if / else语句无法在python中正确执行

Question

我正在尝试获取python脚本以列出AWS账户中的组，然后将这些组添加到用户：

group_list = client.list_groups()
    for group in group_list['Groups']:
        group_name = group['GroupName']
        all_groups.append(group_name)
    for group_name in all_groups:
        print(group_name)
    add_group_name = ''
    while add_group_name != 'quit':
        add_group_name = input("Enter the group name to add to user %s: " % user_name)
        if add_group_name is 'quit':
            break
        else:
            client.add_user_to_group(GroupName=add_group_name,UserName=user_name)

这是一个示例运行：

Enter the group name to add to user tdunphy: jf-admin-group
Enter the group name to add to user tdunphy: quit

脚本确实打印出了组，但是当您尝试退出循环时会出现此错误：

botocore.errorfactory.NoSuchEntityException: An error occurred (NoSuchEntity) when calling the AddUserToGroup operation: The group with name quit cannot be found.

这不符合if语句中的中断。如何正确执行此操作？