bpo-28893: Set __cause__ for errors in async iteration protocol (#407)

Author: 1st1

Lib/test/test_coroutines.py

@@ -1680,6 +1680,44 @@ async def foo():
 warnings.simplefilter("error")
 run_async(foo())
 
+deftest_for_11(self):
+classF:
+def__aiter__(self):
+returnself
+def__anext__(self):
+returnself
+def__await__(self):
+1/0
+
+asyncdefmain():
+asyncfor_inF():
+pass
+
+withself.assertRaisesRegex(TypeError,
+'an invalid object from __anext__') asc:
+main().send(None)
+
+err=c.exception
+self.assertIsInstance(err.__cause__, ZeroDivisionError)
+
+deftest_for_12(self):
+classF:
+def__aiter__(self):
+returnself
+def__await__(self):
+1/0
+
+asyncdefmain():
+asyncfor_inF():
+pass
+
+withself.assertRaisesRegex(TypeError,
+'an invalid object from __aiter__') asc:
+main().send(None)
+
+err=c.exception
+self.assertIsInstance(err.__cause__, ZeroDivisionError)
+
 deftest_for_tuple(self):
 classDone(Exception): pass
 

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.6.1 release candidate 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.
 

Python/ceval.c

@@ -1904,13 +1904,13 @@ _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 {
@@ -1969,7 +1969,7 @@ _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",