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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.
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.
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 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.

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

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()

Note that calling freezeSupport() at some other point in the main module is likely to cause problems.