AttributeError:'NoneType'对象没有属性'open_session'"

运行以下功能时:

def conEnclosure():
    freebay = open("freebay", "w+")
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    password = getpass.getpass("Enter the password for infra: ")

    for host in open("full.json", "r"):
        print host
        ssh.connect("%s" % host, username="infra", password="%s" % password)
        stdin, stdout, stderr = ssh.exec_command("show server info all")

我收到以下错误:

    Traceback (most recent call last):
  File "./findbay_v2.py", line 53, in 
    conEnclosure()
  File "./findbay_v2.py", line 41, in conEnclosure
    ssh.exec_command("show server info all")
  File "build/bdist.macosx-10.9-intel/egg/paramiko/client.py", line 364, in exec_command
AttributeError: 'NoneType' object has no attribute 'open_session'

但是,在python shell中运行时,请执行以下命令:

>>> import paramiko
>>> ssh = paramiko.SSHClient()
>>> ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
>>> ssh.connect("host", username="xxx", password="xxx")
>>> stdin, stdout, stderr = ssh.exec_command("show server info all")

一切都按预期进行,只有当我在file.py中的上述函数中对其进行转换时才会发生错误.有谁知道它可能是什么？