(file-system)=

# Dealing with the file system

Pyodide includes a file system provided by Emscripten.
In JavaScript, the Pyodide file system can be accessed through {any}`pyodide.FS` which re-exports the [Emscripten File System API](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api)

**Example: Reading from the file system**

```js
pyodide.runPython(`
  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**

```js
let data = "hello world!";
pyodide.FS.writeFile("/hello.txt", data, { encoding: "utf8" });
pyodide.runPython(`
  with open("/hello.txt", "r") as fh:
        data = fh.read()
  print(data)
`);
```

## Mounting a file system

The default file system used in Pyodide is [MEMFS](https://emscripten.org/docs/api_reference/Filesystem-API.html#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](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api) for more details.
For instance, to store data persistently between page reloads, one could mount
a folder with the
[IDBFS file system](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api-idbfs)

```js
let mountDir = "/mnt";
pyodide.FS.mkdir(mountDir);
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`.

```js
let mountDir = "/mnt";
pyodide.FS.mkdir(mountDir);
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
```