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python - 进程池中进程之间共享的类属性和内存？

问题描述

我有一个类A，它在启动时会更改一个可变类属性nums。

当通过带有 的进程池启动类时，我注意到它具有几个不同进程的值。这对我来说是一种不良行为。maxtasksperchild= 1nums

我的问题是：

编辑：我猜这个池会腌制它开始的前一个进程（而不是原来的进程），从而保存 的值nums，对吗？如果是这样，我该如何强制它使用原始流程？

这是一个示例代码：

from multiprocessing import Pool


class A:
    nums = []

    def __init__(self, num=None):
        self.__class__.nums.append(num)  # I use 'self.__class__' for the sake of explicitly
        print(self.__class__.nums)
        assert len(self.__class__.nums) < 2  # checking that they don't share memory


if __name__ == '__main__':
    with Pool(maxtasksperchild=1) as pool:
        pool.map(A, range(99))  # the assert is being raised

编辑因为 k.wahome 的回答：使用实例属性不能回答我的问题我需要使用类属性，因为在我的原始代码（此处未显示）中，每个进程有几个实例。我的问题专门关于多处理池的工作原理。

顺便说一句，执行以下操作确实有效

from multiprocessing import Process

if __name__ == '__main__':
    prs = []
    for i in range(99):
        pr = Process(target=A, args=[i])
        pr.start()
        prs.append(pr)
    [pr.join() for pr in prs]
# the assert was not raised

标签： pythonmultiprocessingpython-multiprocessingprocess-pool

解决方案

共享很可能是通过A具有类属性的映射类进入的nums。

类属性是类绑定的，因此属于类本身，是在加载类时创建的，它们将由所有实例共享。所有对象都将具有对类属性的相同内存引用。

与类属性不同，实例属性是实例绑定的，不被各种实例共享。每个实例都有自己的实例属性副本。

查看类 vs 实例属性效果：

1.使用nums作为类属性 class_num.py

from multiprocessing import Pool


class A:
nums = []

def __init__(self, num=None):
    # I use 'self.__class__' for the sake of explicitly
    self.__class__.nums.append(num)
    print("nums:", self.__class__.nums)
    # checking that they don't share memory
    assert len(self.__class__.nums) < 2


if __name__ == '__main__':
with Pool(maxtasksperchild=1) as pool:
    print(pool)
    pool.map(A, range(99))  # the assert is being raised

运行此脚本

>>> python class_num.py
nums: [0]
nums: [0, 1]
nums: [4]
nums: [4, 5]
nums: [8]
nums: [8, 9]
nums: [12]
nums: [12, 13]
nums: [16]
nums: [16, 17]
nums: [20]
nums: [20, 21]
nums: [24]
nums: [24, 25]
nums: [28]
nums: [28, 29]
nums: [32]
nums: [32, 33]
nums: [36]
nums: [36, 37]
nums: [40]
nums: [40, 41]
nums: [44]
nums: [44, 45]
nums: [48]
nums: [48, 49]
nums: [52]
nums: [52, 53]
nums: [56]
nums: [56, 57]
nums: [60]
nums: [60, 61]
nums: [64]
nums: [64, 65]
nums: [68]
nums: [68, 69]
nums: [72]
nums: [72, 73]
nums: [76]
nums: [76, 77]
nums: [80]
nums: [80, 81]
nums: [84]
nums: [84, 85]
nums: [88]
nums: [88, 89]
nums: [92]
nums: [92, 93]
nums: [96]
nums: [96, 97]
multiprocessing.pool.RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 119, in worker
    result = (True, func(*args, **kwds))
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 44, in mapstar
    return list(map(*args))
  File "class_num.py", line 12, in __init__
    assert len(self.__class__.nums) < 2
AssertionError
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "class_num.py", line 18, in <module>
    pool.map(A, range(99))  # the assert is being raised
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 260, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/pool.py", line 608, in get
    raise self._value
AssertionError

2.nums作为实例属性使用 instance_num.py

from multiprocessing import Pool


class A:

    def __init__(self, num=None):
        self.nums = []
        if num is not None:
            self.nums.append(num)
        print("nums:", self.nums)
        # checking that they don't share memory
        assert len(self.nums) < 2


if __name__ == '__main__':
    with Pool(maxtasksperchild=1) as pool:
        pool.map(A, range(99))  # the assert is being raised

运行此脚本

>>> python instance_num.py
nums: [0]
nums: [1]
nums: [2]
nums: [3]
nums: [4]
nums: [5]
nums: [6]
nums: [7]
nums: [8]
nums: [9]
nums: [10]
nums: [11]
nums: [12]
nums: [13]
nums: [14]
nums: [15]
nums: [16]
nums: [17]
nums: [18]
nums: [19]
nums: [20]
nums: [21]
nums: [22]
nums: [23]
nums: [24]
nums: [25]
nums: [26]
nums: [27]
nums: [28]
nums: [29]
nums: [30]
nums: [31]
nums: [32]
nums: [33]
nums: [34]
nums: [35]
nums: [36]
nums: [37]
nums: [38]
nums: [39]
nums: [40]
nums: [41]
nums: [42]
nums: [43]
nums: [44]
nums: [45]
nums: [46]
nums: [47]
nums: [48]
nums: [49]
nums: [50]
nums: [51]
nums: [52]
nums: [53]
nums: [54]
nums: [55]
nums: [56]
nums: [57]
nums: [58]
nums: [59]
nums: [60]
nums: [61]
nums: [62]
nums: [63]
nums: [64]
nums: [65]
nums: [66]
nums: [67]
nums: [68]
nums: [69]
nums: [70]
nums: [71]
nums: [72]
nums: [73]
nums: [74]
nums: [75]
nums: [76]
nums: [77]
nums: [78]
nums: [79]
nums: [80]
nums: [81]
nums: [82]
nums: [83]
nums: [84]
nums: [85]
nums: [86]
nums: [87]
nums: [88]
nums: [89]
nums: [90]
nums: [91]
nums: [92]
nums: [93]
nums: [94]
nums: [95]
nums: [96]
nums: [97]
nums: [98]

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