Stackless Python 101

Stackless Python 101 An advanced level presentation in 20 minutes Kiwi PyCon 9 th November 2009 Richard Tew [email_address] www.stackless.com

What is Stackless?

An extended version of Python.
Backwards compatible.
Stable.

Who uses Stackless

CCP Games
Who the hell knows?

What Stackless provides

Lightweight threading.
More readable code.
Persistence of running code.

Lightweight threading?

The shared thread

More readable code def Function_1(): Function_2() def Function_2(): Current_Function() def Current_Function(): # whatever...

Reasons to block

Allow other tasklets to run.
Reawaken when event occurs.
Reawaken when time has passed.

Example: Python sockets def Function_1(self): self.socket = socket(AF_INET, SOCK_STREAM) ... self.Function_2() def Blocking_Function(self): return self.socket.recv(1000)

stacklesssocket.py Third party library. Monkey patches in place of Python sockets import stacklesssocket stacklesssocket.install()

Example: Stackless sockets def Function_1(self): self.socket = socket(AF_INET, SOCK_STREAM) ... self.Function_2() def Blocking_Function(self): return self.socket.recv(1000)

Where's the readability? No callbacks. No boilerplate. Just a function call. do_something_i_dont_care_if_you_block()

Comparison: generators Generators block to the calling function. Not the scheduler. http://www.dabeaz.com/coroutines

Using Stackless

Tasklets.
The scheduler.
Channels.

The scheduler stackless.run(...)

Scheduling cooperatively Tasklets choose when to block stackless.schedule() channels... Run the scheduler until it is empty. stackless.run()

Scheduling preemptively while 1: interruptedTasklet = stackless.run(100) interruptedTasklet.insert() # Do other stuff.

Tasklets stackless.tasklet(print_args)(1, 2, x=“test“)

Tasklets and threads

Tasklet yielding def print_args(*args, **kwargs): while 1: print 'args', args print 'kwargs', kwargs stackless.schedule()

Channels Used for:

Tasklet blocking.
Tasklet communication.

Channel operations Sending: channel.send(value) Receiving: value = channel.receive()

One blocking tasklet def send_value(channel, value): channel.send(value) channel = stackless.channel() stackless.tasklet(send_value)(channel, 1) stackless.run()

Multiple blocking tasklets def send_value(channel, value): channel.send(value) channel = stackless.channel() stackless.tasklet(send_value)(channel, 1) stackless.tasklet(send_value)(channel, 2) stackless.tasklet(send_value)(channel, 3) stackless.run()

Emptying the channel def empty_channel(channel): print 'tasklets waiting:', channel.balance while channel.balance > 0: value = channel.receive() print 'received', value stackless.tasklet(empty_channel)(channel) stackless.run()

Idiom: Blocking out of the scheduler rapture_channel = stackless.channel() def wait_for_rapture(): rapture_channel.receive() def call_me_when_rapture_occurs(): while rapture_channel.balance < 0: rapture_channel.send(None)

What I have not covered