.. include:: header.txt

========================
 Programming guidelines
========================

There are certain guidelines and idioms which should be adhered to
when using the `processing` package.  


All platforms
-------------

*Avoid shared state*
    As far as possible one should try to avoid shifting large amounts
    of data between processes.
    
    It is probably best to stick to using queues or pipes for
    communication between processes rather than using the lower level
    synchronization primitives from the `threading` module.
    
*Picklability*:
    Ensure that the arguments to the methods of proxies are
    picklable.

*Thread safety of proxies*:
    Do not use a proxy object from more than one thread unless you
    protect it with a lock.

    (There is never a problem with different processes using the
    'same' proxy.)
        
*Joining zombie processes*
    On Unix when a process finishes but has not been joined it becomes
    a zombie.  There should never be very many because each time a new
    process starts (or `activeChildren()` is called) all completed
    processes which have not yet been joined will be joined.  Also
    calling a finished process's `isAlive()` will join the process.
    Even so it is probably good practice to explicitly join all the
    processes that you start.

*Better to inherit than pickle/unpickle*
    On Windows many of types from the `processing` package need to be
    picklable so that child processes can use them.  However, one
    should generally avoid sending shared objects to other processes
    using pipes or queues.  Instead you should arrange the program so
    that a process which need access to a shared resource created
    elsewhere can inherit it from an ancestor process.

*Avoid terminating processes*
    Using the `terminate()` method to stop a process is liable to
    cause any shared resources (such as locks, semaphores, pipes and
    queues) currently being used by the process to become broken or
    unavailable to other processes.

    Therefore it is probably best to only consider using `terminate()`
    on processes which never use any shared resources.

*Joining processes that use queues*
    Bear in mind that a process that has put items in a queue will
    wait before terminating until all the buffered items are fed by
    the "feeder" thread to the underlying pipe.  (The child process
    can call the `cancelJoin()` method of the queue to avoid this
    behaviour.)

    This means that whenever you use a queue you need to make sure
    that all items which have been put on the queue will eventually be
    removed before the process is joined.  Otherwise you cannot be
    sure that processes which have put items on the queue will
    terminate.  Remember also that non-daemonic processes will be
    automatically be joined.

    An example which will deadlock is the following::

        from processing import Process, Queue

        def f(q):
            q.put('X' * 1000000)

        if __name__ == '__main__':
            queue = Queue()
            p = Process(target=f, args=(queue,))
            p.start()
            p.join()                    # this deadlocks
            obj = queue.get()

    A fix here would be to swap the last two lines round (or simply
    remove the `p.join()` line).

*Explicity pass resources to child processes*
    On Unix a child process can make use of a shared resource created
    in a parent process using a global resource.  However, it is
    better to pass the object as an argument to the constructor for
    the child process.  

    Apart from making the code (potentially) compatible with Windows
    this also ensures that as long as the child process is still alive
    the object will not be garbage collected in the parent process.
    This might be important if some resource is freed when the object
    is garbage collected in the parent process.

    So for instance ::

        from processing import Process, Lock

        def f():
            ... do something using "lock" ...

        if __name__ == '__main__':
           lock = Lock()
           for i in range(10):
                Process(target=f).start()

    should be rewritten as ::

        from processing import Process, Lock

        def f(l):
            ... do something using "l" ...

        if __name__ == '__main__':
           lock = Lock()
           for i in range(10):
                Process(target=f, args=(lock,)).start()

 
Windows
-------

Since Windows lacks `os.fork()` it has a few extra restrictions:

*More picklability*:

    Ensure that all arguments to `Process.__init__()` are picklable.
    This means, in particular, that bound or unbound methods cannot be
    used directly as the `target` argument on Windows --- just define
    a function and use that instead.

    Also, if you subclass `Process` then make sure that instances
    will be picklable when the `start()` method is called.

*Global variables*:
    Bear in mind that if code run in a child process tries to access a
    global variable, then the value it sees (if any) may not be the
    same as the value in the parent process at the time that
    `start()` was called.

    However, global variables which are just module level constants
    cause no problems.

*Safe importing of main module*:
    Make sure that the main module can be safely imported by a new
    Python interpreter without causing unintended side effects (such a
    starting a new process).

    For example, under Windows running the following module would
    fail with a `RuntimeError`::

        from processing import Process

        def foo():
            print 'hello'

        p = Process(target=foo)
        p.start()

    Instead one should protect the "entry point" of the program by
    using `if __name__ == '__main__':` as follows::

        from processing import Process

        def foo():
            print 'hello'

        if __name__ == '__main__':
            freezeSupport()
            p = Process(target=foo)
            p.start()

    (The `freezeSupport()` line can be ommitted if the program will
    be run normally instead of frozen.)

    This allows the newly spawned Python interpreter to safely import
    the module and then run the module's `foo()` function.

    Similar restrictions apply if a pool or manager is created in the
    main module.


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