bpo-39048: Look up __aenter__ before __aexit__ in async with (GH-17609)

* Reorder the __aenter__ and __aexit__ checks for async with
* Add assertions for async with body being skipped
* Swap __aexit__ and __aenter__ loading in the documentation

1 files changed, 10 insertions, 9 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 096645aeeb..5e586589e9 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3230,20 +3230,21 @@ main_loop:
         }
 
         case TARGET(BEFORE_ASYNC_WITH): {
-            _Py_IDENTIFIER(__aexit__);
             _Py_IDENTIFIER(__aenter__);
-
+            _Py_IDENTIFIER(__aexit__);
             PyObject *mgr = TOP();
-            PyObject *exit = special_lookup(tstate, mgr, &PyId___aexit__),
-                     *enter;
+            PyObject *enter = special_lookup(tstate, mgr, &PyId___aenter__);
             PyObject *res;
-            if (exit == NULL)
+            if (enter == NULL) {
+                goto error;
+            }
+            PyObject *exit = special_lookup(tstate, mgr, &PyId___aexit__);
+            if (exit == NULL) {
+                Py_DECREF(enter);
                 goto error;
+            }
             SET_TOP(exit);
-            enter = special_lookup(tstate, mgr, &PyId___aenter__);
             Py_DECREF(mgr);
-            if (enter == NULL)
-                goto error;
             res = _PyObject_CallNoArg(enter);
             Py_DECREF(enter);
             if (res == NULL)
@@ -3264,8 +3265,8 @@ main_loop:
         }
 
         case TARGET(SETUP_WITH): {
-            _Py_IDENTIFIER(__exit__);
             _Py_IDENTIFIER(__enter__);
+            _Py_IDENTIFIER(__exit__);
             PyObject *mgr = TOP();
             PyObject *enter = special_lookup(tstate, mgr, &PyId___enter__);
             PyObject *res;