Python Workers are now generally available
blog.cloudflare.com
blog.cloudflare.com
Some context from an urllib3 maintainer:
urllib3 received and merged large contributions adding Pyodide/Emscripten support a few years ago, and later JSPI support, which is what made this work for Requests.
As far as I know, the funding for this work went to the external contributor who implemented it, not to the urllib3 maintainers. We reviewed and merged the changes, and the project is now responsible for maintaining the resulting backend.
This matters because the Emscripten backend is still considered experimental in urllib3, and is explicitly out of scope in our security policy.
CVE-2025-50182 is one example of the problems we've run into. urllib3's redirect controls did not have the expected behavior when requests were routed through `fetch`. There are potentially many more differences like this because browser/`fetch` networking semantics are quite different from urllib3's normal backend.
I'm glad the work was contributed upstream and is useful to Pyodide and Cloudflare. But I think there is a meaningful difference between funding a contribution to an upstream project and funding the upstream maintainers who have to support it afterwards.
Pyodide and Cloudflare don't use the real network stack for http requests (they patched the functions to use JS `fetch` underneath). They also patched Python's async event loop to use JS event loop.
This has a major downside: incompatibility issues. JS event loop is preemptive (async functions get called regardless of you calling await on them) while Python is lazy (async functions only execute when you await them).
In my belief, the network semantics should be preserved. When the behavior differs issues start arising.
I think the word you want is "eager". Preemptive usually refers to things like signal handlers: when the signal is received the handler "preempts" normal execution without waiting for an explicit yield point.
In any case, with the WebLoop, Python coroutines stay lazy. The fundamental primitive a Python event loop needs to implement is call_later() which maps fairly cleanly to `setTimeout()`.
The reason we want to use the JS event loop is that the JS event loop is where all the actual I/O events in a JavaScript runtime happen. If you create a second event loop and run it, it will block actual IO on the JS event loop. So making uvloop work would be pointless.
Yup, thanks for the correction! (and appreciate the extra insight!)
Pyodide in browsers _can't_ use the real network stack because of fundamental security principles of browsers. With direct networking you could get around the CORS restrictions.
Thanks to Gyeongjae Choi's work, Pyodide in Node/Cloudflare can use direct sockets. See this section of the blog post: https://blog.cloudflare.com/python-workers-ga/#using-postgre...
Any of the upstream maintainers consider blocking merge unless Cloudflare signs a support contract?
Not an easy process to navigate, but if you find an internal champion that can help make the case, maintenance can be funded. For critical deps, companies are generally down to budget some financial support if you ask and know how to work the process.
We intentionally keep that module experimental and don't actively maintain it in urllib3. The maintainers don't currently work on new features, bug fixes, or security hardening for it, and we explicitly say so in our security policy.
So this isn't really a case of blocking a merge until someone funds it. The code is already upstream; we just haven't taken on the ongoing maintenance of that backend.
If Cloudflare needs ongoing support for it, we'd be happy to discuss that with them.
I was very excited when they first launched Python Workers two years ago. Even though we have competing products at Wasmer, I think Cloudflare work is always exciting and inspiring.
I went back to the feedback I posted in the original launch thread [1]. It's great to see that they have made meaningful progress since then, particularly around package support: PyEmscripten is now standardized through PEP 783.
That said, some of the main architectural concerns I raised at the time are still present:
* Being tied to use only one version of Python/Pyodide (the one that Workerd embeds)
* Architecturally tied to the JS/v8 world, which may show some challenges as they aim to reduce cold start times (in my opinion, it will be quite hard for them to achieve <100ms startup time with their current architecture).
The GA announcement doesn't seem to include updated cold-start numbers. Could someone from the Cloudflare team share the current p50/p95 cold-start times for Python Workers, ideally both with and without native user packages? (for example, one with FastAPI and other without any dependencies).
[1] https://news.ycombinator.com/item?id=39907120
[2] https://wasmer.io/posts/wasm-clouds-the-world-after-containe...
> Being tied to use only one version of Python/Pyodide (the one that Workerd embeds)
This isn't quite the case, you can choose between different versions using compatibility flags. For example, `python_workers_314` is the compat flag for Python 3.14[1]. You've also got compat flags for 3.13 and 3.12. Though it is worth noting that by using those older versions you will also be using older Pyodide versions too, which have fewer features (for example they lack JSPI support).
> Architecturally tied to the JS/v8 world, which may show some challenges as they aim to reduce cold start times
That is indeed a challenge. But our memory snapshot implementation has improved the cold starts significantly already and we will be working to reduce these even further. We also have sharding these days which reduces cold start frequency a lot. We wrote about cold starts (and sharding) in a previous blog post[2] which includes some numbers.
1 - https://developers.cloudflare.com/workers/configuration/comp...
2 - https://blog.cloudflare.com/python-workers-advancements/
> it is worth noting that by using those older versions you will also be using older Pyodide versions too
Yeah, I think this summarizes properly the issue I mentioned. Basically compat flags are a global version that affects not only the Python version used but workerd as well. I believe you'll see some architectural issues from this design. Following up on your example, users will not be able to use a previous version of Python that has JSPI included, unless you update the old workerd as well (please correct me if I'm wrong), which will make certain things a challenge as workerd evolves.
> We wrote about cold starts (and sharding) in a previous blog post[2] which includes some numbers
Thanks for sharing. On that blogpost [2] Cloudflare Python Workers startup time was reported to be about 1.027 seconds, which is way behind the numbers we have at Wasmer for cold starts in Python apps (60ms, or 16x faster). That's why I was asking if you guys remeasured and have better timings now :)
[1] https://googleappengine.blogspot.com/2008/04/introducing-goo...
There is still more to do here and it’s what we will be focusing on next.
Still a welcome move that they finally added support from Python.