Implement application default credentials (#32)
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
new file mode 100644
index 0000000..9785c34
--- /dev/null
+++ b/google/auth/environment_vars.py
@@ -0,0 +1,32 @@
+# Copyright 2016 Google Inc.
+#
+# 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
+#
+# http://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.
+
+"""Environment variables used by :mod:`google.auth`."""
+
+
+PROJECT = 'GOOGLE_CLOUD_PROJECT'
+"""Environment variable defining default project.
+
+This used by :func:`google.auth.default` to explicitly set a project ID. This
+environment variable is also used by the Google Cloud Python Library.
+"""
+
+CREDENTIALS = 'GOOGLE_APPLICATION_CREDENTIALS'
+"""Environment variable defining the location of Google application default
+credentials."""
+
+# The environment variable name which can replace ~/.config if set.
+CLOUD_SDK_CONFIG_DIR = 'CLOUDSDK_CONFIG'
+"""Environment variable defines the location of Google Cloud SDK's config
+files."""