BigQuery: Add sample for explicitly creating client from service acco… [(#2312)](GoogleCloudPlatform/python-docs-samples#2312)

* BigQuery: Add sample for explicitly creating client from service account credentials.

* Flake8

* Move imports to improve indentation.

* Combine auth and client functions.

* flake8

Author: tswast

samples/snippets/authenticate_service_account.py

@@ -0,0 +1,43 @@
+# Copyright 2019 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+
+def main():
+    key_path = os.environ.get("GOOGLE_APPLICATION_CREDENTIALS")
+
+    # [START bigquery_client_json_credentials]
+    from google.cloud import bigquery
+    from google.oauth2 import service_account
+
+    # TODO(developer): Set key_path to the path to the service account key
+    #                  file.
+    # key_path = "path/to/service_account.json"
+
+    credentials = service_account.Credentials.from_service_account_file(
+        key_path,
+        scopes=["https://www.googleapis.com/auth/cloud-platform"],
+    )
+
+    client = bigquery.Client(
+        credentials=credentials,
+        project=credentials.project_id,
+    )
+    # [END bigquery_client_json_credentials]
+    return client
+
+
+if __name__ == "__main__":
+    main()

samples/snippets/authenticate_service_account_test.py

@@ -0,0 +1,33 @@
+# Copyright 2019 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import google.auth
+
+import authenticate_service_account
+
+
+def mock_credentials(*args, **kwargs):
+    credentials, _ = google.auth.default(
+        ["https://www.googleapis.com/auth/cloud-platform"]
+    )
+    return credentials
+
+
+def test_main(monkeypatch):
+    monkeypatch.setattr(
+        'google.oauth2.service_account.Credentials.from_service_account_file',
+        mock_credentials,
+    )
+    client = authenticate_service_account.main()
+    assert client is not None