subprocess的实例不能pickle

#!/usr/bin/env python

#encoding=utf-8

from subprocess import Popen, PIPE, STDOUT

cmd = '/home/mlzboy/my/ide/test/time_sleep.py'

p = Popen(cmd, shell=False, stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)

print type(p)

import pickle

pickle.dump(p,open("./p.pickle","wb"))

print "done!"