gh-91456: [Enum] Deprecate default auto() behavior with mixed value types (GH-91457)

When used with plain Enum, auto() returns the last numeric value assigned, skipping any incompatible member values (such as strings); starting in 3.13 the default auto() for plain Enums will require all the values to be of compatible types, and will return a new value that is 1 higher than any existing value.

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Doc/library/enum.rst

@@ -761,6 +761,10 @@ Utilities and Decorators
    ``_generate_next_value_`` can be overridden to customize the values used by
    *auto*.
 
+   .. note:: in 3.13 the default ``"generate_next_value_`` will always return
+             the highest member value incremented by 1, and will fail if any
+             member is an incompatible type.
+
 .. decorator:: property
 
    A decorator similar to the built-in *property*, but specifically for

Lib/enum.py

@@ -1205,21 +1205,39 @@ class Enum(metaclass=EnumType):
     def __init__(self, *args, **kwds):
         pass
 
-    def _generate_next_value_(name, start, count, last_values):
+    def _generate_next_value_(name, start, count, last_value):
         """
         Generate the next value when not given.
 
         name: the name of the member
         start: the initial start value or None
         count: the number of existing members
-        last_value: the last value assigned or None
+        last_value: the list of values assigned
         """
-        for last_value in reversed(last_values):
-            try:
-                return last_value + 1
-            except TypeError:
-                pass
-        else:
+        if not last_value:
+            return start
+        try:
+            last = last_value[-1]
+            last_value.sort()
+            if last == last_value[-1]:
+                # no difference between old and new methods
+                return last + 1
+            else:
+                # trigger old method (with warning)
+                raise TypeError
+        except TypeError:
+            import warnings
+            warnings.warn(
+                    "In 3.13 the default `auto()`/`_generate_next_value_` will require all values to be sortable and support adding +1\n"
+                    "and the value returned will be the largest value in the enum incremented by 1",
+                    DeprecationWarning,
+                    stacklevel=3,
+                    )
+            for v in last_value:
+                try:
+                    return v + 1
+                except TypeError:
+                    pass
             return start
 
     @classmethod

Lib/test/test_enum.py

@@ -3953,23 +3953,54 @@ class TestInternals(unittest.TestCase):
         self.assertEqual(Color.blue.value, 'blue')
         self.assertEqual(Color.green.value, 'green')
 
-    def test_auto_garbage(self):
-        class Color(Enum):
-            red = 'red'
-            blue = auto()
+    @unittest.skipIf(
+            python_version >= (3, 13),
+            'mixed types with auto() no longer supported',
+            )
+    def test_auto_garbage_ok(self):
+        with self.assertWarnsRegex(DeprecationWarning, 'will require all values to be sortable'):
+            class Color(Enum):
+                red = 'red'
+                blue = auto()
         self.assertEqual(Color.blue.value, 1)
 
-    def test_auto_garbage_corrected(self):
-        class Color(Enum):
-            red = 'red'
-            blue = 2
-            green = auto()
+    @unittest.skipIf(
+            python_version >= (3, 13),
+            'mixed types with auto() no longer supported',
+            )
+    def test_auto_garbage_corrected_ok(self):
+        with self.assertWarnsRegex(DeprecationWarning, 'will require all values to be sortable'):
+            class Color(Enum):
+                red = 'red'
+                blue = 2
+                green = auto()
 
         self.assertEqual(list(Color), [Color.red, Color.blue, Color.green])
         self.assertEqual(Color.red.value, 'red')
         self.assertEqual(Color.blue.value, 2)
         self.assertEqual(Color.green.value, 3)
 
+    @unittest.skipIf(
+            python_version < (3, 13),
+            'mixed types with auto() will raise in 3.13',
+            )
+    def test_auto_garbage_fail(self):
+        with self.assertRaisesRegex(TypeError, 'will require all values to be sortable'):
+            class Color(Enum):
+                red = 'red'
+                blue = auto()
+
+    @unittest.skipIf(
+            python_version < (3, 13),
+            'mixed types with auto() will raise in 3.13',
+            )
+    def test_auto_garbage_corrected_fail(self):
+        with self.assertRaisesRegex(TypeError, 'will require all values to be sortable'):
+            class Color(Enum):
+                red = 'red'
+                blue = 2
+                green = auto()
+
     def test_auto_order(self):
         with self.assertRaises(TypeError):
             class Color(Enum):
@@ -3991,6 +4022,22 @@ class TestInternals(unittest.TestCase):
         self.assertEqual(Color.red.value, 'pathological case')
         self.assertEqual(Color.blue.value, 'blue')
 
+    @unittest.skipIf(
+            python_version < (3, 13),
+            'auto() will return highest value + 1 in 3.13',
+            )
+    def test_auto_with_aliases(self):
+        class Color(Enum):
+            red = auto()
+            blue = auto()
+            oxford = blue
+            crimson = red
+            green = auto()
+        self.assertIs(Color.crimson, Color.red)
+        self.assertIs(Color.oxford, Color.blue)
+        self.assertIsNot(Color.green, Color.red)
+        self.assertIsNot(Color.green, Color.blue)
+
     def test_duplicate_auto(self):
         class Dupes(Enum):
             first = primero = auto()

Misc/NEWS.d/next/Library/2022-04-11-16-55-41.gh-issue-91456.DK3KKl.rst

@@ -0,0 +1,3 @@
+Deprecate current default auto() behavior:  In 3.13 the default will be for
+for auto() to always return the largest member value incremented by
+1, and to raise if incompatible value types are used.