Coroutines — kaa.base v0.6.0 documentation

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# Coroutines

Coroutines are used to break up large and computationally expensive tasks into smaller tasks, where control is relinquished to the main loop after each smaller task. Coroutines are also very useful in constructing state machines. In the event where blocking is unavoidable, and the duration of the block is unknown (for example, connecting to a remote host, or scaling a very large image), threads can be used. These two different approaches are unified with a very similar API.

A function or method is designated a coroutine by using the @kaa.coroutine decorator. A coroutine allows a larger tasks to be broken down into smaller ones by yielding control back to the “scheduler” (the notifier), implementing a kind of cooperative multitasking. More usefully, coroutines can yield at points where they may otherwise block on resources (e.g. disk or network), and when the resource becomes available, the coroutine resumes where it left off. With coroutines and InProgress objects, it is possible to construct non-trivial state machines, whose state is modified by asynchronous events, using a single coroutine. Without coroutines, this is typically implemented as a series of smaller callback functions. (For more information on coroutines, see Wikipedia’s treatment of the subject.)

Any function decorated with coroutine will return an InProgress object, and the caller can connect a callback to the InProgress object in order to be notified of its return value or any exception.

When a coroutine yields kaa.NotFinished, control is returned to the main loop, and the coroutine will resume after the yield statement at the next main loop iteration, or, if an interval is provided with the decorator, after this time interval. Following the cooperative multitasking analogy, yielding kaa.NotFinished can be thought of as the coroutine releasing a “time slice” so that other tasks may run.

When a coroutine yields any value other than kaa.NotFinished (including None), the coroutine is considered finished and the InProgress returned to the caller will be emitted (i.e. it is finished). As with return, if no value is explicitly yielded and the coroutine terminates, the InProgress is finished with None.

There is an important exception to the above rule: if the coroutine yields an `InProgress` object, the coroutine will be resumed when the InProgress object is finished. This allows a coroutine to be “chained” with other InProgress tasks, including other coroutines.

To recap, if a coroutine yields:

> - kaa.NotFinished: control is returned to the main loop so that other tasks can run (such as other timers, I/O handlers, etc.) and resumed on the next main loop iteration.
> - an `InProgress` object: control is returned to the main loop and the coroutine is resumed with the yielded InProgress is finished. Inside the coroutine, the yield call “returns” the value that InProgress was finished with.
> - any other value: the coroutine terminates, and the InProgress the coroutine returned to the caller is finished with that value (which includes None, if no value was explicitly yielded and the coroutine reaches the end naturally).

Here is a simple example that breaks up a loop into smaller tasks:

    import kaa

    @kaa.coroutine()
    def do_something():
       for i in range(10):
          do_something_expensive()
          yield kaa.NotFinished
       yield 42

    def handle_result(result):
       print "do_something() finished with result:", result

    do_something().connect(handle_result)
    kaa.main.run()

A coroutine can yield other coroutines (or rather, the InProgress object the other coroutine returns):

    @kaa.coroutine()
    def do_something_else():
       try:
          result = yield do_something()
       except:
          print "do_something failed"
          yield

       yield True if result else False

(Note that the above syntax, in which the yield statement returns a value, was introduced in Python 2.5. kaa.base requires Python 2.5 or later.)

Classes in kaa make heavy use of coroutines and (to a lesser extent) threads when methods would otherwise block on some resource. Both coroutines and @threaded-decorated methods return InProgress objects and behave identically. These can be therefore yielded from a coroutine in the same way:

    @kaa.coroutine()
    def fetch_page(host):
        """
        Fetches / from the given host on port 80.
        """
        socket = kaa.Socket()
        # Socket.connect() is implemented as a thread
        yield socket.connect((host, 80))
        # Socket.read() and write() are implemented as single-thread async I/O.
        yield socket.write('GET / HTTP/1.1\n\n')
        print (yield socket.read())

In the above example, the difference between threaded functions (`kaa.Socket.connect()`) and coroutines (`write()` and `read()`) is transparent. Both return InProgress objects. (As an aside, we didn’t really need to yield socket.write() because writes are queued and written to the socket when it becomes writable. However, yielding a write means that when the coroutine resumes, the data has been written.)

To more clearly see the benefit of implementing the above example as a coroutine, consider the following code, which is rewritten using the more traditional approach of connecting callbacks at the various stages of the task:

    def fetch_page(host):
        socket = kaa.Socket()
        socket.connect((host, 80)).connect(finished_connect, socket)

    def finished_connect(result, socket):
        socket.write('GET / HTTP/1.1\n\n').connect(finished_write, socket)

    def finished_write(len, socket):
        socket.read().connect(finished_read)

    def finished_read(data):
        print data

In practice then, coroutines can be seen as an alternative approach to the classic signal/callback pattern, allowing you to achieve the same logic but with a much more intuitive and readable code. This means that if you design your application to use signals and callbacks, it might not be clear where coroutines would be useful.

However, if you make use of the asynchronous plumbing that kaa offers early on in your design – and that includes healthy use of `InProgress` objects, either explicitly or implicitly through the use of the @coroutine and @threaded decorators – you should find that you’re able to produce some surprisingly elegant, non-trivial code.

## Decorator

`kaa.` `coroutine`(interval=0, policy=None, progress=False, group=None)

  Decorated functions (which must be generators) may yield control back to the mainloop and be subsequently resumed at a later time.

  Functions which yield `kaa.NotFinished` will be resumed on the next mainloop iteration; yielding an `InProgress` object will cause the coroutine to be resumed when the InProgress is finished. However, yielding a finished InProgress object will cause the coroutine to be resumed immediately.

  The coroutine is considered finished when the underlying generator yields a value other than `kaa.NotFinished` or an InProgress object.

  Parameters: | interval – Number of seconds to delay before resuming entry into the coroutine. Set to 0 (default) to resume as soon as possible (but not sooner than the next mainloop iteration). policy – None, or one of `POLICY_SYNCHRONIZED`, `POLICY_SINGLETON`, or `POLICY_PASS_LAST` (described below). progress – if True, a Progress object is passed as the first argument to the decorated function, allowing the coroutine to report progress to the caller. (The progress parameter corresponds to the `progress` attribute of the InProgress object returned to the caller.) group – Name of the group this coroutine shares its policy with. For example, multiple coroutines with POLICY_SYNCHRONIZED and the same group name will all be synchronized against each other.
  Returns: | an `InProgress` object representing the coroutine.

  Possible policies are:

  > - `kaa.POLICY_SYNCHRONIZED`: reentry into the coroutine is not permitted, and multiple calls are queued so that they execute sequentially.
  > - `kaa.POLICY_SINGLETON`: only one active instance of the coroutine is allowed to exist. If the coroutine is invoked while another is running, the CoroutineInProgress object returned by the first invocation until it finishes.
  > - `kaa.POLICY_PASS_LAST`: passes the CoroutineInProgress of the most recently called, unfinished invocation of this coroutine as the ‘last’ kwarg. If no such CoroutineInProgress exists, the last kwarg will be None. This is useful to chain multiple invocations of the coroutine together, but unlike `POLICY_SYNCHRONIZED`, the decorated function is entered each invocation.

  A function decorated with this decorator will always return an `InProgress` object. It may already be finished (which happens if the coroutine’s first yielded value is one other than `kaa.NotFinished` or an InProgress object).

  If it is not finished, the coroutine’s life can be controlled via the `InProgress` it returns. It can be `interval()` property.

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