python - Starmap 在传入参数之前修改参数？

Question

我在尝试使用 multiprocessing.Pool.starmap 时遇到了一个奇怪的错误。重现错误所需的最少代码在这里:

from multiprocessing import Pool

# Ignore the fact that this class is useless as-is, it has more code but it wasn't relevant to the bug
class Coordinate(tuple) :                                                                          

    def __new__(cls, *args):                                                                   
        return tuple.__new__(cls, args)                                                        

#Essentially just stores two coordinates
class Move :                                                     

    def __init__(self, oldPos, newPos) :      
        self.oldPos = oldPos                  
        self.newPos = newPos                  

    def __str__(self) :      
        return 'Old pos : ' + str(self.oldPos) + ' -- New pos : ' + str(self.newPos)

#Dummy function to show the problem
def funcThatNeedsTwoParams(move, otherParam) :
    print(move)             
    # Second param ignored, no problem there

p = Pool(2)  
moveOne = Move(Coordinate(0, 2), Coordinate(0, 1))
moveTwo = Move(Coordinate(2, 1), Coordinate(3, 0))
moveThree = Move(Coordinate(22345, -12400), Coordinate(153, 2357))
# The numbers are irrelevant, no effect on whether problem shows up or not

moves = [moveOne, moveTwo, moveThree]
paramsForStarmap = [[move, 'other param'] for move in moves]

print(paramsForStarmap)
#Output : 
#[[<__main__.Move object at 0x1023d4438>, 'other param'], [<__main__.Move object at 0x1023d4470>, 'other param'], [<__main__.Move object at 0x1023d44a8>
for move in [params[0] for params in paramsForStarmap] :
    print(move)
#Output : 
#Old pos : (0, 2) -- New pos : (0, 1)
#Old pos : (2, 1) -- New pos : (3, 0)
#Old pos : (22345, -12400) -- New pos : (153, 2357)
p.starmap(funcThatNeedsTwoParams, paramsForStarmap)
#Output :
#Old pos : ((0, 2),) -- New pos : ((0, 1),)
#Old pos : ((22345, -12400),) -- New pos : ((153, 2357),)
#Old pos : ((2, 1),) -- New pos : ((3, 0),)

基本上，我有一个参数对数组，像这样:[[move, otherParam], [move, otherParam], ...]，我打印出每个第一个参数以表明移动之前是有效的使用星图功能。然后我使用之前创建的池调用 starmap 函数，并告诉它使用我拥有的参数对。然后，莫名其妙地，每一步的坐标都变成了 ((coordinate), ) 形式的元组，而不是 (coordinate)。

我似乎无法弄清楚为什么 starmap 会更改传递给它的对象的属性，非常感谢任何帮助，谢谢。

Answer 1

这很有趣。问题不仅仅在于 starmap。所有 Pool 函数都会发生这种情况 - apply、map 等。事实证明，问题不在于 多重处理。当您 pickle/unpickle Coordinate 类时会发生这种情况:

>>> c = Coordinate(0,2)
>>> print(c)
(0, 2)
>>> str(pickle.loads(pickle.dumps(c)))
'((0, 2),)'

Pickling tuple 子类并不像看起来那么简单，事实证明。您可以通过定义 __reduce__ 来修复它修复酸洗过程的方法:

class Coordinate(tuple):
    def __new__(cls, *args):
        return tuple.__new__(cls, args)

    def __reduce__(self):
        return (self.__class__, tuple(self))

现在泡菜就好了:

>>> c = Coordinate(0,2)
>>> pickle.loads(pickle.dumps(c))
(0, 2)

而且您的示例代码也能正常工作。