AttributeError: 'str' object has no attribute 'value' when set resource_name of manager_link

[jia Jimmy]

As google-ads-python(link_manager_to_client) suggesting codes example showed ,I tried with below code:

manager_link_operation = c_client.get_type(

        'CustomerManagerLinkOperation', version='v2')

 manager_link = manager_link_operation.update

manager_link.resource_name.value = (

        'customers/{}/customerManagerLinks/{}~{}'.format(

            client_id, manager_id, manager_link_id))

But I got error like :

Traceback (most recent call last):

  File "demo.py", line 210, in <module>

    foo4()

  File "demo.py", line 195, in foo4

    link_manager_to_client(m_client, c_client, "xxxx", "xxxx")

  File "demo.py", line 174, in link_manager_to_client

    client_id, manager_id, manager_link_id))

AttributeError: 'str' object has no attribute 'value'

I'm just following the example in   google-ads-python, why I got unexpected  error?