[py_utils] Add empty and optional context managers.

common/py_utils/py_utils/contextlib_ext.py

+# 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.
+
+
+class _OptionalContextManager(object):
+
+  def __init__(self, manager, condition):
+    self._manager = manager
+    self._condition = condition
+
+  def __enter__(self):
+    if self._condition:
+      return self._manager.__enter__()
+    return None
+
+  def __exit__(self, exc_type, exc_val, exc_tb):
+    if self._condition:
+      return self._manager.__exit__(exc_type, exc_val, exc_tb)
+    return None
+
+
+def Optional(manager, condition):
+  """Wraps the provided context manager and runs it if condition is True.
+
+  Args:
+    manager: A context manager to conditionally run.
+    condition: If true, runs the given context manager.
+  Returns:
+    A context manager that conditionally executes the given manager.
+  """
+  return _OptionalContextManager(manager, condition)
+