gh-101541: [Enum] create flag psuedo-member without calling original __new__ (GH-101590)

(cherry picked from commit ef7c2bfcf1)

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

Lib/enum.py

@@ -1430,12 +1430,11 @@ def _missing_(cls, value):
                     % (cls.__name__, value, unknown, bin(unknown))
                     )
         # normal Flag?
-        __new__ = getattr(cls, '__new_member__', None)
-        if cls._member_type_ is object and not __new__:
+        if cls._member_type_ is object:
             # construct a singleton enum pseudo-member
             pseudo_member = object.__new__(cls)
         else:
-            pseudo_member = (__new__ or cls._member_type_.__new__)(cls, value)
+            pseudo_member = cls._member_type_.__new__(cls, value)
         if not hasattr(pseudo_member, '_value_'):
             pseudo_member._value_ = value
         if member_value:

Lib/test/test_enum.py

@@ -2741,6 +2741,46 @@ class NTEnum(Enum):
                 [TTuple(id=0, a=0, blist=[]), TTuple(id=1, a=2, blist=[4]), TTuple(id=2, a=4, blist=[0, 1, 2])],
                 )
 
+    def test_flag_with_custom_new(self):
+        class FlagFromChar(IntFlag):
+            def __new__(cls, c):
+                value = 1 << c
+                self = int.__new__(cls, value)
+                self._value_ = value
+                return self
+            #
+            a = ord('a')
+        #
+        self.assertEqual(FlagFromChar.a, 158456325028528675187087900672)
+        self.assertEqual(FlagFromChar.a|1, 158456325028528675187087900673)
+        #
+        #
+        class FlagFromChar(Flag):
+            def __new__(cls, c):
+                value = 1 << c
+                self = object.__new__(cls)
+                self._value_ = value
+                return self
+            #
+            a = ord('a')
+            z = 1
+        #
+        self.assertEqual(FlagFromChar.a.value, 158456325028528675187087900672)
+        self.assertEqual((FlagFromChar.a|FlagFromChar.z).value, 158456325028528675187087900674)
+        #
+        #
+        class FlagFromChar(int, Flag, boundary=KEEP):
+            def __new__(cls, c):
+                value = 1 << c
+                self = int.__new__(cls, value)
+                self._value_ = value
+                return self
+            #
+            a = ord('a')
+        #
+        self.assertEqual(FlagFromChar.a, 158456325028528675187087900672)
+        self.assertEqual(FlagFromChar.a|1, 158456325028528675187087900673)
+
 class TestOrder(unittest.TestCase):
     "test usage of the `_order_` attribute"
 

Misc/NEWS.d/next/Library/2023-02-05-14-39-49.gh-issue-101541.Mo3ppp.rst

@@ -0,0 +1 @@
+[Enum] - fix psuedo-flag creation