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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.

Alternatively another copy of the proxy can be created using the copy.copy() function.

(There is never a problem with different processes using the 'same' proxy.)

Catching ProcessExit
If a ProcessExit exception is raised in a child process then that process should end (almost) immediately; otherwise the stop() method will not work as expected. Note that ProcessExit is a subclass of SystemExit.
Not letting the manager die early
When the main process terminates any manager process it has created will also terminate. Make sure that no child process still needs the manager when this happens --- the easiest way is just to join all child processes you create.
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 will already have been joined. 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, on Unix the following types are not picklable:

Lock, RLock, Semaphore, BoundedSemaphore, Condition, Event, Queue, SharedValue, SharedStruct, SharedArray.

For the sake of compatibility it is better not to rely on these types being picklable.

Windows

Platforms such as Windows which lack os.fork() have a few extra restrictions:

More picklability:

Ensure that all arguments to Process.__init__() are picklable.

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:

Make sure that the module containing the target of a Process instance (or the definition of a subclass of Process you are using) 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 recursively create new processes until you run out of memory or get a crash:

from processing import Process

def foo():
    print 'hello'

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

Instead one should protect creation of the new process by using if __name__ == '__main__': as follows:

from processing import Process

def foo():
    print 'hello'

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

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

Freezing:

One can produce Windows executables from a python program by using py2exe, PyInstaller, cx_Freeze etc. However, if the program uses processing then one needs to call freezeSupport() immediately after the if __name__ == '__main__': line of the main module. Otherwise one will probably get the same problems mentioned above concerning Safe importing. For example

from processing import Process, freezeSupport

def foo():
    print 'hello'

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