python - 为什么我收到多处理的递归错误？

Question

我希望使用多处理对大量地址进行地理编码。我有以下代码:

import multiprocessing
import geocoder

addresses = ['New York City, NY','Austin, TX', 'Los Angeles, CA', 'Boston, MA'] # and on and on

def geocode_worker(address):
    return geocoder.arcgis(address)

def main_process():
    pool = multiprocessing.Pool(processes=multiprocessing.cpu_count())
    return pool.map(geocode_worker, addresses)

if __name__ == '__main__':
    main_process()

但它给了我这个错误:

Traceback (most recent call last):
  File "/opt/anaconda3/lib/python3.7/threading.py", line 926, in _bootstrap_inner
    self.run()
  File "/opt/anaconda3/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/anaconda3/lib/python3.7/multiprocessing/pool.py", line 470, in _handle_results
    task = get()
  File "/opt/anaconda3/lib/python3.7/multiprocessing/connection.py", line 251, in recv
    return _ForkingPickler.loads(buf.getbuffer())
  File "/opt/anaconda3/lib/python3.7/site-packages/geocoder/base.py", line 599, in __getattr__
    if not self.ok:
  File "/opt/anaconda3/lib/python3.7/site-packages/geocoder/base.py", line 536, in ok
    return len(self) > 0
  File "/opt/anaconda3/lib/python3.7/site-packages/geocoder/base.py", line 422, in __len__
    return len(self._list)

错误的最后3行一遍又一遍的重复，然后traceback的最后一行是:

RecursionError: maximum recursion depth exceeded while calling a Python object

谁能帮我弄清楚为什么？

Answer 1

问题是 geocoder 返回的 ArcgisQuery 对象不可 picklable - 或者更确切地说，它不可 unpicklable。 unpickle 进程由于使用 __getattr__ 而进入无限循环，它在内部尝试访问 self.ok，最终依赖于 self._list 待定义，unpickling 时未定义，因为它仅在 __init__ 中定义，并且 __init__ is not called while unpickling .由于它未定义，它会尝试使用 __getattr__ 找到它，这会尝试再次访问 self.ok，并创建无限循环。

您可以通过不在工作进程和主进程之间传递 ArcgisQuery 对象本身来解决此问题，而是仅传递其底层 __dict__。然后，在主进程中重建 ArcgisQuery 对象:

import multiprocessing
import geocoder
from geocoder.arcgis import ArcgisQuery

addresses = ['New York City, NY','Austin, TX', 'Los Angeles, CA', 'Boston, MA'] # and on and on

def geocode_worker(address):
    out = geocoder.arcgis(address)
    return out.__dict__ # Only return the object's __dict__

def main_process():
    pool = multiprocessing.Pool(processes=multiprocessing.cpu_count())
    l = pool.map(geocode_worker, addresses)
    out = []
    for d in l:
        q = ArcgisQuery(d['location'])  # location is a required constructor arg
        q.__dict__.update(d)  # Load the rest of our state into the new object
        out.append(q)
    return out

if __name__ == '__main__':
    print(main_process())

如果您实际上不需要整个 ArcgisQuery 对象，而只需要它的某些部分，您也可以只从工作进程中返回这些对象，以避免需要这种 hack。

就其值(value)而言，看起来 geocoder 可以通过在 ArcgisQuery 或其基类上实现 __getstate__ 和 __setstate__ 来解决其酸洗问题，像这样:

    def __getstate__(self):
        return self.__dict__

    def __setstate__(self, state):
        self.__dict__.update(state)