bpo-43162: [Enum] deprecate enum member.member access

Lib/enum.py

@@ -139,28 +139,38 @@ def __get__(self, instance, ownerclass=None):
                 return ownerclass._member_map_[self.name]
             except KeyError:
                 raise AttributeError(
-                        '%s: no attribute %r' % (ownerclass.__name__, self.name)
+                        '%s: no class attribute %r' % (ownerclass.__name__, self.name)
                         )
         else:
             if self.fget is None:
+                # check for member
+                if self.name in ownerclass._member_map_:
+                    import warnings
+                    warnings.warn(
+                            "accessing one member from another is not supported, "
+                            " and will be disabled in 3.11",
+                            DeprecationWarning,
+                            stacklevel=2,
+                            )
+                    return ownerclass._member_map_[self.name]
                 raise AttributeError(
-                        '%s: no attribute %r' % (ownerclass.__name__, self.name)
+                        '%s: no instance attribute %r' % (ownerclass.__name__, self.name)
                         )
             else:
                 return self.fget(instance)
 
     def __set__(self, instance, value):
         if self.fset is None:
             raise AttributeError(
-                    "%s: cannot set attribute %r" % (self.clsname, self.name)
+                    "%s: cannot set instance attribute %r" % (self.clsname, self.name)
                     )
         else:
             return self.fset(instance, value)
 
     def __delete__(self, instance):
         if self.fdel is None:
             raise AttributeError(
-                    "%s: cannot delete attribute %r" % (self.clsname, self.name)
+                    "%s: cannot delete instance attribute %r" % (self.clsname, self.name)
                     )
         else:
             return self.fdel(instance)

Lib/test/test_enum.py

@@ -2185,6 +2185,29 @@ class Private(Enum):
         self.assertEqual(Private._Private__corporal, 'Radar')
         self.assertEqual(Private._Private__major_, 'Hoolihan')
 
+    @unittest.skipUnless(
+            sys.version_info[:2] == (3, 10),
+            'member-member access now raises an exception',
+            )
+    def test_warning_for_member_from_member_access(self):
+        with self.assertWarns(DeprecationWarning):
+            class Di(Enum):
+                YES = 1
+                NO = 0
+            nope = Di.YES.NO
+        self.assertIs(Di.NO, nope)
+
+    @unittest.skipUnless(
+            sys.version_info[:2] > (3, 10),
+            'member-member access currently issues a warning',
+            )
+    def test_exception_for_member_from_member_access(self):
+        with self.assertRaisesRegex(AttributeError, "Di: no instance attribute .NO."):
+            class Di(Enum):
+                YES = 1
+                NO = 0
+            nope = Di.YES.NO
+
     def test_strenum_auto(self):
         class Strings(StrEnum):
             ONE = auto()

Misc/NEWS.d/next/Library/2021-02-08-16-27-00.bpo-43162.t-W7h3.rst

@@ -0,0 +1,2 @@
+deprecate unsupported ability to access enum members as attributes of other
+enum members