python运行多线程库pathos时，pymongo递归深度溢出

版本:pymongo 3.2.2;python 2.7;mongodb 3.0.12；pathos 0.2a1.dev0
在使用pathos的python多线程/进程库时，遇到了如下错误，注释掉“self.db_userinfo_table = MongoClient('localhost',27017).collection.example”这一行，就可以正常运行，如果不注释数据库初始化这一行，“result = ProcessPool(4).map(r.compute, range(100))”这句就会出错，但数据库还没使用呀，仅仅是初始化，难道pymongo本身有bug,还是pathos有bug？

from pathos.pools import ProcessPool, ThreadPool
import logging
from pymongo import MongoClient

class PMPExample(object):
    def __init__(self):
        self.cache = {}
        self.db_userinfo_table  = MongoClient('localhost',27017).collection.example
    def compute(self, x):
        self.cache[x] = x ** 3
        return self.cache[x]

if __name__ == '__main__':
    logging.basicConfig()
    log = logging.getLogger(__name__)
    log.setLevel(logging.INFO)
    r = PMPExample()
    #result = ThreadPool(4).map(r.compute, range(100))
    result = ProcessPool(4).map(r.compute, range(100))
    log.info("result processpooled caches: {}".format(result))
    

出现了好多行同样的错误提示：