bpo-40025: Require _generate_next_value_ to be defined before members (GH-19098)

require `_generate_next_value_` to be defined before members

Doc/library/enum.rst

@@ -273,6 +273,10 @@ overridden::
     the next :class:`int` in sequence with the last :class:`int` provided, but
     the way it does this is an implementation detail and may change.
 
+.. note::
+
+    The :meth:`_generate_next_value_` method must be defined before any members.
+
 Iteration
 ---------
 

Lib/enum.py

@@ -60,6 +60,7 @@ class _EnumDict(dict):
         self._member_names = []
         self._last_values = []
         self._ignore = []
+        self._auto_called = False
 
     def __setitem__(self, key, value):
         """Changes anything not dundered or not a descriptor.
@@ -77,6 +78,9 @@ class _EnumDict(dict):
                     ):
                 raise ValueError('_names_ are reserved for future Enum use')
             if key == '_generate_next_value_':
+                # check if members already defined as auto()
+                if self._auto_called:
+                    raise TypeError("_generate_next_value_ must be defined before members")
                 setattr(self, '_generate_next_value', value)
             elif key == '_ignore_':
                 if isinstance(value, str):
@@ -100,6 +104,7 @@ class _EnumDict(dict):
                 # enum overwriting a descriptor?
                 raise TypeError('%r already defined as: %r' % (key, self[key]))
             if isinstance(value, auto):
+                self._auto_called = True
                 if value.value == _auto_null:
                     value.value = self._generate_next_value(key, 1, len(self._member_names), self._last_values[:])
                 value = value.value

Lib/test/test_enum.py

@@ -1751,6 +1751,16 @@ class TestEnum(unittest.TestCase):
         self.assertEqual(Color.blue.value, 2)
         self.assertEqual(Color.green.value, 3)
 
+    def test_auto_order(self):
+        with self.assertRaises(TypeError):
+            class Color(Enum):
+                red = auto()
+                green = auto()
+                blue = auto()
+                def _generate_next_value_(name, start, count, last):
+                    return name
+
+
     def test_duplicate_auto(self):
         class Dupes(Enum):
             first = primero = auto()

Misc/ACKS

@@ -1240,6 +1240,7 @@ Bryan Olson
 Grant Olson
 Furkan Onder
 Koray Oner
+Ethan Onstott
 Piet van Oostrum
 Tomas Oppelstrup
 Jason Orendorff

Misc/NEWS.d/next/Library/2020-03-21-05-26-38.bpo-40025.DTLtyq.rst

@@ -0,0 +1 @@
+Raise TypeError when _generate_next_value_ is defined after members. Patch by Ethan Onstott.