gh-95077: [Enum] add code-based deprecation warnings for member.member access

Doc/howto/enum.rst

@@ -945,23 +945,12 @@ but remain normal attributes.
 """"""""""""""""""""
 
 Enum members are instances of their enum class, and are normally accessed as
-``EnumClass.member``.  In Python versions ``3.5`` to ``3.10`` you could access
-members from other members -- this practice was discouraged, and in ``3.11``
-:class:`Enum` returns to not allowing it::
-
-    >>> class FieldTypes(Enum):
-    ...     name = 0
-    ...     value = 1
-    ...     size = 2
-    ...
-    >>> FieldTypes.value.size
-    Traceback (most recent call last):
-    ...
-    AttributeError: <enum 'FieldTypes'> member has no attribute 'size'
-
+``EnumClass.member``.  In Python versions starting with ``3.5`` you could access
+members from other members -- this practice is discouraged, is deprecated
+in ``3.12``, and will be removed in ``3.14``.
 
 .. versionchanged:: 3.5
-.. versionchanged:: 3.11
+.. versionchanged:: 3.12
 
 
 Creating members that are mixed with other data types

Doc/library/enum.rst

@@ -176,13 +176,6 @@ Data Types
         >>> dir(Color)
         ['BLUE', 'GREEN', 'RED', '__class__', '__contains__', '__doc__', '__getitem__', '__init_subclass__', '__iter__', '__len__', '__members__', '__module__', '__name__', '__qualname__']
 
-   .. method:: EnumType.__getattr__(cls, name)
-
-      Returns the Enum member in *cls* matching *name*, or raises an :exc:`AttributeError`::
-
-        >>> Color.GREEN
-        <Color.GREEN: 2>
-
    .. method:: EnumType.__getitem__(cls, name)
 
       Returns the Enum member in *cls* matching *name*, or raises an :exc:`KeyError`::

Lib/enum.py

@@ -185,19 +185,35 @@ class property(DynamicClassAttribute):
     a corresponding enum member.
     """
 
+    member = None
+
     def __get__(self, instance, ownerclass=None):
         if instance is None:
-            try:
-                return ownerclass._member_map_[self.name]
-            except KeyError:
+            if self.member is not None:
+                return self.member
+            else:
                 raise AttributeError(
                         '%r has no attribute %r' % (ownerclass, self.name)
                         )
         else:
             if self.fget is None:
-                raise AttributeError(
-                        '%r member has no attribute %r' % (ownerclass, self.name)
+                if self.member is None:   # not sure this can happen, but just in case
+                    raise AttributeError(
+                            '%r has no attribute %r' % (ownerclass, self.name)
+                            )
+                # issue warning deprecating this behavior
+                import warnings
+                warnings.warn(
+                        "`member.member` access (e.g. `Color.RED.BLUE`) is "
+                        "deprecated and will be removed in 3.14.",
+                        DeprecationWarning,
+                        stacklevel=2,
                         )
+                return self.member
+                # XXX: uncomment in 3.14 and remove warning above
+                # raise AttributeError(
+                #         '%r member has no attribute %r' % (ownerclass, self.name)
+                #         )
             else:
                 return self.fget(instance)
 
@@ -299,30 +315,20 @@ def __set_name__(self, enum_class, member_name):
                 enum_class._member_names_.append(member_name)
         # get redirect in place before adding to _member_map_
         # but check for other instances in parent classes first
-        need_override = False
         descriptor = None
         for base in enum_class.__mro__[1:]:
             descriptor = base.__dict__.get(member_name)
             if descriptor is not None:
                 if isinstance(descriptor, (property, DynamicClassAttribute)):
                     break
-                else:
-                    need_override = True
-                    # keep looking for an enum.property
-        if descriptor and not need_override:
-            # previous enum.property found, no further action needed
-            pass
-        elif descriptor and need_override:
-            redirect = property()
-            redirect.__set_name__(enum_class, member_name)
-            # Previous enum.property found, but some other inherited attribute
-            # is in the way; copy fget, fset, fdel to this one.
-            redirect.fget = descriptor.fget
-            redirect.fset = descriptor.fset
-            redirect.fdel = descriptor.fdel
-            setattr(enum_class, member_name, redirect)
-        else:
-            setattr(enum_class, member_name, enum_member)
+        redirect = property()
+        redirect.member = enum_member
+        redirect.__set_name__(enum_class, member_name)
+        if descriptor:
+            redirect.fget = getattr(descriptor, 'fget', None)
+            redirect.fset = getattr(descriptor, 'fset', None)
+            redirect.fdel = getattr(descriptor, 'fdel', None)
+        setattr(enum_class, member_name, redirect)
         # now add to _member_map_ (even aliases)
         enum_class._member_map_[member_name] = enum_member
         try:
@@ -740,22 +746,6 @@ def __dir__(cls):
             # return whatever mixed-in data type has
             return sorted(set(dir(cls._member_type_)) | interesting)
 
