5.3 KiB
— title: Some Python asyncio disambiguation author: Chris Hodapp date: March 9, 2018 tags: technobabble —
Recently I needed to work a little more in-depth with asyncio in Python 3.x. While some people (including me) might scoff at this because cooperative threading is a model that's fresh out of the '90s and because Python still has the GIL, it is still preferable to manually writing code in continuation-passing-style (that's all callbacks are), and last time I had to write that many callbacks, I hated it enough that I wrote my own EDSL to avoid it. But I digress.
I found the Concurrency with Processes, Threads, and Coroutines tutorials to be approachable and thorough, and I highly recommend them.
However, I still had a few stumbling blocks in understanding, and below I give some notes I wrote to check my understanding. I put together a table to try to classify what method to use in different circumstances. As I use it here, calling "now" means turning control over to some other code, whereas calling "whenever" means retaining control but queuing up some code to be run in the background asychronously (as much as possible).
| Call from | Call to | When/where | How |
|---|---|---|---|
| Either | Function | Now, same thread | Normal function call |
| Function | Coroutine | Now, same thread | .run_* in event loop |
| Coroutine | Coroutine | Now, same thread | await |
| Either | Function | Whenever, same thread | Event loop .call_*() |
| Either | Coroutine | Whenever, same thread | Event loop .create_task() |
asyncio.ensure_future() |
|||
| Either | Function | Now, another thread | .run_in_executor() on ThreadPoolExecutor |
| Either | Function | Now, another process | .run_in_executor() on ProcessPoolExecutor |
Futures & Coroutines
The documentation was also sometimes vague on the relation between coroutines and futures. My summary on what I figured out is below.
Coroutines and Futures are mostly independent.
It just happens that both allow you to call things asychronously.
However, you can use coroutines/asyncio without ever touching a
Future. Likewise, you can use a Future without ever touching a
coroutine or asyncio. Note that its .result() call isn't a
coroutine.
They can still encapsulate each other.
A coroutine can encapsulate a Future simply by using await on it.
A Future can encapsulate a coroutine with asyncio.ensure\_future() or the event loop's .create\_task().
Futures can implement asychronicity(?) differently
The ability to make a Future from a coroutine was mentioned above; that's asyncio.Task, an implementation of asyncio.Future, but it's not the only way to make a Future.
concurrent.futures.Future is another mostly-compatible way. Its ThreadPoolExecutor provides Futures based on separate threads, and its ProcessPoolExecutor provides Futures based on separate processes.
Futures are always paired with some running context.
That is, a Future is already "started" - running, or scheduled to run, or already ran, or something along those lines, and this is why it has semantics for things like cancellation.
A coroutine by itself is not. The closest analogue is asyncio.Handle which is available only when a coroutine has been scheduled to run.
Other Event Loops
Quamash implements an asyncio event loop inside of Qt, and I used this
on a project. I ran into many issues with this combination. Qt's
juggling of multiple event loops seemed to cause many problems here,
and I still have some unsolved issues in which calls
run_until_complete cause coroutines to die early with an exception
because the event loop appears to have died. This came up regularly
for me because of how often I would want a Qt slot to queue a task in
the background, and it seems this is an acknowledge issue.
There is also uvloop. I presently have no need for extra performance (nor could I really use it alongside Qt), but it's helpful to know about.