# Module k.tasklet

Part of kiwi

Pseudo-thread (coroutines) framework

# Introduction

This module adds infrastructure for managing tasklets. In this context, a tasklet is defined as a routine that explicitly gives back control to the main program a certain points in the code, while waiting for certain events. Other terms that may be used to describe tasklets include coroutines, or cooperative threads.

The main advantages of tasklets are:

- Eliminates the danger of unexpected race conditions or deadlocks that happen with preemptive (regular) threads;
- Reduces the number of callbacks in your code, that sometimes are so many that you end up with spaghetti code.

The fundamental block used to create tasklets is Python's generators. Generators are objects that are defined as functions, and when called produce iterators that return values defined by the body of the function, specifically `yield` statements.

The neat thing about generators are not the iterators themselves but the fact that a function's state is completely frozen and restored between one call to the iterator's `next()` and the following one. This allows the function to return control to a program's main loop while waiting for an event, such as IO on a socket, thus allowing other code to run in the mean time. When the specified event occurs, the function regains control and continues executing as if nothing had happened.

# Structure of a tasklet

At the outset, a tasklet is simply a python generator function, i.e. a function or method containing one or more `yield` statements. Tasklets add a couple more requirements to regular generator functions:

1. The values contained in `yield` statements cannot be arbitrary (see below);
2. After each `yield` that indicates events, the function `kiwi.tasklet.get_event` must be called to retrieve the event that just occurred.

# Syntax for yield in tasklets

Inside tasklet functions, `yield` statements are used to suspend execution of the tasklet while waiting for certain events. Valid `yield` values are:

- A single `Message` object, with a correctly set dest parameter. With this form, a message is sent to the indicated tasklet. When `yield` returns, no event is generated, so the tasklet should not call `get_event`.
- One, or a sequence of:
  - `WaitCondition`, meaning to wait for that specific condition
  - `Tasklet`, with the same meaning as `WaitForTasklet` `(tasklet)`
  - generator, with the same meaning as `WaitForTasklet` `(Tasklet(gen))`
  In this case, the tasklet is suspended until either one of the indicated events occurs. The tasklet must call `get_event` in this case.

# Launching a tasklet

To start a tasklet, the `Tasklet` constructor must be used:

     from kiwi import tasklet

     def my_task(x):
         [...]

     tasklet.Tasklet(my_task(x=0))

Alternatively, `kiwi.tasklet.run` can be used to the same effect:

     from kiwi import tasklet
     tasklet.run(my_task(x=0))

Yet another approach is to use the @tasklet.task decorator:

     from kiwi import tasklet

     @tasklet.task
     def my_task(x):
         [...]
         raise StopIteration("return value")

     yield my_task(x=0)
     retval = tasklet.get_event().retval

# Examples

## Background timeout task

This example demonstrates basic tasklet structure and timeout events:

     import gobject
     from kiwi import tasklet

     mainloop = gobject.MainLoop()

     def simple_counter(numbers):
         timeout = tasklet.WaitForTimeout(1000)
         for x in xrange(numbers):
             print x
             yield timeout
             tasklet.get_event()
         mainloop.quit()

     tasklet.run(simple_counter(10))
     mainloop.run()

## Message passing

This example extends the previous one and demonstrates message passing:

     import gobject
     from kiwi import tasklet

     mainloop = gobject.MainLoop()

     @tasklet.task
     def printer():
         msgwait = tasklet.WaitForMessages(accept=("quit", "print"))
         while True:
             yield msgwait
             msg = tasklet.get_event()
             if msg.name == "quit":
                 return
             assert msg.name == 'print'
             print ">>> ", msg.value

     @tasklet.task
     def simple_counter(numbers, task):
         timeout = tasklet.WaitForTimeout(1000)
         for x in xrange(numbers):
             yield tasklet.Message('print', dest=task, value=x)
             yield timeout
             tasklet.get_event()
         yield tasklet.Message('quit', dest=task)
         mainloop.quit()

     task = printer()
     simple_counter(10, task)
     mainloop.run()

Class | task | A decorator that modifies a tasklet function to avoid the need
Function | get_event | Return the last event that caused the current tasklet to regain control.
Function | run | Start running a generator as a `Tasklet`.
Class | WaitCondition | Base class for all wait-able condition objects.
Class | WaitForCall | An object that waits until it is called.
Class | WaitForIO | An object that waits for IO conditions on sockets or file
Class | WaitForTimeout | An object that waits for a specified ammount of time (a timeout)
Class | WaitForIdle | An object that waits for the main loop to become idle
Class | WaitForTasklet | An object that waits for a tasklet to complete
Class | WaitForSignal | An object that waits for a signal emission
Class | WaitForProcess | An object that waits for a process to end
Class | Message | A message that can be received by or sent to a tasklet.
Function | _normalize_list_argument | returns a list of strings from an argument that can be either
Class | WaitForMessages | An object that waits for messages to arrive
Class | Tasklet | An object that launches and manages a tasklet.

def get_event():
Return the last event that caused the current tasklet to regain control.

Note | this function should be called exactly once after each yield that includes a wait condition.

def run(gen):
Start running a generator as a `Tasklet`.

Parameters | gen | generator object that implements the tasklet body.
Returns | a new `Tasklet` instance, already running.
Note | this is strictly equivalent to calling `Tasklet(gen)`.

def _normalize_list_argument(arg, name):
returns a list of strings from an argument that can be either list of strings, None (returns []), or a single string returns ([arg])