-    def __getattr__(cls, name):
-        """
-        Return the enum member matching `name`
-
-        We use __getattr__ instead of descriptors or inserting into the enum
-        class' __dict__ in order to support `name` and `value` being both
-        properties for enum members (which live in the class' __dict__) and
-        enum members themselves.
-        """
-        if _is_dunder(name):
-            raise AttributeError(name)
-        try:
-            return cls._member_map_[name]
-        except KeyError:
-            raise AttributeError(name) from None
-
     def __getitem__(cls, name):
         """
         Return the member matching `name`.
@@ -1200,10 +1190,10 @@ def __reduce_ex__(self, proto):
     # enum.property is used to provide access to the `name` and
     # `value` attributes of enum members while keeping some measure of
     # protection from modification, while still allowing for an enumeration
-    # to have members named `name` and `value`.  This works because enumeration
-    # members are not set directly on the enum class; they are kept in a
-    # separate structure, _member_map_, which is where enum.property looks for
-    # them
+    # to have members named `name` and `value`.  This works because each
+    # instance of enum.property saves its companion member, which it returns
+    # on class lookup; on instance lookup it either executes a provided function
+    # or raises an AttributeError.
 
     @property
     def name(self):
@@ -1677,10 +1667,12 @@ def convert_class(cls):
                     value = gnv(name, 1, len(member_names), gnv_last_values)
                 if value in value2member_map:
                     # an alias to an existing member
+                    member = value2member_map[value]
                     redirect = property()
+                    redirect.member = member
                     redirect.__set_name__(enum_class, name)
                     setattr(enum_class, name, redirect)
-                    member_map[name] = value2member_map[value]
+                    member_map[name] = member
                 else:
                     # create the member
                     if use_args:
@@ -1696,6 +1688,7 @@ def convert_class(cls):
                     member.__objclass__ = enum_class
                     member.__init__(value)
                     redirect = property()
+                    redirect.member = member
                     redirect.__set_name__(enum_class, name)
                     setattr(enum_class, name, redirect)
                     member_map[name] = member
@@ -1723,10 +1716,12 @@ def convert_class(cls):
                     value = value.value
                 if value in value2member_map:
                     # an alias to an existing member
+                    member = value2member_map[value]
                     redirect = property()
+                    redirect.member = member
                     redirect.__set_name__(enum_class, name)
                     setattr(enum_class, name, redirect)
-                    member_map[name] = value2member_map[value]
+                    member_map[name] = member
                 else:
                     # create the member
                     if use_args:
@@ -1743,6 +1738,7 @@ def convert_class(cls):
                     member.__init__(value)
                     member._sort_order_ = len(member_names)
                     redirect = property()
+                    redirect.member = member
                     redirect.__set_name__(enum_class, name)
                     setattr(enum_class, name, redirect)
                     member_map[name] = member

Lib/test/test_enum.py

@@ -2646,14 +2646,28 @@ class Private(Enum):
         self.assertEqual(Private._Private__corporal, 'Radar')
         self.assertEqual(Private._Private__major_, 'Hoolihan')
 
-    @unittest.skip("Accessing all values retained for performance reasons, see GH-93910")
+    @unittest.skipIf(
+            python_version <= (3, 13),
+            'member.member access currently deprecated',
+            )
     def test_exception_for_member_from_member_access(self):
         with self.assertRaisesRegex(AttributeError, "<enum .Di.> member has no attribute .NO."):
             class Di(Enum):
                 YES = 1
                 NO = 0
             nope = Di.YES.NO
 
+    @unittest.skipIf(
+            python_version > (3, 13),
+            'member.member access now raises',
+            )
+    def test_warning_for_member_from_member_access(self):
+        with self.assertWarnsRegex(DeprecationWarning, '`member.member` access .* is deprecated and will be removed in 3.14'):
+            class Di(Enum):
+                YES = 1
+                NO = 0
+            warn = Di.YES.NO
+        self.assertIs(warn, Di.NO)
 
     def test_dynamic_members_with_static_methods(self):
         #

Misc/NEWS.d/next/Library/2022-07-22-00-58-49.gh-issue-95077.4Z6CNC.rst

@@ -0,0 +1 @@
+Add deprecation warning for enum ``member.member`` access (e.g. ``Color.RED.BLUE``).