authenticate.py

from apiclient.discovery import build
from oauth2client.client import flow_from_clientsecrets
from oauth2client.file import Storage
from oauth2client.tools import run
import httplib2

def authenticate():

    # Path to the client_secret.json file downloaded from the Developer Console
    CLIENT_SECRET_FILE = 'emailer_key.json'

    # Check https://developers.google.com/gmail/api/auth/scopes for all available scopes
    OAUTH_SCOPE = 'https://www.googleapis.com/auth/gmail.compose'

    # Location of the credentials storage file
    STORAGE = Storage('gmail.storage')

    # Start the OAuth flow to retrieve credentials
    flow = flow_from_clientsecrets(CLIENT_SECRET_FILE, scope=OAUTH_SCOPE)
    http = httplib2.Http()

    # Try to retrieve credentials from storage or run the flow to generate them
    credentials = STORAGE.get()
    if credentials is None or credentials.invalid:
          credentials = run(flow, STORAGE, http=http)

    # Authorize the httplib2.Http object with our credentials
    http = credentials.authorize(http)

    # Build the Gmail service from discovery
    gmail_service = build('gmail', 'v1', http=http)
    return(gmail_service)