python · 1st1 · Mar 3, 2017 · Mar 2, 2017
diff --git a/Lib/test/test_coroutines.py b/Lib/test/test_coroutines.py
@@ -1680,6 +1680,44 @@ async def foo():
                warnings.simplefilter("error")
                run_async(foo())

+    def test_for_11(self):
+        class F:
+            def __aiter__(self):
+                return self
+            def __anext__(self):
+                return self
+            def __await__(self):
+                1 / 0
+
+        async def main():
+            async for _ in F():
+                pass
+
+        with self.assertRaisesRegex(TypeError,
+                                    'an invalid object from __anext__') as c:
+            main().send(None)
+
+        err = c.exception
+        self.assertIsInstance(err.__cause__, ZeroDivisionError)
+
+    def test_for_12(self):
+        class F:
+            def __aiter__(self):
+                return self
+            def __await__(self):
+                1 / 0
+
+        async def main():
+            async for _ in F():
+                pass
+
+        with self.assertRaisesRegex(TypeError,
+                                    'an invalid object from __aiter__') as c:
+            main().send(None)
+
+        err = c.exception
+        self.assertIsInstance(err.__cause__, ZeroDivisionError)
+
    def test_for_tuple(self):
        class Done(Exception): pass


diff --git a/Misc/NEWS b/Misc/NEWS
@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------

+- bpo-28893: Set correct __cause__ for errors about invalid awaitables
+  returned from __aiter__ and __anext__.
+
 - bpo-29683: Fixes to memory allocation in _PyCode_SetExtra.  Patch by
  Brian Coleman.


diff --git a/Python/ceval.c b/Python/ceval.c
@@ -1855,13 +1855,13 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)

            awaitable = _PyCoro_GetAwaitableIter(iter);
            if (awaitable == NULL) {
-                SET_TOP(NULL);
-                PyErr_Format(
+                _PyErr_FormatFromCause(
                    PyExc_TypeError,
                    "'async for' received an invalid object "
                    "from __aiter__: %.100s",
                    Py_TYPE(iter)->tp_name);

+                SET_TOP(NULL);
                Py_DECREF(iter);
                goto error;
            } else {
@@ -1920,7 +1920,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)

                awaitable = _PyCoro_GetAwaitableIter(next_iter);
                if (awaitable == NULL) {
-                    PyErr_Format(
+                    _PyErr_FormatFromCause(
                        PyExc_TypeError,
                        "'async for' received an invalid object "
                        "from __anext__: %.100s",