bpo-29922: Improve error messages in 'async with' (GH-6352)

when __aenter__() or __aexit__() return non-awaitable object.
(cherry picked from commit a68f2f0578bbf812fa2264d0e0bb388340d6e230)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

1 files changed, 25 insertions, 0 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 9ad582b..6252e89 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -66,6 +66,7 @@ static PyObject * unicode_concatenate(PyObject *, PyObject *,
 static PyObject * special_lookup(PyObject *, _Py_Identifier *);
 static int check_args_iterable(PyObject *func, PyObject *vararg);
 static void format_kwargs_mapping_error(PyObject *func, PyObject *kwargs);
+static void format_awaitable_error(PyTypeObject *, int);
 
 #define NAME_ERROR_MSG \
     "name '%.200s' is not defined"
@@ -2040,6 +2041,11 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
             PyObject *iterable = TOP();
             PyObject *iter = _PyCoro_GetAwaitableIter(iterable);
 
+            if (iter == NULL) {
+                format_awaitable_error(Py_TYPE(iterable),
+                                       _Py_OPCODE(next_instr[-2]));
+            }
+
             Py_DECREF(iterable);
 
             if (iter != NULL && PyCoro_CheckExact(iter)) {
@@ -5403,6 +5409,25 @@ format_exc_unbound(PyCodeObject *co, int oparg)
     }
 }
 
+static void
+format_awaitable_error(PyTypeObject *type, int prevopcode)
+{
+    if (type->tp_as_async == NULL || type->tp_as_async->am_await == NULL) {
+        if (prevopcode == BEFORE_ASYNC_WITH) {
+            PyErr_Format(PyExc_TypeError,
+                         "'async with' received an object from __aenter__ "
+                         "that does not implement __await__: %.100s",
+                         type->tp_name);
+        }
+        else if (prevopcode == WITH_CLEANUP_START) {
+            PyErr_Format(PyExc_TypeError,
+                         "'async with' received an object from __aexit__ "
+                         "that does not implement __await__: %.100s",
+                         type->tp_name);
+        }
+    }
+}
+
 static PyObject *
 unicode_concatenate(PyObject *v, PyObject *w,
                     PyFrameObject *f, const _Py_CODEUNIT *next_instr)