在python中使用多处理共享对象数组

Question

我正在尝试在 python 程序的不同进程之间创建一个共享对象数组（类的实例）。这些对象中的每一个都将在程序中进行修改。为此，我使用如下多重处理：

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，我在上述代码中的错误是什么？提前谢谢你。

Answer 1

ctypes.py_object 代表 C 中的 PyObject *。它是指向代表 Python 对象的 struct 的指针，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；试图在进程之间共享指针是没有用的。

另见this answer by Alex Martelli。

Answer 2

您可能想要使用multiprocessing.Queue，您可以在其中转储对象而无需担心类型。它也是线程安全和进程安全的。

这是一个简单的队列示例，用于解决生产者-消费者问题（original source，比萨饼是我的小礼物）。

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。如果您运行它，生产者和消费者打印语句可能会因并行进程的不确定性执行而不同地交错。

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”，但它们可以是任何东西。