Interrupting execution

The native Python interrupt system is based on preemptive multitasking but Web Assembly has no support for preemptive multitasking. Because of this, interrupting execution in Pyodide must be achieved via a different mechanism which takes some effort to set up.

Setting up interrupts

In order to use interrupts you must be using Pyodide in a webworker. You also will need to use a SharedArrayBuffer, which means that your server must set appropriate security headers. See the MDN docs for more information.

To use the interrupt system, you should create a SharedArrayBuffer on either the main thread or the worker thread and share it with the other thread. You should use pyodide.setInterruptBuffer to set the interrupt buffer on the Pyodide thread. When you want to indicate an interrupt, write a 2 into the interrupt buffer. When the interrupt signal is processed, Pyodide will set the value of the interrupt buffer back to 0.

By default, when the interrupt fires, a KeyboardInterrupt is raised. Using the signal module, it is possible to register a custom Python function to handle SIGINT. If you register a custom handler function it will be called instead.

Here is a very basic example. Main thread code:

let pyodideWorker = new Worker("pyodideWorker.js");
let interruptBuffer = new Uint8Array(new SharedArrayBuffer(1));
pyodideWorker.postMessage({ cmd: "setInterruptBuffer", interruptBuffer });
function interruptExecution() {
  // 2 stands for SIGINT.
  interruptBuffer[0] = 2;
// imagine that interruptButton is a button we want to trigger an interrupt.
interruptButton.addEventListener("click", interruptExecution);
async function runCode(code) {
  // Clear interruptBuffer in case it was accidentally left set after previous code completed.
  interruptBuffer[0] = 0;
  pyodideWorker.postMessage({ cmd: "runCode", code });

Worker code:

self.addEventListener("message", (msg) => {
  if ( === "setInterruptBuffer") {
  if ( === "runCode") {

Allowing JavaScript code to be interrupted

The interrupt system above allows interruption of Python code and also of C code that opts to allow itself to be interrupted by periodically calling PyErr_CheckSignals. There is also a function pyodide.checkInterrupt that allows JavasSript functions called from Python to check for an interrupt. As a simple example, we can implement an interruptable sleep function using Atomics.wait:

let blockingSleepBuffer = new Int32Array(new SharedArrayBuffer(4));
function blockingSleep(t) {
  for (let i = 0; i < t * 20; i++) {
    // This Atomics.wait call blocks the thread until the buffer changes or a 50ms timeout ellapses.
    // Since we won't change the value in the buffer, this blocks for 50ms.
    Atomics.wait(blockingSleepBuffer, 0, 0, 50);
    // Periodically check for an interrupt to allow a KeyboardInterrupt.