在Python中使用多处理的对象的共享数组

问题描述：

我想在python程序的不同进程之间创建一个共享对象（类的实例）数组。这些对象中的每一个都将在程序中修改。为此我使用的多为：在Python中使用多处理的对象的共享数组

import multiprocessing 
import numpy as np 
sh = multiprocessing.RawArray (ctypes.py_object, 10) 
f = np.frombuffer(sh, dtype=object)

，我得到的错误是：

Traceback (most recent call last): 
    File "<pyshell#14>", line 1, in <module> 
    f = np.frombuffer(sh, dtype=object) 
ValueError: cannot create an OBJECT array from memory buffer

现在的问题是，首先，这是一般解决这个问题的正确方法第二，在上面的代码中我的错误是什么？先谢谢你。

答

ctypes.py_object代表C中的PyObject *。它是一个指向struct的指针，它表示一个Python对象，驻留在进程的私有内存中，并且包含更多指针。另一个进程无法访问它;试图在进程之间共享指针是没有用的。请参阅this answer by Alex Martelli。

实际上，使用Array和RawArray（在多处理中）是一种在内存中创建共享数组以供多个进程访问的方法。我可以写上面的代码让我们说一个共享的整数数组，但是有问题写它的一组对象。例如，如果我写： >>> import multiprocessing as mp >>> import numpy as np >>> sh = mp.RawArray（ctypes.c_int，10） >>> f = np.frombuffer（sh， dtype = int） >>> f [0] = 1 >>> sh [0] 它完美地工作！ – user823743 2014-11-07 15:13:53

@ user823743正确，但你不能用指针做同样的事情。 – 2014-11-07 18:04:19

你是对的;幸运的是，我发现了另一种避免多处理的方法。谢谢。 – user823743 2014-11-08 19:24:27

答

您可能想要使用multiprocessing.Queue，您可以在其中转储对象而不用担心类型。它也是线程安全的和流程安全的。

下面是一个简单的队列示例，用于简化生产者 - 消费者问题（original source，比萨饼对我有点帮助）。

from multiprocessing import Process, Queue 

class Pizza(object): 
    def __init__(self, pizza_num): 
     self.pizza_num = pizza_num 
     self.num_slices = 8 

sentinel = "NO PIZZA" 

def producer(initial_num_pizzas, total_num_pizzas, q): 
    """Cooks Pizzas to be consumed and waits for the consumer to finish eating.""" 
    print("Producer: I am cooking %s Pizzas and putting them on the Queue!"%(total_num_pizzas-initial_num_pizzas)) 
    for i in range(q.qsize(), total_num_pizzas): 
     print("Producer: Behold, for I have cooked Pizza no. %s"%i) 
     q.put(Pizza(i)) 
    q.put(sentinel) 

def consumer(q): 
    """Consumes some Pizza. In this case, all it does is set the number of slices to 0.""" 
    while True: 
     pizza = q.get() 
     pizza.num_slices = 0 
     if pizza == sentinel: 
      break 
     print("Comsumer: Pizza no. %s was found! It has %s slices, yum!"%(pizza.pizza_num, pizza.num_slices)) 

if __name__ == '__main__': 
    q = Queue() 
    total_num_pizzas = 10 
    initial_num_pizzas = 4 
    ## Let's add some Pizzas beforehand: 
    for i in range(0, initial_num_pizzas): 
     q.put(Pizza(i)) 
    print("Main: I have precooked %s Pizzas."%q.qsize()) 

    producer_proc = Process(target=producer, args=(initial_num_pizzas, total_num_pizzas, q)) 
    consumer_proc = Process(target=consumer, args=(q,)) 
    producer_proc.start() 
    consumer_proc.start() 

    q.close() ## Shop is closed, no more Pizzas will be added to Queue! 
    q.join_thread() 

    producer_proc.join() 
    consumer_proc.join()

下面是一个输出示例。如果运行它，生产者和消费者打印语句可能会以不同方式进行交错，因为并行进程的非确定性执行。

Main: I have precooked 4 Pizzas. 
Producer: I am cooking 6 Pizzas and putting them on the Queue! 
Producer: Behold, for I have cooked Pizza no. 4 
Producer: Behold, for I have cooked Pizza no. 5 
Producer: Behold, for I have cooked Pizza no. 6 
Producer: Behold, for I have cooked Pizza no. 7 
Comsumer: Pizza no. 0 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 1 was found! It has 8 slices, yum! 
Producer: Behold, for I have cooked Pizza no. 8 
Comsumer: Pizza no. 2 was found! It has 8 slices, yum! 
Producer: Behold, for I have cooked Pizza no. 9 
Comsumer: Pizza no. 3 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 4 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 5 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 6 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 7 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 8 was found! It has 8 slices, yum! 
Comsumer: Pizza no. 9 was found! It has 8 slices, yum!

请注意，您应该use Sentinels to mark the end of your Queue。我在这里使用了“NO PIZZA”，但他们可以做任何事情。

在Python中使用多处理的对象的共享数组

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