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

  with open("/hello.txt", "w") as fh:
      fh.write("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" });
  with open("/hello.txt", "r") as fh:
        data =

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, { root: "." }, 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