gh-154817: Fix OrderedDict.pop() crash with inconsistent equality

Lib/test/test_ordered_dict.py

@@ -1014,6 +1014,28 @@ def test_weakref_list_is_not_traversed(self):
 
         gc.collect()
 
+    def test_pop_inconsistent_eq(self):
+        class K:
+            def __init__(self):
+                self.calls = 0
+
+            def __hash__(self):
+                return 12345
+
+            # Behave inconsistently across repeated equality checks.
+            def __eq__(self, other):
+                self.calls += 1
+                return self.calls <= 2
+
+        k1 = K()
+        k2 = K()
+
+        od = self.OrderedDict()
+        od[k1] = "value"
+
+        with self.assertRaises(KeyError):
+            od.pop(k2)
+
 
 class PurePythonOrderedDictSubclassTests(PurePythonOrderedDictTests):
 

Misc/NEWS.d/next/Library/2026-07-28-17-56-37.gh-issue-154817.PYfwpm.rst

@@ -0,0 +1,4 @@
+Fix a crash in collections.OrderedDict.pop() when a key's __eq__
+implementation returns inconsistent results across repeated comparisons.
+OrderedDict.pop() now raises KeyError instead of crashing when no default
+value is provided.

Objects/odictobject.c

@@ -1095,7 +1095,12 @@ _odict_popkey_hash(PyObject *od, PyObject *key, PyObject *failobj,
         /* Now delete the value from the dict. */
         if (_PyDict_Pop_KnownHash((PyDictObject *)od, key, hash,
                                   &value) == 0) {
-            value = Py_NewRef(failobj);
+            if (failobj) {
+                value = Py_NewRef(failobj);
+            }
+            else {
+                PyErr_SetObject(PyExc_KeyError, key);
+            }
         }
     }
     else if (value == NULL && !PyErr_Occurred()) {