Python多处理池apply_async错误

Question

我正在尝试评估多处理池中的多个进程，但一直遇到错误，我无法弄清楚为什么......下面是代码的简化版本:

class Object_1():

    def add_godd_spd_column()

        def calculate_correlations(arg1, arg2, arg3):
            return {'a': 1}

        processes = {}
        pool = Pool(processes=6)
        for i in range(1, 10):
            processes[i] = pool.apply_async(calculate_correlations,
                                            args=(arg1, arg2, arg3,))

        correlations = {}
        for i in range(0, 10):
            correlations[i] = processes[i].get()

这会返回以下错误:

Traceback (most recent call last):
  File "./02_results.py", line 116, in <module>
    correlations[0] = processes[0].get()
  File "/opt/anaconda3/lib/python3.5/multiprocessing/pool.py", line 608, in get
    raise self._value
  File "/opt/anaconda3/lib/python3.5/multiprocessing/pool.py", line 385, in 
_handle_tasks
    put(task)
  File "/opt/anaconda3/lib/python3.5/multiprocessing/connection.py", line 206, in send
     self._send_bytes(ForkingPickler.dumps(obj))
   File "/opt/anaconda3/lib/python3.5/multiprocessing/reduction.py", line 50, in dumps
    cls(buf, protocol).dump(obj)
AttributeError: Can't pickle local object 'SCADA.add_good_spd_column.<locals>.calculate_correlations

当我调用以下内容时: 相关[0].成功()我收到以下错误:

Traceback (most recent call last):
  File "./02_results.py", line 116, in <module>
    print(processes[0].successful())
  File "/opt/anaconda3/lib/python3.5/multiprocessing/pool.py", line 595, in 
successful
    assert self.ready()
AssertionError

这是因为在调用 .get() 之前进程实际上并未完成吗？被评估的函数只返回一个字典，它绝对应该是 pickle-able...

干杯，

Answer 1

错误发生是因为 pickling a function nested in another function is not supported ，并且 multiprocessing.Pool 需要挑选您作为参数传递给 apply_async 的函数，以便在工作进程中执行它。您必须将函数移动到模块的顶层，或者使其成为类的实例方法。请记住，如果您将其设为实例方法，则类本身的实例也必须是可挑选的。

是的，调用 successful() 时发生断言错误是因为您在结果准备好之前调用它。 From the docs :

successful()

Return whether the call completed without raising an exception. Will raise AssertionError if the result is not ready.