Python: вывод функции в файл

f.write(тут текст)
f.close()

for fn in ftp.dir(directory):
    f.write(fn + '\n')

import ftplib
f = ftplib.FTP()
f.connect("ftp.ee.debian.org")
f.login("anonymous", "anonymous")
f.cwd('/debian/dists/stable/main/source')
ls = []
f.retrlines('LIST', ls.append)
for entry in ls:
    print entry

$ python ftp.py 
-rw-r--r--    1 ftp      ftp            95 Apr 25 11:11 Release
-rw-rw-r--    1 ftp      ftp       9133176 Apr 26 20:16 Sources.gz
-rw-r--r--    1 ftp      ftp       7065016 Apr 26 20:17 Sources.xz

import ftplib
import os
import sys


class Redirect(object):
    def __init__(self, stream):
        self.__origin_stdout = sys.stdout
        self.__new_stdout = stream
    def __enter__(self):
        sys.stdout = self.__new_stdout
    def __exit__(self, *args):
        sys.stdout = self.__origin_stdout

ftp = ftplib.FTP("ftp.ee.debian.org")
ftp.login("anonymous", "anonymous")
with open(os.path.expanduser('~/list.txt'), 'w') as fd:
    with Redirect(fd):
        ftp.dir('/debian/dists/stable/main/source')
ftp.quit()
print 'OK'

from __future__ import print_function

...

print('-', line, file=f)