当字典（可变数据类型）是通过多处理池实现的函数的参数时，为什么 python 使用 'CallByValue' 函数调用？

Question

注意：这是针对更大问题的人为示例

from multiprocessing import Pool

dict1 = {'key1':1}

def alterDict(dict_num):
    for key in dict_num:
        dict_num[key] = 20000

alterDict(dict1)
print(dict1) # output is {'key1': 20000}



dict1 = {'key1':1}


with Pool(2) as p:
    p.map(alterDict,[dict1])

print(dict1) # output is {'key1': 1}

为什么输出不同？有没有办法避免 Pool 使用 'call by value' 风格的函数调用？我想让池使用函数调用的引用调用方式

Answer 1

当您使用 multiprocessing 并且您想要更改 dict、list 等对象时。（共享数据）您需要使用 Sharing state between process。

import multiprocessing as mp

def alterDict(dict_num): 
    for key, _ in dict_num.items(): 
        dict_num[key] = 20000 

with mp.Manager() as manager: 
    d = manager.dict() 
    d['key'] = 1 
    with manager.Pool() as pool: 
        pool.map(alterDict, [d]) 
    print(dict(d)) 

# {'key': 20000} # output

顺便说一句，你应该使用 dict_num.items() 和 items 否则你会得到错误：

/usr/local/lib/python3.8/multiprocessing/managers.py in _callmethod(self, methodname, args, kwds)
    848             dispatch(conn, None, 'decref', (token.id,))
    849             return proxy
--> 850         raise convert_to_error(kind, result)
    851 
    852     def _getvalue(self):

AttributeError: 'NoneType' object has no attribute '_registry'

当字典（可变数据类型）是通过多处理池实现的函数的参数时，为什么 python 使用 'CallByValue' 函数调用？

Why python uses 'CallByValue' function call when a dict (mutable dtype) is an argument to a function that is implemented via multiprocessing Pool?

python

python-3.x

python-multiprocessing

pool