added more documentation

Author: mhaseebtariq

doc/source/whatsnew/v0.19.0.txt

@@ -361,7 +361,7 @@ Google BigQuery Enhancements
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
-- The ``.get_credentials()`` method of ``GbqConnector`` can now first try to fetch the default credentials for Google Compute Engine without the need to run ``OAuth2WebServerFlow`` - if private_key is not provided (:issue:`13577`)
+- The ``.get_credentials()`` method of ``GbqConnector`` can now first try to fetch the application default credentials (https://developers.google.com/identity/protocols/application-default-credentials). This is only valid if the parameter ``private_key`` is not provided. Using this functionality, the ``OAuth2WebServerFlow`` call can be skipped (:issue:`13577`).
 
 - The ``.tz_localize()`` method of ``DatetimeIndex`` and ``Timestamp`` has gained the ``errors`` keyword, so you can potentially coerce nonexistent timestamps to ``NaT``. The default behaviour remains to raising a ``NonExistentTimeError`` (:issue:`13057`)
 - ``pd.to_numeric()`` now accepts a ``downcast`` parameter, which will downcast the data if possible to smallest specified numerical dtype (:issue:`13352`)

pandas/io/gbq.py

@@ -52,7 +52,10 @@ def _test_google_api_imports():
         except:
             from apiclient.discovery import build  # noqa
             from apiclient.errors import HttpError  # noqa
+        from oauth2client.client import GoogleCredentials  # noqa
         from oauth2client.client import AccessTokenRefreshError  # noqa
+        from oauth2client.client import HttpAccessTokenRefreshError  # noqa
+        from oauth2client.client import ApplicationDefaultCredentialsError  # noqa
         from oauth2client.client import OAuth2WebServerFlow  # noqa
         from oauth2client.file import Storage  # noqa
         from oauth2client.tools import run_flow, argparser  # noqa
@@ -168,25 +171,37 @@ def get_credentials(self):
 
     def get_application_default_credentials(self):
         """
-        This method tries to retrieve the "default application credentials"
-        Could be useful for running code on Google Cloud Platform
+        This method tries to retrieve the "default application credentials".
+        This could be useful for running code on Google Cloud Platform.
+
+        .. versionadded:: 0.19.0
+
+        Parameters
+        ----------
+        None
+
+        Returns
+        -------
+        - GoogleCredentials,
+            If the default application credentials can be retrieved
+            from the environment. The retrieved credentials should also
+            have access to the project (self.project_id) on BigQuery.
+        - OR None,
+            If default application credentials can not be retrieved
+            from the environment. Or, the retrieved credentials do not
+            have access to the project (self.project_id) on BigQuery.
         """
-        oauth2client_library_imported = False
         try:
-            from oauth2client.client import GoogleCredentials
-            from oauth2client.client import AccessTokenRefreshError
-            from oauth2client.client import HttpAccessTokenRefreshError
-            from oauth2client.client import ApplicationDefaultCredentialsError
-            oauth2client_library_imported = True
             from googleapiclient.discovery import build
             from googleapiclient.errors import HttpError
         except ImportError:
-            if not oauth2client_library_imported:
-                return None
             from apiclient.discovery import build
             from apiclient.errors import HttpError
+        from oauth2client.client import GoogleCredentials
+        from oauth2client.client import AccessTokenRefreshError
+        from oauth2client.client import HttpAccessTokenRefreshError
+        from oauth2client.client import ApplicationDefaultCredentialsError
 
-        credentials = None
         try:
             credentials = GoogleCredentials.get_application_default()
         except ApplicationDefaultCredentialsError:
@@ -198,10 +213,10 @@ def get_application_default_credentials(self):
         job_data = {'configuration': {'query': {'query': 'SELECT 1'}}}
         try:
             jobs.insert(projectId=self.project_id, body=job_data).execute()
+            return credentials
         except (HttpAccessTokenRefreshError, AccessTokenRefreshError,
                 HttpError, TypeError):
             return None
-        return credentials
 
     def get_user_account_credentials(self):
         from oauth2client.client import OAuth2WebServerFlow