问题
之前在调用class内的函数用multiprocessing模块的pool函数进行多线程处理的时候报了以下下错误信息:
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed
查了下官方文档发现python默认只能pickle以下的类型:
- None, True, and False
- integers, floating point numbers, complex numbers
- strings, bytes, bytearrays
- tuples, lists, sets, and dictionaries containing only picklable objects
- functions defined at the top level of a module (using def, not lambda)
- built-in functions defined at the top level of a module
- classes that are defined at the top level of a module
- instances of such classes whose dict or the result of calling getstate() is picklable (see section -
- Pickling Class Instances for details).
函数只能pickle在顶层定义的函数，很明显的class内的函数无法被pickle因此会报错。
import multiprocessing
def work(): # top-level 函数
print "work!"
class Foo():
def work(self): # 非top-level函数
print "work"
pool1 = multiprocessing.Pool(processes=4)
foo = Foo()
pool1.apply_async(foo.work)
pool1.close()
pool1.join()
# 此时报错
pool2 = multiprocessing.Pool(processes=4)
pool2.apply_async(work)
pool2.close()
pool2.join()
# 此时工作正常
解决方案
调用pathos包下的multiprocessing模块代替原生的multiprocessing。pathos中multiprocessing是用dill包改写过的，dill包可以将几乎所有python的类型都serialize，因此都可以被pickle。或者也可以自己用dill写一个（有点重复造轮子之嫌啊）