使用多处理时出现 PicklingError

问题描述

在多处理模块中使用 Pool.map_async()(以及 Pool.map())时遇到问题.我已经实现了一个并行循环函数，只要 Pool.map_async 的函数输入是常规"函数，它就可以正常工作.功能.当功能是例如一个类的方法，然后我得到一个 PicklingError:

I am having trouble when using the Pool.map_async() (and also Pool.map()) in the multiprocessing module. I have implemented a parallel-for-loop function that works fine as long as the function input to Pool.map_async is a "regular" function. When the function is e.g. a method to a class, then I get a PicklingError:

cPickle.PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed

我只使用 Python 进行科学计算，所以我对酸洗的概念不是很熟悉，今天刚刚了解了一点.我看过几个以前的答案，比如 Can't pickle <type 'instancemethod'>使用多处理 Pool.map() 时，但我无法弄清楚如何使其工作，即使按照答案中提供的链接进行操作也是如此.

I use Python only for scientific computing so I am not so familiar with the concept of pickling, have just learned a bit about it today. I have looked at a couple of previous answers, like Can't pickle <type 'instancemethod'> when using multiprocessing Pool.map(), but I cannot figure out how to make it work, even when following the link provided in the answer.

我的代码，其目标是使用多核模拟 Normal r.v 的向量.请注意，这只是一个示例，甚至可能无法在多核上运行.

My code, where the objective is to simulate a vector of Normal r.v's with the use of multiple cores. Note that this is just an example and maybe it does not even payoff to run on multiple cores.

import multiprocessing as mp
import scipy as sp
import scipy.stats as spstat

def parfor(func, args, static_arg = None, nWorkers = 8, chunksize = None):
    """
    Purpose: Evaluate function using Multiple cores.

    Input:
        func       - Function to evaluate in parallel
        arg        - Array of arguments to evaluate func(arg)  
        static_arg - The "static" argument (if any), i.e. the variables that are      constant in the evaluation of func.
        nWorkers   - Number of Workers to process computations.
    Output:
        func(i, static_arg) for i in args.
    
    """
    # Prepare arguments for func: Collect arguments with static argument (if any)
    if static_arg != None:
        arguments = [[arg] + static_arg for arg in list(args)]
    else:
        arguments = args
    
    # Initialize workers
    pool = mp.Pool(processes = nWorkers) 

    # Evaluate function
    result = pool.map_async(func, arguments, chunksize = chunksize)
    pool.close()
    pool.join()

    return sp.array(result.get()).flatten() 

# First test-function. Freeze location and scale for the Normal random variates generator.
# This returns a function that is a method of the class Norm_gen. Methods cannot be pickled
# so this will give an error.
def genNorm(loc, scale):
    def subfunc(a):
        return spstat.norm.rvs(loc = loc, scale = scale, size = a)
    return subfunc

# Second test-function. The same as above but does not return a method of a class. This is a "plain" function and can be 
# pickled
def test(fargs):
    x, a, b = fargs
    return spstat.norm.rvs(size = x, loc = a, scale = b)

# Try it out.
N = 1000000

# Set arguments to function. args1 = [1, 1, 1,... ,1], the purpose is just to generate a random variable of size 1 for each 
# element in the output vector.
args1 = sp.ones(N)
static_arg = [0, 1] # standarized normal.

# This gives the PicklingError
func = genNorm(*static_arg)
sim = parfor(func, args1, static_arg = None, nWorkers = 12, chunksize = None)

# This is OK:
func = test
sim = parfor(func, args1, static_arg = static_arg, nWorkers = 12, chunksize = None)

按照 不能腌制<输入'instancemethod'>在使用多处理 Pool.map() 时，Steven Bethard(几乎在最后)建议使用 copy_reg 模块.他的代码是:

Following the link provided in the answer to the question in Can't pickle <type 'instancemethod'> when using multiprocessing Pool.map(), Steven Bethard (almost at the end) suggests using the copy_reg module. His code is:

def _pickle_method(method):
    func_name = method.im_func.__name__
    obj = method.im_self
    cls = method.im_class
    return _unpickle_method, (func_name, obj, cls)

def _unpickle_method(func_name, obj, cls):
    for cls in cls.mro():
        try:
            func = cls.__dict__[func_name]
        except KeyError:
            pass
        else:
            break
    return func.__get__(obj, cls)

import copy_reg
import types

copy_reg.pickle(types.MethodType, _pickle_method, _unpickle_method)

我真的不明白如何使用它.我唯一能想到的就是把它放在我的代码之前，但它没有帮助.一个简单的解决方案当然是只使用可行的解决方案，避免涉及 copy_reg.我更感兴趣的是让 copy_reg 正常工作以充分利用多处理，而不必每次都解决问题.

