Implement application default credentials (#32)

diff --git a/tests/test__cloud_sdk.py b/tests/test__cloud_sdk.py
new file mode 100644
index 0000000..35ee426
--- /dev/null
+++ b/tests/test__cloud_sdk.py
@@ -0,0 +1,146 @@
+# 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.
+
+import json
+import os
+
+import mock
+import pytest
+
+from google.auth import _cloud_sdk
+from google.auth import environment_vars
+import google.oauth2.credentials
+
+
+DATA_DIR = os.path.join(os.path.dirname(__file__), 'data')
+AUTHORIZED_USER_FILE = os.path.join(DATA_DIR, 'authorized_user.json')
+
+with open(AUTHORIZED_USER_FILE) as fh:
+    AUTHORIZED_USER_FILE_DATA = json.load(fh)
+
+SERVICE_ACCOUNT_FILE = os.path.join(DATA_DIR, 'service_account.json')
+
+with open(SERVICE_ACCOUNT_FILE) as fh:
+    SERVICE_ACCOUNT_FILE_DATA = json.load(fh)
+
+with open(os.path.join(DATA_DIR, 'cloud_sdk.cfg')) as fh:
+    CLOUD_SDK_CONFIG_DATA = fh.read()
+
+CONFIG_PATH_PATCH = mock.patch('google.auth._cloud_sdk.get_config_path')
+
+
+@pytest.fixture
+def config_file(tmpdir):
+    config_dir = tmpdir.join(
+        '.config', _cloud_sdk._CONFIG_DIRECTORY)
+    config_file = config_dir.join(
+        _cloud_sdk._ACTIVE_CONFIG_FILENAME)
+
+    with CONFIG_PATH_PATCH as mock_get_config_dir:
+        mock_get_config_dir.return_value = str(config_dir)
+        yield config_file
+
+
+def test_get_project_id(config_file):
+    config_file.write(CLOUD_SDK_CONFIG_DATA, ensure=True)
+    project_id = _cloud_sdk.get_project_id()
+    assert project_id == 'example-project'
+
+
+def test_get_project_id_non_existent(config_file):
+    project_id = _cloud_sdk.get_project_id()
+    assert project_id is None
+
+
+def test_get_project_id_bad_file(config_file):
+    config_file.write('<<<badconfig', ensure=True)
+    project_id = _cloud_sdk.get_project_id()
+    assert project_id is None
+
+
+def test_get_project_id_no_section(config_file):
+    config_file.write('[section]', ensure=True)
+    project_id = _cloud_sdk.get_project_id()
+    assert project_id is None
+
+
+@CONFIG_PATH_PATCH
+def test_get_application_default_credentials_path(mock_get_config_dir):
+    config_path = 'config_path'
+    mock_get_config_dir.return_value = config_path
+    credentials_path = _cloud_sdk.get_application_default_credentials_path()
+    assert credentials_path == os.path.join(
+        config_path, _cloud_sdk._CREDENTIALS_FILENAME)
+
+
+def test_get_config_path_env_var(monkeypatch):
+    config_path_sentinel = 'config_path'
+    monkeypatch.setenv(
+        environment_vars.CLOUD_SDK_CONFIG_DIR, config_path_sentinel)
+    config_path = _cloud_sdk.get_config_path()
+    assert config_path == config_path_sentinel
+
+
+@mock.patch('os.path.expanduser')
+def test_get_config_path_unix(mock_expanduser):
+    mock_expanduser.side_effect = lambda path: path
+
+    config_path = _cloud_sdk.get_config_path()
+
+    assert os.path.split(config_path) == (
+        '~/.config', _cloud_sdk._CONFIG_DIRECTORY)
+
+
+@mock.patch('os.name', new='nt')
+def test_get_config_path_windows(monkeypatch):
+    appdata = 'appdata'
+    monkeypatch.setenv(_cloud_sdk._WINDOWS_CONFIG_ROOT_ENV_VAR, appdata)
+
+    config_path = _cloud_sdk.get_config_path()
+
+    assert os.path.split(config_path) == (
+        appdata, _cloud_sdk._CONFIG_DIRECTORY)
+
+
+@mock.patch('os.name', new='nt')
+def test_get_config_path_no_appdata(monkeypatch):
+    monkeypatch.delenv(_cloud_sdk._WINDOWS_CONFIG_ROOT_ENV_VAR, raising=False)
+    monkeypatch.setenv('SystemDrive', 'G:')
+
+    config_path = _cloud_sdk.get_config_path()
+
+    assert os.path.split(config_path) == (
+        'G:/\\', _cloud_sdk._CONFIG_DIRECTORY)
+
+
+def test_load_authorized_user_credentials():
+    credentials = _cloud_sdk.load_authorized_user_credentials(
+        AUTHORIZED_USER_FILE_DATA)
+
+    assert isinstance(credentials, google.oauth2.credentials.Credentials)
+
+    assert credentials.token is None
+    assert (credentials._refresh_token ==
+            AUTHORIZED_USER_FILE_DATA['refresh_token'])
+    assert credentials._client_id == AUTHORIZED_USER_FILE_DATA['client_id']
+    assert (credentials._client_secret ==
+            AUTHORIZED_USER_FILE_DATA['client_secret'])
+    assert credentials._token_uri == _cloud_sdk._GOOGLE_OAUTH2_TOKEN_ENDPOINT
+
+
+def test_load_authorized_user_credentials_bad_format():
+    with pytest.raises(ValueError) as excinfo:
+        _cloud_sdk.load_authorized_user_credentials({})
+
+    assert excinfo.match(r'missing fields')