In our API reference, under the Workbook section, we demo how to use get() to acquire all the workbooks on the server. The code for that example looks like the one below. The last 2 lines are the matter.
import tableauserverclient as TSC
tableau_auth = TSC.TableauAuth('username', 'password', site_id='site')
server = TSC.Server('http://servername')
with server.auth.sign_in(tableau_auth):
all_workbook_items, pagination_item = server.workbooks.get()
print([workbook.name for workbook in all_workbooks])
There seems to be a typo if the code snippet above is to be a standalone example. The second to last assigns all the workbook items to the variable all_workbook_items, however, in the last line we reference to that assignment with the variable all_workbooks, which was not defined anywhere in the code snippet.