I don't really understand how I can make use of this. The only thing I could come up with was putting it just before my code but it did not help. A simple solution is of course to just go with the one that works and avoid getting involved with copy_reg. I am more interested in getting copy_reg to work properly to take fully advantage of multiprocessing without having to go around the problem each time.

推荐答案

这里的问题与其说是pickle"错误信息，不如说是概念性的:多进程确实在工人"不同的进程中分叉你的代码以执行它的魔力.

The problem here is less of the "pickle" error message than conceptual: multiprocess does fork your code in "worker" different processes in order to perform its magic.

然后，它通过无缝序列化和反序列化数据(即使用 pickle 的部分)将数据发送到不同的进程或从不同的进程发送数据.

It then sends data to and from the different process by seamlessly serializing and de-serializing the data (that is the part that uses the pickle).

当来回传递的部分数据是一个函数时 - 它假设被调用进程中存在同名的函数，并且(我猜)将函数名作为字符串传递.由于函数是无状态的，被调用的工作进程只是用它收到的数据调用同一个函数.(Python函数不能通过pickle进行序列化，所以只在主进程和工作进程之间传递引用)

When part of the data passed back and forth is a function - it assumes a function with the same name exists in the callee process, and (I guess) passes the function name, as a string. Since functions are stateless, the called worker-process just calls that same function with the data it has received. (Python functions can't be serialized through pickle, so just the reference is passed between the master and the worker processes)

当你的函数是实例中的一个方法时——尽管当我们编写 python 代码时，它与函数很像，带有一个自动"self 变量，但在下面就不一样了.因为实例(对象)是有状态的.这意味着工作进程没有对象的副本，该对象是您要在另一端调用的方法的所有者.

When your function is a method in an instance - although when we code python it is much like the same thing as a function, with an "automatic" self variable, it is not the same underneath. Because instances (objects) are stateful. That means the worker process does not have a copy of the object that is the owner of the method you want to call on the other side.

解决将方法作为函数传递给 map_async 调用的方法也不起作用 - 因为多进程只使用函数引用，而不是传递它时的实际函数.

Working around ways of passing your method as a function to the map_async call won't work either - as multiprocess just uses a function reference, not the actual function when passing it around.

因此，您应该 (1) 更改您的代码，以便将函数(而不是方法)传递给工作进程，将对象保持的任何状态转换为要调用的新参数.(2) 为 map_async 调用创建一个目标"函数，该函数在工作进程端重建所需的对象，然后调用其中的函数.Python 中最直接的类本身是可挑选的，因此您可以在 map_async 调用中传递作为函数所有者本身的对象 - 目标"函数将在工作端调用适当的方法本身.

So, you should (1) either change your code so that you do pass a function - and not a method - to the worker processes, converting whatever states the object keeps to new parameters to be called. (2) Create a "target" function for the map_async call that reconstructs the needed object on the worker-process side, and then calls the function inside it. Most straightforward classes in Python are pickable themselves, so you could pass the object that is the function owner itself on the map_async call - and the "target" function would call the appropriate method itself on the worker side.

(2) 可能听起来困难"，但可能就是这样——除非你的对象的类不能被腌制:

(2) may sound "difficult" but it is probably just something like this - unless your object's class can't be pickled:

import types

def target(object, *args, **kw):
    method_name = args[0]
    return getattr(object, method_name)(*args[1:])
(...)    
#And add these 3 lines prior to your map_async call:


    # Evaluate function
    if isinstance (func, types.MethodType):
        arguments.insert(0, func.__name__)
        func = target
    result = pool.map_async(func, arguments, chunksize = chunksize)

*免责声明:我没有测试过这个

*disclaimer: I haven't tested this

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