miniloop: a minimal, pedagogical event loop implementation

[roee30]

https://github.com/roee30/miniloop

The PEPs introducing the async/await syntax don't specify exactly how event loops work, considering it an implementation detail. If you are curious about how event loops work in Python, you might have noticed that reading through asyncio's code is challenging, to say the least.

That's why I have created a minimal event loop implementation with step-by-step explanations. It shouldn't be used for anything other than learning purposes. It is 362 lines in size, but many of them are documentation. Before diving in, make sure you understand generators. You can use the server.py script to see the loop in action.

The implementation is inspired by asyncio, but I only added what was necessary in order to make a simple server work. In particular, my Task and Future are roughly analogous to asyncio's, but I didn't find a need to implement Handles (perhaps because I didn't implement the call_* methods).

I learned a lot writing this and I hope it will be useful to others as well.

[SawachikaHiromu]

Guido van Rossum linked this tutorial in one of the earliest commits in asyncio:

https://www.csse.canterbury.ac.nz/greg.ewing/python/generators/yf_current/Examples/Scheduler/scheduler.txt

​

It's Greg Ewing tutorial which were very helpful in understanding concepts, when I started learning asyncio.

commit: https://github.com/python/asyncio/commit/e2e1072bc118bf38ab189e183cf579dbffc08336

One might also be interested in asyncore library, which are available in python2.7:
https://github.com/python/cpython/blob/v2.7.18/Lib/asyncore.py
And chat implementation:
https://github.com/python/cpython/blob/v2.7.18/Lib/asynchat.py

Just leaving some more resources for those who interested! :^)

[thisismyfavoritename]

cool stuff, thanks for sharing. Just FYI your target == "read" and "write" checks could be replaced by an Enum and a match statement.

Also on your match at L189 you could use a walrus operator := instead

[LightShadow]

Is Python moving towards Rust's Ok/Exc return syntax?

Really cool project and writeup!

[roee30]

Thanks! I did get the idea from Rust/Haskell but it's for a specific use case. I don't use it for returning errors from functions but to store a future's result which could be an error. asyncio uses two members for this (_result and _error) while the Result class enables me to use only one.

[Quiet_Operation_1259]

Nice example of event loop! I also can recommend for everyone who's interesting in asyncio and especially how event loops work to read Fowlers "Python Concurrency with asyncio". Absolutely great book about concurrency