Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:37:50) [MSC v.1916 64 bit (AMD64)] on win32Type "help", "copyright", "credits" or "license()" for more information.>>> list(reversed({1: 1}))[1]>>> list(reversed({}))================================ RESTART: Shell ================================>>>

Outside of IDLE, the example causes a segfault.  With debug build of current master HEAD:Assertion failed: (value != NULL), function dictreviter_iternext, file ../../source/Objects/dictobject.c, line 3834.    if (d->ma_values) {        if (i < 0) {            goto fail;        }        key = DK_ENTRIES(k)[i].me_key;        value = d->ma_values[i];        assert (value != NULL);    }

Thanks for the report, by the way!

This issue is related to reversed dict iter.>>> a = reversed({})>>> next(a)Assertion failed: (value != NULL), function dictreviter_iternext, fileObjects/dictobject.c, line 3834.[1]    1366 abort      ./python.exe

>>> reversed({}.items())<dict_reverseitemiterator object at 0x106a2bc50>>>> a = reversed({}.items())>>> next(a)Assertion failed: (value != NULL), function dictreviter_iternext, fileObjects/dictobject.c, line 3834.[1]    4106 abort      ./python.exe

Can I take look at this issue?

Wops, we made a PR at the same time, Dong-hee Na.I will close mine :)

> Wops, we made a PR at the same time, Dong-hee Na.lol> I will close mine :)Oh.. thank you for giving me a chance.

The proposed fix fixes a crash, but there is other bug in iterating shared dicts.class A:    def __init__(self, x, y):        if x: self.x = x        if y: self.y = ya = A(1, 2)print(list(iter(a.__dict__)))print(list(reversed(a.__dict__)))b = A(1, 0)print(list(iter(b.__dict__)))print(list(reversed(b.__dict__)))WithPR 16846 the last print outputs [] instead of expected ['x']. It crashes withoutPR 16846, so this issue is not only about empty dicts.

> The proposed fix fixes a crash, but there is other bug in iterating shared dicts.Yes, you are right.But I can not work on this issue today.(I have to finalize my paperwork by today :-()since priority is critical if other core developers finalize this issue today. It will be okay for me.

FYIc = A(0, 1)print(type(c.__dict__))print(list(iter(c.__dict__)))print(list(reversed(c.__dict__)))works as we expected.

When dict is empty, di_pos of reverse iterator must be -1, not 0.Additionally, di_pos must be initialized from ma_used when dict is key sharing dict.diff --git a/Objects/dictobject.c b/Objects/dictobject.cindex64876e0519..6c4b41700b 100644--- a/Objects/dictobject.c+++ b/Objects/dictobject.c@@ -3452,10 +3452,15 @@ dictiter_new(PyDictObject *dict, PyTypeObject *itertype)     di->di_dict = dict;     di->di_used = dict->ma_used;     di->len = dict->ma_used;-    if ((itertype == &PyDictRevIterKey_Type ||-         itertype == &PyDictRevIterItem_Type ||-         itertype == &PyDictRevIterValue_Type) && dict->ma_used) {+    if (itertype == &PyDictRevIterKey_Type ||+            itertype == &PyDictRevIterItem_Type ||+            itertype == &PyDictRevIterValue_Type) {+        if (dict->ma_values) {+            di->di_pos = dict->ma_used - 1;+        }+        else {             di->di_pos = dict->ma_keys->dk_nentries - 1;+        }     }     else {

New changeset24dc2f8c56697f9ee51a4887cf0814b6600c1815 by Pablo Galindo (Dong-hee Na) in branch 'master':bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict (GH-16846)https://github.com/python/cpython/commit/24dc2f8c56697f9ee51a4887cf0814b6600c1815

New changesetd73205d788a32148ba9a2beaa27badbd94ab65ff by Miss Islington (bot) in branch '3.8':bpo-38525: Fix a segmentation fault when using reverse iterators of empty dict (GH-16846)https://github.com/python/cpython/commit/d73205d788a32148ba9a2beaa27badbd94ab65ff

Closing this.Thanks, Dong-hee Na and everyone involved in the issue!