Credentials check before read

sheets/quickstart/quickstart.py

@@ -15,7 +15,8 @@
 # [START sheets_quickstart]
 from __future__ import print_function
 import pickle
-import os.path
+from sys import exit as sysexit
+from os.path import exists
 from googleapiclient.discovery import build
 from google_auth_oauthlib.flow import InstalledAppFlow
 from google.auth.transport.requests import Request
@@ -32,21 +33,40 @@ def main():
     Prints values from a sample spreadsheet.
     """
     creds = None
+    updated_creds = False
+
+    # Check to see if the program can obtain credentials for authorizing api,
+    # either from previous token.pickle or from credentials.json
+    if not (exists('token.pickle') or exists('credentials.json')):
+        print('\nPrerequisite - \'credentials.json\' - file not found. Before'
+        ' running again, please follow the Quickstart to'
+        ' download the \'credentials.json\' file here: \n\n'
+        'https://developers.google.com/sheets/api/quickstart/'
+        'python#step_1_turn_on_the\n')
+        sysexit()
+
     # The file token.pickle stores the user's access and refresh tokens, and is
     # created automatically when the authorization flow completes for the first
-    # time.
-    if os.path.exists('token.pickle'):
+    # time. First, try to read the token.pickle from prior login attempt.
+    elif exists('token.pickle'):
         with open('token.pickle', 'rb') as token:
             creds = pickle.load(token)
-    # If there are no (valid) credentials available, let the user log in.
-    if not creds or not creds.valid:
+
+    # If no token.pickle, try to create creds from the credentials.json file
+    else:
+        flow = InstalledAppFlow.from_client_secrets_file(
+        'credentials.json', SCOPES)
+        creds = flow.run_local_server()
+        updated_creds = True
+
+    # If the credentials are not valid, let the user log in.
+    if not creds.valid:
         if creds and creds.expired and creds.refresh_token:
             creds.refresh(Request())
-        else:
-            flow = InstalledAppFlow.from_client_secrets_file(
-                'credentials.json', SCOPES)
-            creds = flow.run_local_server()
-        # Save the credentials for the next run
+            updated_creds = True
+
+    # If updated the credentials, save for next run
+    if updated_creds:
         with open('token.pickle', 'wb') as token:
             pickle.dump(creds, token)
 
@@ -64,7 +84,7 @@ def main():
         print('Name, Major:')
         for row in values:
             # Print columns A and E, which correspond to indices 0 and 4.
-            print('%s, %s' % (row[0], row[4]))
+            print('\n%s, %s' % (row[0], row[4]))
 
 if __name__ == '__main__':
     main()