First draft of Python asyncio post

drafts/2018-03-09-python-asyncio.org

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+---
+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 [[https://docs.python.org/3/library/asyncio.html][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 [[https://wiki.python.org/moin/GlobalInterpreterLock][GIL]], it is still preferable to
+manually writing code in [[https://en.wikipedia.org/wiki/Continuation-passing_style][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 [[https://github.com/HaskellEmbedded/ion][EDSL]] to avoid it.  But I digress.
+
+I found the [[https://pymotw.com/3/concurrency.html][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     | How                         |
+|-----------+-----------+----------+-----------------------------|
+| Either    | Function  | Now      | Normal function call        |
+| Function  | Coroutine | Now      | `.run_*` in event loop      |
+| Coroutine | Coroutine | Now      | `await`                     |
+| Either    | Function  | Whenever | Event loop `.call_*()`      |
+| Either    | Coroutine | Whenever | Event loop `.create_task()` |
+|           |           |          | `asyncio.ensure_future()`   |
+
+* 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 `await`ing it.
+
+A Future can encapsulate a coroutine with [[https://docs.python.org/3/library/asyncio-task.html#asyncio.ensure_future][asyncio.ensure_future()]] or
+the event loop's [[https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.AbstractEventLoop.create_task][.create_task()]].
+
+** Futures can implement asychronicity(?) differently
+
+The ability to make a Future from a coroutine was mentioned above;
+that's [[https://docs.python.org/3/library/asyncio-task.html#task][asyncio.Task]], an implementation of [[https://docs.python.org/3/library/asyncio-task.html#future][asyncio.Future]], but it's not
+the only way to make a Future.
+
+[[https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Future][concurrent.futures.Future]] is another mostly-compatible way.  Its
+[[https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor][ThreadPoolExecutor]] provides Futures based on separate threads, and its
+[[https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor][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 [[https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.Handle][asyncio.Handle]]
+which is available only when a coroutine has been scheduled to run.
+
+* Other Event Loops
+
+[[https://pypi.python.org/pypi/Quamash][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 [[https://github.com/harvimt/quamash/issues/33][issue]].
+
+There is also [[https://github.com/MagicStack/uvloop\][uvloop]].  I have no need for extra performance (nor could
+I really use it alongside Qt), but it's helpful to know about.
+
+# Also: What about coroutine generators?
+# Are they anything special?