[py_utils] Add empty and optional context managers.

common/py_utils/py_utils/contextlib_ext_unittest.py

Index: common/py_utils/py_utils/contextlib_ext_unittest.py
diff --git a/common/py_utils/py_utils/contextlib_ext_unittest.py b/common/py_utils/py_utils/contextlib_ext_unittest.py
new file mode 100644
index 0000000000000000000000000000000000000000..b83e7e5e0183c5248c61c745bf2e22d66016fdb3
--- /dev/null
+++ b/common/py_utils/py_utils/contextlib_ext_unittest.py
@@ -0,0 +1,34 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import unittest
+
+from py_utils import contextlib_ext
+
+
+class OptionalUnittest(unittest.TestCase):
+
+  class SampleContextMgr(object):
+
+    def __init__(self):
+      self.entered = False
+      self.exited = False
+
+    def __enter__(self):
+      self.entered = True
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+      self.exited = True
+
+  def testConditionTrue(self):
+    c = self.SampleContextMgr()
+    with contextlib_ext.Optional(c, True):
+      self.assertTrue(c.entered)
+    self.assertTrue(c.exited)
+
+  def testConditionFalse(self):
+    c = self.SampleContextMgr()
+    with contextlib_ext.Optional(c, False):
+      self.assertFalse(c.entered)
+    self.assertFalse(c.exited)