");

Accessing JavaScript scope from Python#

The JavaScript scope can be accessed from Python using the js module (see Importing JavaScript objects into Python). We can use it to access global variables and functions from Python. For instance, we can directly manipulate the DOM:

import js

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