Asynchronous I/O in Python 3


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Feihong talks about PEP 3156 and basic usage of Tulip, the reference implementation.


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Asynchronous I/O in Python 3

  1. 1. Asychronous I/O in Python 3 Feihong Hsu Chicago Python User Group July 11, 2013
  2. 2. What is PEP 3156? • "Asynchronous IO Support Rebooted" • Describes a new event loop API • Aims to be compatible with current popular event loops (Twisted,Tornado, etc.) • Should land in the standard library in 3.4 (early 2014)
  3. 3. The big take-away We're going to be able to write asynchronous code without threads or callbacks!
  4. 4. Threads vs event loop • I'm not going to talk about this • Twisted being around for more than a decade kinda validates the event loop model • For a detailed discussion see An Intro to Asynchronous Programming and Twisted
  5. 5. Twisted vs Tulip Ugh, how to explain this? I could just give you a boring list of differences... Or I could use an analogy...
  6. 6. Programming with Twisted is like... Being a mystical ninja warrior!
  7. 7. Mystical Ninja Warrior • Can defeat demons and demigods by deploying an army of shadow clones • Mental gymnastics required to coordinate shadow clones requires years of brutal training • Ninja magic comes with a terrible price: the more clones you make, the more insane you become
  8. 8. Programming with Tulip is like... Playing PlantsVs Zombies
  9. 9. Playing PlantsVs Zombies • Can defeat a zombie horde by deploying and managing a large number of horticultural specimens • Not particularly mentally taxing • Somehow end up playing for hours without eating or sleeping
  10. 10. Alrighty With the analogies out of the way, let's look at some examples.
  11. 11. Fetching a web page in Twisted master/examples/
  12. 12. Control flow diagram I was going to draw one but then I got sleepy.
  13. 13. Problems with callback- based asynch code • Can be hard to understand control flow • Error messages are not friendly • Debugging is harder
  14. 14. Fetching a web page in Tulip master/examples/
  15. 15. Fetching a web page in Tulip w/ callbacks master/examples/3-tulip-download-
  16. 16. Fetching a web page in Twisted w/ inline callbacks master/examples/4-twisted-download-inline-
  17. 17. The Tulip API • Basics are documented in PEP 3156 • Not available in Python Package Index • Includes sockets, file I/O, etc. • Includes concurrency data structures like locks, queues, semaphores • Includes SSL and HTTP (with support for websockets!)
  18. 18. Coroutines vs tasks When I first started, the most confusing thing about Tulip. Coroutines are executed by "yield from". Tasks are not executed by "yield from".
  19. 19. Coroutine • Basically a function that contains at least one "yield from" statement. • Not the same thing as a generator function, which is a function that contains a yield expression. • Tulip will barf if you try to make it execute a generator.
  20. 20. Coroutine @tulip.coroutine def download(url): response = yield from tulip.http.request('GET', url) return yield from Calling "download()" returns a generator object, but otherwise does nothing!You need to do "yield from download()" to run the body of a coroutine.
  21. 21. Task • An object that manages a coroutine • Roughly equivalent to the Deferred object in Twisted
  22. 22. Task @tulip.task def download(url): response = yield from tulip.http.request('GET', url) return yield from Calling "download()" actually does run the body of the function.The "yield from" part is done implicitly for you.
  23. 23. Task In Tulip, there are two ways of creating tasks: tulip.task is a decorator that produces a task-wrapping function tulip.Task is a constructor that accepts a coroutine
  24. 24. Why bother to use tasks? • To interoperate with callback-based frameworks like Twisted • To cancel an already-running coroutine • To start a new coroutine from within another coroutine
  25. 25. Wait, so what's a Future? PEP 3156 makes frequent mention of Futures. But talking about Futures is a little confusing, since there are two Future classes: tulip.Future and concurrent.futures.Future.
  26. 26. Future is the superclass of Task • Future don't necessarily manage a coroutine • In practice you never create Future objects, only Task objects • Futures are acceptable to yield from expressions
  27. 27. Methods on Future • cancel(), cancelled() • running(), done() • result(), set_result() • add_done_callback(), remove_done_callback() • ...
  28. 28. Web development with Tulip • All the classes you need are in the tulip.http module • Make subclass of tulip.http.ServerHttpProtocol • Override the handle_request() method
  29. 29. Hello World! master/examples/
  30. 30. Some observations The HTTP API is fairly simple, but a bit low level for everyday web programming. Expect a thousand microframeworks to bloom in the near future. Speaking of microframeworks...
  31. 31. Introducing viol • Tiny web framework based on Tulip • After initial page load, messages between client and server are exchanged via websockets • Makes code demos a bit more visual
  32. 32. Tulip API demos Now for a bunch of demos...
  33. 33. Tulip API demos
  34. 34. Serial version=3&autoplay=1&&start=1546
  35. 35. Simultaneous version=3&autoplay=1&&start=1666
  36. 36. Wait for both coroutines to finish version=3&autoplay=1&&start=1713
  37. 37. Loop through coroutine results in order of finishing version=3&autoplay=1&&start=1869
  38. 38. Execute synchronous code in another thread version=3&autoplay=1&&start=1930
  39. 39. Talk to event loop from another thread version=3&autoplay=1&&start=2060
  40. 40. A regex debugging tool version=3&autoplay=1&&start=2158
  41. 41. Chat application version=3&autoplay=1&&start=2236
  42. 42. Tulip Links • PEP 3156 • Guido's PyCon 2013 keynote • Tulip project page on GoogleCode
  43. 43. Other links • An Introduction to Asynchronous Programming and Twisted • Code for this talk on Github • Viol project page on Github
  44. 44. Questions?