modified documentation + restructured tests

Author: mhaseebtariq

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

@@ -361,7 +361,7 @@ 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). This is only valid if the parameter ``private_key`` is not provided. Using this functionality, the ``OAuth2WebServerFlow`` call can be skipped (: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/tests/test_gbq.py

@@ -153,6 +153,9 @@ def test_requirements():
 
 
 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()
     from oauth2client.client import GoogleCredentials
     try:
@@ -170,18 +173,6 @@ def _check_if_can_get_correct_default_credentials():
         return False
 
 
-def _skip_if_cant_get_correct_default_credentials():
-    if not _check_if_can_get_correct_default_credentials():
-        raise nose.SkipTest("Cannot get default_credentials "
-                            "from the environment!")
-
-
-def _skip_if_can_get_correct_default_credentials():
-    if _check_if_can_get_correct_default_credentials():
-        raise nose.SkipTest("Can get default_credentials "
-                            "from the environment!")
-
-
 def clean_gbq_environment(private_key=None):
     dataset = gbq._Dataset(PROJECT_ID, private_key=private_key)
 
@@ -249,12 +240,16 @@ def test_should_be_able_to_get_results_from_query(self):
         self.assertTrue(pages is not None)
 
     def test_get_application_default_credentials_does_not_throw_error(self):
-        _skip_if_can_get_correct_default_credentials()
+        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):
-        _skip_if_cant_get_correct_default_credentials()
+        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))