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.

1 files changed, 6 insertions, 6 deletions

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index e91cf46..5e47054 100644
--- a/Lib/contextlib.py
+++ b/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):