set-gmail-signature.py

#
# pipenv:
#
# [packages]
# google-api-python-client = "*"
# google-auth = "*"

from google.oauth2 import service_account
from googleapiclient.discovery import build

secret_file = 'service-account-credentials.json'
scopes = [
    "https://www.googleapis.com/auth/gmail.settings.basic",
]
 
client_email = "regular-user@domain.com"

# impersonate service accont to be a regular user
credentials = service_account.Credentials.from_service_account_file(
    secret_file,
    scopes=scopes,
    subject=client_email,
)
service = build('gmail', 'v1', credentials=credentials)

r = service.users().settings().sendAs().patch(
    userId='me',  # referres to the credentials user which is regular user
    sendAsEmail=client_email,
    body={"signature":"I can't believe it worked"},
).execute()
print(r)

#> {'sendAsEmail': 'regular-user@domain.com', 'displayName': '', 'replyToAddress': '', 'signature': 'I can't believe it worked', 'isPrimary': True, 'isDefault': True}