Pyodide, Emscripten, and PyScript

From the Pyodide documentation, Pyodide is a Python distribution for the browser and Node.js based on WebAssembly and Emscripten. This technology also underpins the PyScript framework and Jupyterlite, so should work in those environments too.

Starting in version 2.2.0 urllib3 supports being used in a Pyodide runtime utilizing the JavaScript fetch API or falling back on XMLHttpRequest if the fetch API isn’t available (such as when cross-origin isolation isn’t active). This means you can use Python libraries to make HTTP requests from your browser!

Because urllib3’s Emscripten support is API-compatible, this means that libraries that depend on urllib3 may now be usable from Emscripten and Pyodide environments, too.


Support for Emscripten and Pyodide is experimental. Report all bugs to the urllib3 issue tracker. Currently only supports browsers, does not yet support running in Node.js.

It’s recommended to run Pyodide in a Web Worker in order to take full advantage of features like the fetch API which enables streaming of HTTP response bodies.

Getting started

Using urllib3 with Pyodide means you need to get started with Pyodide first. The Pyodide project provides a useful online REPL to try in your browser without any setup or installation to test out the code examples below.

urllib3’s Emscripten support is automatically enabled if sys.platform is "emscripten", so no setup is required beyond installation and importing the module.

You can install urllib3 in a Pyodide environment using micropip. Try using the following code in a Pyodide console or <script> tag:

import micropip
await micropip.install("urllib3")

import urllib3
resp = urllib3.request("GET", "")

print(resp.status)  # 200
print(resp.headers) # HTTPHeaderDict(...)
print(resp.json())  # {"headers": {"Accept": "*/*", ...}, ...}

Because Requests is built on urllib3, Requests also works out of the box:

import micropip
await micropip.install("requests")

import requests
resp = requests.request("GET", "")

print(resp.status_code)  # 200


Because we use JavaScript APIs under the hood, it’s not possible to use all of urllib3 features. Features which are usable with Emscripten support are:

  • Requests over HTTP and HTTPS

  • Timeouts

  • Retries

  • Streaming (with Web Workers and Cross-Origin Isolation)

  • Redirects

  • Decompressing response bodies

Features which don’t work with Emscripten:

  • Proxies, both forwarding and tunneling

  • Customizing TLS and certificates (uses browsers’ configuration)

  • Configuring low-level socket options or source address

Streaming with Web Workers

To access the fetch API and do HTTP response streaming with urllib3 you must be running the code within a Web Worker and set specific HTTP headers for the serving website to enable Cross-Origin Isolation.

You can verify whether a given environment is cross-origin isolated by evaluating the global crossOriginIsolated JavaScript property.