bpo-29692: contextlib.contextmanager may incorrectly unchain RuntimeError (GH-949)

contextlib._GeneratorContextManager.__exit__ includes a special case to deal with
PEP 479 RuntimeErrors created when `StopIteration` is thrown into the context
manager body.

Previously this check was too permissive, and undid one level of chaining on *all*
RuntimeError instances, not just those that wrapped a StopIteration instance.

Lib/contextlib.py

@@ -88,7 +88,7 @@ class _GeneratorContextManager(ContextDecorator, AbstractContextManager):
             try:
                 next(self.gen)
             except StopIteration:
-                return
+                return False
             else:
                 raise RuntimeError("generator didn't stop")
         else:
@@ -110,7 +110,7 @@ class _GeneratorContextManager(ContextDecorator, AbstractContextManager):
                 # Likewise, avoid suppressing if a StopIteration exception
                 # was passed to throw() and later wrapped into a RuntimeError
                 # (see PEP 479).
-                if exc.__cause__ is value:
+                if type is StopIteration and exc.__cause__ is value:
                     return False
                 raise
             except:
@@ -121,10 +121,10 @@ class _GeneratorContextManager(ContextDecorator, AbstractContextManager):
                 # fixes the impedance mismatch between the throw() protocol
                 # and the __exit__() protocol.
                 #
-                if sys.exc_info()[1] is not value:
-                    raise
-            else:
-                raise RuntimeError("generator didn't stop after throw()")
+                if sys.exc_info()[1] is value:
+                    return False
+                raise
+            raise RuntimeError("generator didn't stop after throw()")
 
 
 def contextmanager(func):

Lib/test/test_contextlib.py

@@ -152,6 +152,29 @@ def woohoo():
         else:
             self.fail('StopIteration was suppressed')
 
+    def test_contextmanager_do_not_unchain_non_stopiteration_exceptions(self):
+        @contextmanager
+        def test_issue29692():
+            try:
+                yield
+            except Exception as exc:
+                raise RuntimeError('issue29692:Chained') from exc
+        try:
+            with test_issue29692():
+                raise ZeroDivisionError
+        except Exception as ex:
+            self.assertIs(type(ex), RuntimeError)
+            self.assertEqual(ex.args[0], 'issue29692:Chained')
+            self.assertIsInstance(ex.__cause__, ZeroDivisionError)
+
+        try:
+            with test_issue29692():
+                raise StopIteration('issue29692:Unchained')
+        except Exception as ex:
+            self.assertIs(type(ex), StopIteration)
+            self.assertEqual(ex.args[0], 'issue29692:Unchained')
+            self.assertIsNone(ex.__cause__)
+
     def _create_contextmanager_attribs(self):
         def attribs(**kw):
             def decorate(func):

Misc/NEWS

@@ -306,6 +306,9 @@ Extension Modules
 
 Library
 -------
+- bpo-29692: Fixed arbitrary unchaining of RuntimeError exceptions in 
+  contextlib.contextmanager.
+  Patch by Siddharth Velankar.
 
 - bpo-26187: Test that sqlite3 trace callback is not called multiple
   times when schema is changing.  Indirectly fixed by switching to