Asychronous I/O in Python 3

Feihong HsuChicago Python User Group

July 11, 2013

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)

The big take-away

We're going to be able to write asynchronous code without threads or callbacks!

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

Twisted vs Tulip

Ugh, how to explain this?

I could just give you a boring list of differences...

Or I could use an analogy...

Programming with Twisted is like...

Being a mystical ninja warrior!

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

Programming with Tulip is like...

Playing Plants Vs Zombies

Playing Plants Vs 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

Alrighty

With the analogies out of the way, let's look at some examples.

Fetching a web page in Twisted

https://github.com/feihong/tulip-talk/blob/master/examples/1-twisted-download.py

Control flow diagram

I was going to draw one but then I got sleepy.

Problems with callback-based asynch code

• Can be hard to understand control flow

• Error messages are not friendly

• Debugging is harder

Fetching a web page in Tulip

https://github.com/feihong/tulip-talk/blob/master/examples/2-tulip-download.py

Fetching a web page in Tulip w/ callbacks

https://github.com/feihong/tulip-talk/blob/master/examples/3-tulip-download-callbacks.py

Fetching a web page in Twisted w/ inline

callbackshttps://github.com/feihong/tulip-talk/blob/master/examples/4-twisted-download-inline-callbacks.py

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!)

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".

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.

Coroutine

@tulip.coroutine

def download(url):

response = yield from tulip.http.request('GET', url)

return yield from response.read()

Calling "download()" returns a generator object, but otherwise does nothing! You need to do "yield from download()" to run the body of a coroutine.

Task

• An object that manages a coroutine

• Roughly equivalent to the Deferred object in Twisted

Task@tulip.task

def download(url):

response = yield from tulip.http.request('GET', url)

return yield from response.read()

Calling "download()" actually does run the body of the function. The "yield from" part is done implicitly for you.

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

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

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.

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

Methods on Future

• cancel(), cancelled()

• running(), done()

• result(), set_result()

• add_done_callback(), remove_done_callback()

• ...

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

Hello World!

https://github.com/feihong/tulip-talk/blob/master/examples/5-hello-world-http.py

Some observationsThe 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...

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

Tulip API demos

Now for a bunch of demos...

Tulip API demos

Serial

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=1546

Simultaneous

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=1666

Wait for both coroutines to finish

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=1713

Loop through coroutine results in order of finishing

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=1869

Execute synchronous code in another thread

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=1930

Talk to event loop from another thread

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=2060

A regex debugging tool

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=2158

Chat application

http://www.youtube.com/v/jMgRUI7V_mk?version=3&autoplay=1&&start=2236

Tulip Links

• PEP 3156

• Guido's PyCon 2013 keynote

• Tulip project page on GoogleCode

Other links

• An Introduction to Asynchronous Programming and Twisted

• Code for this talk on Github

• Viol project page on Github

Questions?