Estoy intentando descargar un directorio ftp completo en paralelo.

#!/usr/bin/python
import sys
import datetime
import os
from multiprocessing import Process, Pool
from ftplib import FTP
curYear=""
remotePath =""
localPath = ""

def downloadFiles (remotePath,localPath):
        splitted = remotePath.split('/');
        host= splitted[2]
        path='/'+'/'.join(splitted[3:])
        ftp = FTP(host)
        ftp.login()
        ftp.cwd(path)
        filenames =  ftp.nlst()
        total=len(filenames)
        i=0
        pool = Pool()
        for filename in filenames:
                        local_filename = os.path.join(localPath,filename)
                        pool.apply_async(downloadFile, (filename,local_filename,ftp))
                        #downloadFile(filename,local_filename,ftp);
                        i=i+1

        pool.close()
        pool.join()
        ftp.close()

def downloadFile(filename,local_filename,ftp):
        file = open(local_filename, 'wb')
        ftp.retrbinary('RETR '+ filename, file.write)
        file.close()

def getYearFromArgs():
        if len(sys.argv) >= 2 and sys.argv[1] == "Y":
                year = sys.argv[2]
                del sys.argv[1:2]
        else:
                year = str(datetime.datetime.now().year)
        return year

def assignGlobals():
        global p
        global remotePath
        global localPath
        global URL
        global host
        global user
        global password
        global sqldb
        remotePath = 'ftp://ftp3.ncdc.noaa.gov/pub/data/noaa/isd-lite/%s/' % (curYear)
        localPath = '/home/isd-lite/%s/' % (curYear)

def main():
        global curYear
        curYear=getYearFromArgs()
        assignGlobals()
        downloadFiles(remotePath,localPath)

if __name__ == "__main__":
        main()

Pero me sale esta excepción:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.6/multiprocessing/pool.py", line 225, in _handle_tasks
    put(task)
TypeError: expected string or Unicode object, NoneType found

Si comento fuera de esta línea:

pool.apply_async(downloadFile, (filename,local_filename,ftp)

y quitar el comentario en esta línea:

downloadFile(filename,local_filename,ftp);

Entonces funciona bien pero es lento y no multiproceso.