When running the following minimal code to execute the GMail API watch() method. A 'Backend Error' occurs. This is true both when running on a client machine, and when run directly on Google Cloud as a Cloud Function.
from google.oauth2 import service_account
import googleapiclient.discovery
SCOPES = ["https://mail.google.com/"]
SERVICE_ACCOUNT_FILE = '<JSON KEY FILE REMOVED>'
TARGET='<GSUITE DOMAIN USER EMAIL ADDRESS>'
credentials = service_account.Credentials.from_service_account_file(SERVICE_ACCOUNT_FILE, scopes=SCOPES)
credentials_delegated = credentials.with_subject(TARGET)
service = googleapiclient.discovery.build('gmail', 'v1', credentials=credentials_delegated)
val = { 'topicName': '<TOPIC NAME COPIED FROM GCLOUD PUB/SUB>' }
watch_resp = service.users().watch(userId='me', body=val).execute()
print(watch_resp)
Other GMail APIs such as service.users().getProfile(userId='me').execute() work as expected. This error occurs after ensuring that the service account and the delegated user have the pub/sub publisher role and that the topic name is correct and already created. The exact output of the error is as follows:
Traceback (most recent call last):
File "test.py", line 12, in <module>
watch_resp = service.users().watch(userId='me', body=val).execute()
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/googleapiclient/_helpers.py", line 130, in positional_wrapper
return wrapped(*args, **kwargs)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/googleapiclient/http.py", line 842, in execute
raise HttpError(resp, content, uri=self.uri)
googleapiclient.errors.HttpError: <HttpError 500 when requesting https://www.googleapis.com/gmail/v1/users/me/watch?alt=json returned "Backend Error">