我可以在类的方法中使用multiprocessing.Pool吗？

Question

我打算multiprocessing在我的代码中使用以获得更好的性能。

但是，出现以下错误：

Traceback (most recent call last):
  File "D:\EpubBuilder\TinyEpub.py", line 49, in <module>
    e.epub2txt()
  File "D:\EpubBuilder\TinyEpub.py", line 43, in epub2txt
    tempread = self.get_text()
  File "D:\EpubBuilder\TinyEpub.py", line 29, in get_text
    txtlist = pool.map(self.char2text,charlist)
  File "C:\Python34\lib\multiprocessing\pool.py", line 260, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
  File "C:\Python34\lib\multiprocessing\pool.py", line 599, in get
    raise self._value
  File "C:\Python34\lib\multiprocessing\pool.py", line 383, in _handle_tasks
    put(task)
  File "C:\Python34\lib\multiprocessing\connection.py", line 206, in send
    self._send_bytes(ForkingPickler.dumps(obj))
  File "C:\Python34\lib\multiprocessing\reduction.py", line 50, in dumps
    cls(buf, protocol).dump(obj)
TypeError: cannot serialize '_io.BufferedReader' object

我尝试了另一种方式，并收到此错误：

TypeError: cannot serialize '_io.TextIOWrapper' object

我的代码如下所示：

from multiprocessing import Pool
class Book(object):
    def __init__(self, arg):
        self.namelist = arg
    def format_char(self,char):
        char = char + "a"
        return char
    def format_book(self):
        self.tempread = ""
        charlist = [f.read() for f in self.namelist] #list of char
        with Pool() as pool:
            txtlist = pool.map(self.format_char,charlist)
        self.tempread = "".join(txtlist)
        return self.tempread

if __name__ == '__main__':
    import os
    b = Book([open(f) for f in os.listdir()])
    t = b.format_book()
    print(t)

我认为由于未Pool在main函数中使用而引起了错误。

我的猜想对吗？以及如何修改代码以修复错误？

Answer 1

问题是您在实例中有一个无法拾取的实例变量（namelist）Book。因为您正在调用pool.map实例方法，并且您正在Windows上运行，所以整个实例都必须是可腌制的，才能将其传递给子进程。Book.namelist是一个打开的文件对象（_io.BufferedReader），无法对其进行腌制。您可以通过两种方法解决此问题。根据html" target="_blank">示例代码，您似乎可以制作format_char一个顶级函数：

def format_char(char):
    char = char + "a"
    return char


class Book(object):
    def __init__(self, arg):
        self.namelist = arg

    def format_book(self):
        self.tempread = ""
        charlist = [f.read() for f in self.namelist] #list of char
        with Pool() as pool:
            txtlist = pool.map(format_char,charlist)
        self.tempread = "".join(txtlist)
        return self.tempread

但是，实际上，如果您需要format_char成为实例方法，则可以在腌制之前从实例中删除参数，从而使用__getstate__/__setstate__使其可腌制：Book``namelist

class Book(object):
    def __init__(self, arg):
        self.namelist = arg

    def __getstate__(self):
        """ This is called before pickling. """
        state = self.__dict__.copy()
        del state['namelist']
        return state

    def __setstate__(self, state):
        """ This is called while unpickling. """
        self.__dict__.update(state)

    def format_char(self,char):
        char = char + "a"

    def format_book(self):
        self.tempread = ""
        charlist = [f.read() for f in self.namelist] #list of char
        with Pool() as pool:
            txtlist = pool.map(self.format_char,charlist)
        self.tempread = "".join(txtlist)
        return self.tempread

只要您不需要namelist在子进程中访问，就可以。