python - 为什么 multiprocessing.Process 能够 pickle 修饰函数？

Question

所以我读了here不能 pickle 装饰函数。确实:

import multiprocessing as mp

def deco(f):
    def wrapper(*args, **kwargs):
        try:
            f(*args, **kwargs)
        except:
            print 'Exception caught!'
    return wrapper

@deco
def f(x):
    print x
    raise OverflowError 

if __name__ == '__main__':
    pool = mp.Pool(processes=1)
    for _ in pool.imap_unordered(f, range(10)):
        pass
    pool.close()
    pool.join()
    print 'All done'

输出:

Traceback (most recent call last):
  File "deco0.py", line 19, in <module>
    for _ in pool.imap_unordered(f, range(10)):
  File "/Users/usualme/anaconda/lib/python2.7/multiprocessing/pool.py", line 659, in next
    raise value
cPickle.PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

但是现在，如果我用 Process 替换 map:

import multiprocessing as mp

def deco(f):
    def wrapper(*args, **kwargs):
        try:
            f(*args, **kwargs)
        except:
            print 'Exception caught!'
    return wrapper

@deco
def f(x):
    print x
    raise OverflowError 

if __name__ == '__main__':
    p = mp.Process(target=f, args=(1,))
    p.start()
    p.join()
    print 'All done'

输出:

1
Exception caught!
All done

为什么有效？进程不需要 pickle 装饰函数吗？

Answer 1

之所以有效，是因为您在 Linux 上运行，不需要 pickle f 以通过 Process.__init__ 在子进程中调用它。这是因为 f 通过 os.fork 被 child 继承。如果您在 Windows 上运行相同的代码(缺少 fork)，或者尝试将 f 传递给 Pool.apply/Pool/map(两者都需要 pickle f 才能在子进程中调用它)，你会得到一个错误。

无论您使用什么平台，这个例子都会失败:

import multiprocessing as mp

def deco(f):
    def wrapper(*args, **kwargs):
        try:
            f(*args, **kwargs)
        except:
            print 'Exception caught!'
    return wrapper

@deco
def f(x):
    print x
    raise OverflowError 

if __name__ == '__main__':
    p = mp.Pool()
    p.apply(f, args=(1,))  # f needs to be pickled here.
    print 'All done'

输出:

1
Exception caught!
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 504, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 319, in _handle_tasks
    put(task)
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed