Fix wrapping into StopIteration of return values in generators and coroutines (#644)

Lib/test/test_coroutines.py

@@ -975,6 +975,21 @@ async def waiter(coro):
                                     "coroutine is being awaited already"):
             waiter(coro).send(None)
 
+    def test_await_16(self):
+        # See https://bugs.python.org/issue29600 for details.
+
+        async def f():
+            return ValueError()
+
+        async def g():
+            try:
+                raise KeyError
+            except:
+                return await f()
+
+        _, result = run_async(g())
+        self.assertIsNone(result.__context__)
+
     def test_with_1(self):
         class Manager:
             def __init__(self, name):

Misc/NEWS

@@ -10,6 +10,8 @@ Release date: XXXX-XX-XX
 Core and Builtins
 -----------------
 
+- bpo-29600: Fix wrapping coroutine return values in StopIteration.
+
 - Issue #29537: Restore runtime compatibility with bytecode files generated by
   CPython 3.5.0 to 3.5.2, and adjust the eval loop to avoid the problems that
   could be caused by the malformed variant of the BUILD_MAP_UNPACK_WITH_CALL

Objects/genobject.c

@@ -466,8 +466,7 @@ _PyGen_SetStopIterationValue(PyObject *value)
     PyObject *e;
 
     if (value == NULL ||
-        (!PyTuple_Check(value) &&
-         !PyObject_TypeCheck(value, (PyTypeObject *) PyExc_StopIteration)))
+        (!PyTuple_Check(value) && !PyExceptionInstance_Check(value)))
     {
         /* Delay exception instantiation if we can */
         PyErr_SetObject(PyExc_StopIteration, value);