Dealing with the file system#

Pyodide includes a file system provided by Emscripten. In JavaScript, the Pyodide file system can be accessed through pyodide.FS which re-exports the Emscripten File System API

Example: Reading from the file system

  from pathlib import Path

  Path("/hello.txt").write_text("hello world!")

let file = pyodide.FS.readFile("/hello.txt", { encoding: "utf8" });
console.log(file); // ==> "hello world!"

Example: Writing to the file system

let data = "hello world!";
pyodide.FS.writeFile("/hello.txt", data, { encoding: "utf8" });
  from pathlib import Path


Mounting a file system#

The default file system used in Pyodide is MEMFS, which is a virtual in-memory file system. The data stored in MEMFS will be lost when the page is reloaded.

If you wish for files to persist, you can mount other file systems. Other file systems provided by Emscripten are IDBFS, NODEFS, PROXYFS, WORKERFS. Note that some filesystems can only be used in specific runtime environments. See Emscripten File System API for more details. For instance, to store data persistently between page reloads, one could mount a folder with the IDBFS file system

let mountDir = "/mnt";
pyodide.FS.mount(pyodide.FS.filesystems.IDBFS, {}, mountDir);

If you are using Node.js you can access the native file system by mounting NODEFS.

let mountDir = "/mnt";
pyodide.FS.mount(pyodide.FS.filesystems.NODEFS, { root: "." }, mountDir);
pyodide.runPython("import os; print(os.listdir('/mnt'))");
// ==> The list of files in the Node working directory

(Experimental) Using the native file system in the browser#

You can access the native file system from the browser using the File System Access API.

This is experimental

The File System Access API is only supported in Chromium based browsers: Chrome and Edge (as of 2022/08/18).

Mounting a directory#

Pyodide provides an API pyodide.mountNativeFS() which mounts a FileSystemDirectoryHandle into the Pyodide Python file system.

const dirHandle = await showDirectoryPicker();
const permissionStatus = await dirHandle.requestPermission({
  mode: "readwrite",

if (permissionStatus !== "granted") {
  throw new Error("readwrite access to directory not granted");

const nativefs = await pyodide.mountNativeFS("/mount_dir", dirHandle);

  import os

Synchronizing changes to native file system#

Due to browser limitations, the changes in the mounted file system is not synchronized by default. In order to persist any operations to an native file system, you must call

// nativefs is the returned from: await pyodide.mountNativeFS('/mount_dir', dirHandle)
  with open('/mount_dir/new_file.txt', 'w') as f:

// new_file.txt does not exist in native file system

await nativefs.syncfs();

// new_file.txt will now exist in native file system


pyodide.FS.syncfs(false, callback_func);