python multiprocessing pickle属性查询

Question

我正在尝试进行如下所示的多重处理：

from multiprocessing import Pool
def main():
   df1 = function1()
   df2 = function2(df1)
   df3 = list(df2.groupby('key')

   p = Pool(2)
   p.map(model, df3)

上面的方法很完美。但是，当我添加以下内容时：

from multiprocessing import Pool
def main():
   df1 = function1()
   df2 = function2(df1)
   df3 = list(df2.groupby('key')

   df4 = [x+ (df1 [x[0]][0],)+ (df1 [company[0]][1],) for x in df3 ]

   p = Pool(2)
   p.map(model, df4)

现在我收到此错误：

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

我不明白那多余的线怎么使它失败？没有外部函数调用。