python通过定义一个类实例作为ftp回调方法

class Writer:
  def __init__(self, file):
    self.f = open(file, "w")
  def __call__(self, data):
    self.f.write(data)
    self.f.write('\n')
    print data
FILENAME = "AutoIndent.py"
writer = Writer(FILENAME)
import ftplib
ftp = ftplib.FTP('127.0.0.1', 'book', 'bookpw')
ftp.retrlines("RETR %s" % FILENAME, writer)