Use context manager to manage OS environment in tests

test/lib/TestGyp.py

Index: test/lib/TestGyp.py
diff --git a/test/lib/TestGyp.py b/test/lib/TestGyp.py
index 306bf3d3ad2caadb7a6826d0118feeca8e387031..af75f96cd81616bc2eebc96161d15b12794a0a25 100644
--- a/test/lib/TestGyp.py
+++ b/test/lib/TestGyp.py
@@ -7,6 +7,7 @@ TestGyp.py:  a testing framework for GYP integration tests.
 """
 
 import collections
+from contextlib import contextmanager
 import itertools
 import os
 import re
@@ -24,9 +25,10 @@ __all__.extend([
   'TestGyp',
 ])
 
+
 def remove_debug_line_numbers(contents):
   """Function to remove the line numbers from the debug output
-  of gyp and thus remove the exremem fragility of the stdout
+  of gyp and thus reduce the extreme fragility of the stdout
   comparison tests.
   """
   lines = contents.splitlines()
@@ -37,12 +39,24 @@ def remove_debug_line_numbers(contents):
   lines = [len(l) > 3 and ":".join(l[3:]) or l for l in lines]
   return "\n".join(lines)
 
+
 def match_modulo_line_numbers(contents_a, contents_b):
   """File contents matcher that ignores line numbers."""
   contents_a = remove_debug_line_numbers(contents_a)
   contents_b = remove_debug_line_numbers(contents_b)
   return TestCommon.match_exact(contents_a, contents_b)
 
+
+@contextmanager

[Nils Barth (inactive), 2014/04/08 08:56:32]

This is a simple single-use context manager (CM), which is fine:
these nest correctly.
We only need a re-entrant CM if reusing the same CM multiple times, but we want
to set the local environment (formally, new frame) each time.

https://docs.python.org/3/library/contextlib.html#single-use-reusable-and-ree...

+def LocalEnv(local_env):
+  """Context manager to provide a local OS environment."""
+  env_copy = os.environ.copy()
+  os.environ.update(local_env)
+  yield

[Stefan Haller, 2014/04/08 17:05:41]

Sorry for jumping in here: for correctness, this should read

try:
  yield
finally:
  os.environ.clear()
  os.environ.update(env_copy)

Otherwise the environment won't be restored for failing tests, possibly
affecting the results of other tests that run after it.

[Nils Barth (inactive), 2014/04/09 03:09:18]

Oh, good catch -- thanks!
*don't* need try: ... finally: ... for manually written context managers
with cleanup in __exit__,
but *do* need try: ... finally: ... for @contextlib.contextmanager

+  os.environ.clear()
+  os.environ.update(env_copy)
+
+
 class TestGypBase(TestCommon.TestCommon):
   """
   Class for controlling end-to-end tests of gyp generators.