GbqConnector should be able to fetch default credentials on Google Compute Engine

doc/source/io.rst

@@ -4475,6 +4475,15 @@ Additional information on service accounts can be found
 
 You will need to install an additional dependency: `oauth2client <https://github.com/google/oauth2client>`__.
 
+.. versionadded:: 0.19.0
+
+Authentication via ``application default credentials`` is also possible. This is only valid
+if the parameter ``private_key`` is not provided. This method also requires that
+the credentials can be fetched from the environment the code is running in.
+Otherwise, the OAuth2 client-side authentication is used.
+Additional information on ``application default credentials`` can be found
+`here <https://developers.google.com/identity/protocols/application-default-credentials>`__.
+
 .. note::
 
    The `'private_key'` parameter can be set to either the file path of the service account key

doc/source/whatsnew/v0.19.0.txt

@@ -361,6 +361,8 @@ Google BigQuery Enhancements
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
+- 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). See the :ref:`docs <io.bigquery_authentication>` for more details (: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

@@ -160,7 +160,57 @@ def get_credentials(self):
         if self.private_key:
             return self.get_service_account_credentials()
         else:
-            return self.get_user_account_credentials()
+            # Try to retrieve Application Default Credentials
+            credentials = self.get_application_default_credentials()
+            if not credentials:
+                credentials = self.get_user_account_credentials()
+            return credentials
+
+    def get_application_default_credentials(self):
+        """
+        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.
+        """
+        try:
+            from googleapiclient.discovery import build
+        except ImportError:
+            from apiclient.discovery import build
+        try:
+            from oauth2client.client import GoogleCredentials
+        except ImportError:
+            return None
+
+        try:
+            credentials = GoogleCredentials.get_application_default()
+        except:
+            return None
+
+        # Check if the application has rights to the BigQuery project
+        bigquery_service = build('bigquery', 'v2', credentials=credentials)
+        jobs = bigquery_service.jobs()
+        job_data = {'configuration': {'query': {'query': 'SELECT 1'}}}
+        try:
+            jobs.insert(projectId=self.project_id, body=job_data).execute()
+            return credentials
+        except:
+            return None
 
     def get_user_account_credentials(self):
         from oauth2client.client import OAuth2WebServerFlow
@@ -578,10 +628,16 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
     https://developers.google.com/api-client-library/python/apis/bigquery/v2
 
     Authentication to the Google BigQuery service is via OAuth 2.0.
-    By default user account credentials are used. You will be asked to
-    grant permissions for product name 'pandas GBQ'. It is also posible
-    to authenticate via service account credentials by using
-    private_key parameter.
+    - If "private_key" is not provided:
+        By default "application default credentials" are used.
+
+        .. versionadded:: 0.19.0
+
+        If default application credentials are not found or are restrictive,
+        user account credentials are used. In this case, you will be asked to
+        grant permissions for product name 'pandas GBQ'.
+    - If "private_key" is provided:
+        Service account credentials will be used to authenticate.
 
     Parameters
     ----------
@@ -689,10 +745,16 @@ def to_gbq(dataframe, destination_table, project_id, chunksize=10000,
     https://developers.google.com/api-client-library/python/apis/bigquery/v2
 
     Authentication to the Google BigQuery service is via OAuth 2.0.
-    By default user account credentials are used. You will be asked to
-    grant permissions for product name 'pandas GBQ'. It is also posible
-    to authenticate via service account credentials by using
-    private_key parameter.
+    - If "private_key" is not provided:
+        By default "application default credentials" are used.
+
+        .. versionadded:: 0.19.0
+
+        If default application credentials are not found or are restrictive,
+        user account credentials are used. In this case, you will be asked to
+        grant permissions for product name 'pandas GBQ'.
+    - If "private_key" is provided:
+        Service account credentials will be used to authenticate.
 
     Parameters
     ----------

pandas/io/tests/test_gbq.py

@@ -151,6 +151,27 @@ def test_requirements():
         raise nose.SkipTest(import_exception)
 
 
+def _check_if_can_get_correct_default_credentials():
+    # Checks if "Application Default Credentials" can be fetched
+    # from the environment the tests are running in.
+    # See Issue #13577
+    test_requirements()
+    try:
+        from googleapiclient.discovery import build
+    except ImportError:
+        from apiclient.discovery import build
+    try:
+        from oauth2client.client import GoogleCredentials
+        credentials = GoogleCredentials.get_application_default()
+        bigquery_service = build('bigquery', 'v2', credentials=credentials)
+        jobs = bigquery_service.jobs()
+        job_data = {'configuration': {'query': {'query': 'SELECT 1'}}}
+        jobs.insert(projectId=PROJECT_ID, body=job_data).execute()
+        return True
+    except:
+        return False
+
+
 def clean_gbq_environment(private_key=None):
     dataset = gbq._Dataset(PROJECT_ID, private_key=private_key)
 
@@ -217,6 +238,21 @@ def test_should_be_able_to_get_results_from_query(self):
         schema, pages = self.sut.run_query('SELECT 1')
         self.assertTrue(pages is not None)
 
+    def test_get_application_default_credentials_does_not_throw_error(self):
+        if _check_if_can_get_correct_default_credentials():
+            raise nose.SkipTest("Can get default_credentials "
+                                "from the environment!")
+        credentials = self.sut.get_application_default_credentials()
+        self.assertIsNone(credentials)
+
+    def test_get_application_default_credentials_returns_credentials(self):
+        if not _check_if_can_get_correct_default_credentials():
+            raise nose.SkipTest("Cannot get default_credentials "
+                                "from the environment!")
+        from oauth2client.client import GoogleCredentials
+        credentials = self.sut.get_application_default_credentials()
+        self.assertTrue(isinstance(credentials, GoogleCredentials))
+
 
 class TestGBQConnectorServiceAccountKeyPathIntegration(tm.TestCase):
     def setUp(self):