bpo-40025: Require _generate_next_value_ to be defined before members(GH-19763)

require `_generate_next_value_` to be defined before members

1 files changed, 5 insertions, 0 deletions

diff --git a/Lib/enum.py b/Lib/enum.py
index 108d389d948..14cc00e7839 100644
--- a/Lib/enum.py
+++ b/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