From 4ba801360af443813233131252100c35825288c9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Walter=20D=C3=B6rwald?= <walter@livinglogic.de>
Date: Sat, 25 Apr 2009 12:48:43 +0000
Subject: Merged revisions 71875 via svnmerge from
 svn+ssh://pythondev@svn.python.org/python/trunk

........
  r71875 | walter.doerwald | 2009-04-25 14:15:07 +0200 (Sa, 25 Apr 2009) | 7 lines

  Issue #5837: Certain sequences of calls to set() and unset() for
  support.EnvironmentVarGuard objects restored the environment variables
  incorrectly on __exit__.

  Fix this by recording the initial value of each environment variable on the
  first access in set() or unset().
........
---
 Lib/test/support.py | 31 ++++++++++++++++---------------
 Misc/NEWS           |  4 ++++
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/Lib/test/support.py b/Lib/test/support.py
index ebb3495..09e8574 100644
--- a/Lib/test/support.py
+++ b/Lib/test/support.py
@@ -516,30 +516,31 @@ class EnvironmentVarGuard(object):
     a context manager."""
 
     def __init__(self):
-        self._environ = os.environ
-        self._unset = set()
-        self._reset = dict()
+        self._changed = {}
 
     def set(self, envvar, value):
-        if envvar not in self._environ:
-            self._unset.add(envvar)
-        else:
-            self._reset[envvar] = self._environ[envvar]
-        self._environ[envvar] = value
+        # Remember the initial value on the first access
+        if envvar not in self._changed:
+            self._changed[envvar] = os.environ.get(envvar)
+        os.environ[envvar] = value
 
     def unset(self, envvar):
-        if envvar in self._environ:
-            self._reset[envvar] = self._environ[envvar]
-            del self._environ[envvar]
+        # Remember the initial value on the first access
+        if envvar not in self._changed:
+            self._changed[envvar] = os.environ.get(envvar)
+        if envvar in os.environ:
+            del os.environ[envvar]
 
     def __enter__(self):
         return self
 
     def __exit__(self, *ignore_exc):
-        for envvar, value in self._reset.items():
-            self._environ[envvar] = value
-        for unset in self._unset:
-            del self._environ[unset]
+        for (k, v) in self._changed.items():
+            if v is None:
+                if k in os.environ:
+                    del os.environ[k]
+            else:
+                os.environ[k] = v
 
 class TransientResource(object):
 
diff --git a/Misc/NEWS b/Misc/NEWS
index e647965..b9fc50a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -124,6 +124,10 @@ Tests
   other modules with both C and Python implementations in the stdlib
   can be adjusted to use it over time.
 
+- Issue #5837: Certain sequences of calls to set() and unset() for
+  support.EnvironmentVarGuard objects restored the environment variables
+  incorrectly on __exit__.
+
 
 What's New in Python 3.1 alpha 2?
 =================================
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