1、问题：
群中有同学贴了如下一段代码，问为何 list 最后打印的是空值？
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from multiprocessing import Process, Managerimport os manager = Manager()vip_list = []#vip_list = manager.list() def testFunc(cc): vip_list.append(cc) print 'process id:', os.getpid() if __name__ == '__main__': threads = [] for ll in range(10): t = Process(target=testFunc, args=(ll,)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print vip_list |
其实如果你了解 python 的多线程模型，GIL 问题，然后了解多线程、多进程原理，上述问题不难回答，不过如果你不知道也没关系，跑一下上面的代码你就知道是什么问题了。
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python aa.pyprocess id: 632process id: 635process id: 637process id: 633process id: 636process id: 634process id: 639process id: 638process id: 641process id: 640------------------------process id: 619[] |
将第 6 行注释开启，你会看到如下结果：
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process id: 32074process id: 32073process id: 32072process id: 32078process id: 32076process id: 32071process id: 32077process id: 32079process id: 32075process id: 32080------------------------process id: 32066[3, 2, 1, 7, 5, 0, 6, 8, 4, 9] |
2、python 多进程共享变量的几种方式:
（1）Shared memory：
Data can be stored in a shared memory map using Value or Array. For example, the following code
http://docs.python.org/2/library/multiprocessing.html#sharing-state-between-processes
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from multiprocessing import Process, Value, Array def f(n, a): n.value = 3.1415927 for i in range(len(a)): a[i] = -a[i] if __name__ == '__main__': num = Value('d', 0.0) arr = Array('i', range(10)) p = Process(target=f, args=(num, arr)) p.start() p.join() print num.value print arr[:] |
结果：
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3.1415927[0, -1, -2, -3, -4, -5, -6, -7, -8, -9] |
（2）Server process：
A manager object returned by Manager() controls a server process which holds Python objects and allows other processes to manipulate them using proxies.
A manager returned by Manager() will support types list, dict, Namespace, Lock, RLock, Semaphore, BoundedSemaphore, Condition, Event, Queue, Value and Array.
代码见开头的例子。
http://docs.python.org/2/library/multiprocessing.html#managers
3、多进程的问题远不止这么多：数据的同步
看段简单的代码：一个简单的计数器：
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from multiprocessing import Process, Managerimport os manager = Manager()sum = manager.Value('tmp', 0) def testFunc(cc): sum.value += cc if __name__ == '__main__': threads = [] for ll in range(100): t = Process(target=testFunc, args=(1,)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print sum.value |
结果：
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------------------------process id: 1737897 |
也许你会问：WTF？其实这个问题在多线程时代就存在了，只是在多进程时代又杯具重演了而已：Lock！
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from multiprocessing import Process, Manager, Lockimport os lock = Lock()manager = Manager()sum = manager.Value('tmp', 0) def testFunc(cc, lock): with lock: sum.value += cc if __name__ == '__main__': threads = [] for ll in range(100): t = Process(target=testFunc, args=(1, lock)) t.daemon = True threads.append(t) for i in range(len(threads)): threads[i].start() for j in range(len(threads)): threads[j].join() print "------------------------" print 'process id:', os.getpid() print sum.value |
这段代码性能如何呢？跑跑看，或者加大循环次数试一下。。。
4、最后的建议：
Note that usually sharing data between processes may not be the best choice, because of all the synchronization issues; an approach involving actors exchanging messages is usually seen as a better choice. See also Python documentation: As mentioned above, when doing concurrent programming it is usually best to avoid using shared state as far as possible. This is particularly true when using multiple processes. However, if you really do need to use some shared data then multiprocessing provides a couple of ways of doing so.
5、Refer：
http://stackoverflow.com/questions/14124588/python-multiprocessing-shared-memory
http://eli.thegreenplace.net/2012/01/04/shared-counter-with-pythons-multiprocessing/
http://docs.python.org/2/library/multiprocessing.html#multiprocessing.sharedctypes.synchronized
