Use context manager to manage OS environment in tests

test/cxxflags/gyptest-cxxflags.py

 #!/usr/bin/env python
 
 # Copyright (c) 2012 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
-Verifies build of an executable with C++ define specified by a gyp define, and
-the use of the environment during regeneration when the gyp file changes.
+Verifies the use of the environment during regeneration when the gyp file
+changes, specifically via build of an executable with C++ flags specified by
+CXXFLAGS.
+
+In this test, gyp happens within a local environment, but build outside of it.
 """
 
-import os
 import TestGyp
 
-env_stack = []
+FORMATS = ('ninja',)
 
+test = TestGyp.TestGyp(formats=FORMATS)
 
-def PushEnv():
-  env_copy = os.environ.copy()
-  env_stack.append(env_copy)
-
-def PopEnv():
-  os.eniron=env_stack.pop()
-
-# Regenerating build files when a gyp file changes is currently only supported
-# by the make generator.
-test = TestGyp.TestGyp(formats=['make'])
-
-try:
-  PushEnv()
-  os.environ['CXXFLAGS'] = '-O0'
+# We reset the environ after calling gyp. When the auto-regeneration happens,
+# the same define should be reused anyway.
+with TestGyp.LocalEnv({'CXXFLAGS': '-O0'}):
   test.run_gyp('cxxflags.gyp')
-finally:
-  # We clear the environ after calling gyp.  When the auto-regeneration happens,
-  # the same define should be reused anyway.  Reset to empty string first in
-  # case the platform doesn't support unsetenv.
-  PopEnv()
 
 test.build('cxxflags.gyp')
 
 expect = """\
 Using no optimization flag
 """
 test.run_built_executable('cxxflags', stdout=expect)
 
 test.sleep()
 
-try:
-  PushEnv()
-  os.environ['CXXFLAGS'] = '-O2'
+with TestGyp.LocalEnv({'CXXFLAGS': '-O2'}):
   test.run_gyp('cxxflags.gyp')
-finally:
-  # We clear the environ after calling gyp.  When the auto-regeneration happens,
-  # the same define should be reused anyway.  Reset to empty string first in
-  # case the platform doesn't support unsetenv.
-  PopEnv()
 
 test.build('cxxflags.gyp')
 
 expect = """\
 Using an optimization flag
 """
 test.run_built_executable('cxxflags', stdout=expect)
 
 test.pass_test()