python multiprocessing pickle属性查询

Question

i am trying to do multiprocessing as shown:

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

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

The above works perfectly. However, when I add in this:

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)

Now i get this error:

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

I dont understand how that extra line makes it fail? There is no external function call.