Um, I thought I'd already checked this in.

Anyway, this is the changes to the with-statement
so that __exit__ must return a true value in order
for a pending exception to be ignored.
The PEP (343) is already updated.

6 files changed, 28 insertions, 27 deletions

diff --git a/Lib/compiler/pyassem.py b/Lib/compiler/pyassem.py
index b59661f..82ff396 100644
--- a/Lib/compiler/pyassem.py
+++ b/Lib/compiler/pyassem.py
@@ -779,7 +779,7 @@ class StackDepthTracker:
         'SETUP_EXCEPT': 3,
         'SETUP_FINALLY': 3,
         'FOR_ITER': 1,
-        'WITH_CLEANUP': 3,
+        'WITH_CLEANUP': -1,
         }
     # use pattern match
     patterns = [
diff --git a/Lib/compiler/pycodegen.py b/Lib/compiler/pycodegen.py
index a1236de..2b3a24f 100644
--- a/Lib/compiler/pycodegen.py
+++ b/Lib/compiler/pycodegen.py
@@ -858,8 +858,6 @@ class CodeGenerator:
         self.nextBlock(final)
         self.setups.push((END_FINALLY, final))
         self.emit('WITH_CLEANUP')
-        self.emit('CALL_FUNCTION', 3)
-        self.emit('POP_TOP')
         self.emit('END_FINALLY')
         self.setups.pop()
         self.__with_count -= 1
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 33c302d..0a5d608 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -30,8 +30,9 @@ class GeneratorContextManager(object):
         else:
             try:
                 self.gen.throw(type, value, traceback)
+                return True
             except StopIteration:
-                pass
+                return True
 
 
 def contextmanager(func):
@@ -91,6 +92,7 @@ def nested(*contexts):
     """
     exits = []
     vars = []
+    exc = (None, None, None)
     try:
         try:
             for context in contexts:
@@ -102,17 +104,14 @@ def nested(*contexts):
             yield vars
         except:
             exc = sys.exc_info()
-        else:
-            exc = (None, None, None)
     finally:
         while exits:
             exit = exits.pop()
             try:
-                exit(*exc)
+                if exit(*exc):
+                    exc = (None, None, None)
             except:
                 exc = sys.exc_info()
-            else:
-                exc = (None, None, None)
         if exc != (None, None, None):
             raise
 
diff --git a/Lib/decimal.py b/Lib/decimal.py
index 49f8115..967f101 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -2196,8 +2196,6 @@ class ContextManager(object):
         return self.new_context
     def __exit__(self, t, v, tb):
         setcontext(self.saved_context)
-        if t is not None:
-            raise t, v, tb
 
 class Context(object):
     """Contains the context for a Decimal instance.
diff --git a/Lib/test/test_with.py b/Lib/test/test_with.py
index 36035e3..4854436 100644
--- a/Lib/test/test_with.py
+++ b/Lib/test/test_with.py
@@ -78,8 +78,8 @@ class Nested(object):
                 vars.append(mgr.__enter__())
                 self.entered.appendleft(mgr)
         except:
-            self.__exit__(*sys.exc_info())
-            raise
+            if not self.__exit__(*sys.exc_info()):
+                raise
         return vars
 
     def __exit__(self, *exc_info):
@@ -89,7 +89,8 @@ class Nested(object):
         ex = exc_info
         for mgr in self.entered:
             try:
-                mgr.__exit__(*ex)
+                if mgr.__exit__(*ex):
+                    ex = (None, None, None)
             except:
                 ex = sys.exc_info()
         self.entered = None
@@ -574,9 +575,7 @@ class AssignmentTargetTestCase(unittest.TestCase):
         class C:
             def __context__(self): return self
             def __enter__(self): return 1, 2, 3
-            def __exit__(self, t, v, tb):
-                if t is not None:
-                    raise t, v, tb
+            def __exit__(self, t, v, tb): pass
         targets = {1: [0, 1, 2]}
         with C() as (targets[1][0], targets[1][1], targets[1][2]):
             self.assertEqual(targets, {1: [1, 2, 3]})
@@ -594,17 +593,30 @@ class AssignmentTargetTestCase(unittest.TestCase):
 
 class ExitSwallowsExceptionTestCase(unittest.TestCase):
 
-    def testExitSwallowsException(self):
-        class AfricanOrEuropean:
+    def testExitTrueSwallowsException(self):
+        class AfricanSwallow:
             def __context__(self): return self
             def __enter__(self): pass
-            def __exit__(self, t, v, tb): pass
+            def __exit__(self, t, v, tb): return True
         try:
-            with AfricanOrEuropean():
+            with AfricanSwallow():
                 1/0
         except ZeroDivisionError:
             self.fail("ZeroDivisionError should have been swallowed")
 
+    def testExitFalseDoesntSwallowException(self):
+        class EuropeanSwallow:
+            def __context__(self): return self
+            def __enter__(self): pass
+            def __exit__(self, t, v, tb): return False
+        try:
+            with EuropeanSwallow():
+                1/0
+        except ZeroDivisionError:
+            pass
+        else:
+            self.fail("ZeroDivisionError should have been raised")
+
 
 def test_main():
     run_unittest(FailureTestCase, NonexceptionalTestCase,
diff --git a/Lib/threading.py b/Lib/threading.py
index 5b485d5..cc1adce 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -128,8 +128,6 @@ class _RLock(_Verbose):
 
     def __exit__(self, t, v, tb):
         self.release()
-        if t is not None:
-            raise t, v, tb
 
     # Internal methods used by condition variables
 
@@ -190,8 +188,6 @@ class _Condition(_Verbose):
 
     def __exit__(self, t, v, tb):
         self.release()
-        if t is not None:
-            raise t, v, tb
 
     def __repr__(self):
         return "<Condition(%s, %d)>" % (self.__lock, len(self.__waiters))
@@ -321,8 +317,6 @@ class _Semaphore(_Verbose):
 
     def __exit__(self, t, v, tb):
         self.release()
-        if t is not None:
-            raise t, v, tb
 
 
 def BoundedSemaphore(*args, **kwargs):