在python中使用多处理的共享对象数组

Question

我试图在python程序中的不同进程之间创建一个共享对象（类的实例）的数组。 这些对象中的每一个都将在程序中进行修改。 为此，我使用多处理如下：

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

现在我的问题是，首先，这通常是解决这个问题的正确方法，其次，我在上面的代码中的错误是什么？ 先感谢您。

Answer 1

ctypes.py_object表示C中的PyObject * 。它是一个指向struct的指针，表示Python对象，驻留在进程的私有内存中，并包含更多指针。 另一个进程无法访问它; 尝试在进程之间共享指针是没有用的。

请参阅Alex Martelli的回答 。

Answer 2

您可能希望使用multiprocessing.Queue ，您可以在其中转储对象而无需担心类型。 它还具有线程安全性和过程安全性。

这是一个简单的Queue用于促进生产者 - 消费者问题的例子（ 原始来源 ，Pizzas是我的一点奖励）。

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()

下面是一个示例输出。 如果运行它，Producer和Consumer打印语句可能会以不同方式交错，因为并行进程的执行不确定。

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，您应该使用Sentinels标记队列的结尾 。 我在这里使用过“NO PIZZA”，但它们可以是任何东西。