Asyncio: A library with too many sharp corners

Summary

A red garden tulip. Photo by my friend Jamie. One of the headliner features of Python 3.4 (released in 2014) was a new library in the standard library: asyncio, provisionally introduced for feedback as an import of the external tulip library. In Python 3.5 (released in 2015), async and await were added as keywords to the language specifically for usage with asynchronous libraries, replacing the usage of yield from. The asyncio module was also made non-provisional in this release, heralding an entire new ecosystem of asynchronous libraries in the Python world. But asyncio has so many sharp corners and design issues it is far too difficult to use, bordering on being fundamentally broken. Some of these were only realised in hindsight because other languages (Kotlin, Swift) and libraries did what asyncio does significantly better; but most of these issues were bad at release and it is baffling how the library made it out of provisional status with such glaring flaws. I mention the Trio library a lot in this post, but there's also the AnyIO library that implements Trio-like semantics on top of asyncio, fixing most of the issues described here whilst retaining a level of compatibility with regular asyncio libraries. Contents Major Problem #1: Cancellation is broken Major Problem #2: Task was destroyed but it is pending! Major Problem #3: I/O has pointless landmines Major Problem #4: asyncio.Queue is difficult to use Other, less major problems Major Problem #1: Cancellation is broken In the traditional model of concurrent programming using threads, there is no clean way to do cancellation. In the standard pthreads model, the only way to do cancellation is to brutally murder a thread using pthread_kill, which is nearly always a bad idea because anything the thread was using (such as locks) will be in an unknown and inconsistent state if the thread was killed in the middle of an operation. If you want a better mechanism, you need to implement it yourself by constantly pollin...