From 8f982ede281073403a2389ba086706ee1bd01ef6 Mon Sep 17 00:00:00 2001
From: Ethan Furman <ethan@stoneleaf.us>
Date: Mon, 21 Apr 2025 21:01:24 -0700
Subject: [PATCH 1/2] only call _missing_ in __contains__ for Flags

---
 Lib/enum.py           | 20 ++++++++++++--------
 Lib/test/test_enum.py | 11 +++++++++++
 2 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/Lib/enum.py b/Lib/enum.py
index b5f3ca7ae111d6..ea27c9ef9d2190 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -731,14 +731,18 @@ def __contains__(cls, value):
         """
         if isinstance(value, cls):
             return True
-        try:
-            cls(value)
-            return True
-        except ValueError:
-            return (
-                    value in cls._unhashable_values_    # both structures are lists
-                    or value in cls._hashable_values_
-                    )
+        if issubclass(cls, Flag):
+            try:
+                result = cls._missing_(value)
+                if isinstance(result, cls):
+                    return True
+                return False
+            except ValueError:
+                pass
+        return (
+                value in cls._unhashable_values_    # both structures are lists
+                or value in cls._hashable_values_
+                )
 
     def __delattr__(cls, attr):
         # nicer error message when someone tries to delete an attribute
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index dde674164f4a52..68cedc666a59ef 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -1569,6 +1569,17 @@ class IntFlag1(IntFlag):
         self.assertIn(IntEnum1.X, IntFlag1)
         self.assertIn(IntFlag1.X, IntEnum1)
 
+    def test_contains_does_not_call_missing(self):
+        class AnEnum(Enum):
+            UNKNOWN = None
+            LUCKY = 3
+            @classmethod
+            def _missing_(cls, *values):
+                return cls.UNKNOWN
+        self.assertTrue(None in AnEnum)
+        self.assertTrue(3 in AnEnum)
+        self.assertFalse(7 in AnEnum)
+
     def test_inherited_data_type(self):
         class HexInt(int):
             __qualname__ = 'HexInt'

From 2122e59cee84c576fdf14dac6cc1cbd914de6b1b Mon Sep 17 00:00:00 2001
From: Ethan Furman <ethan@stoneleaf.us>
Date: Thu, 24 Apr 2025 13:25:59 -0700
Subject: [PATCH 2/2] simplify code
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
---
 Lib/enum.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/Lib/enum.py b/Lib/enum.py
index ea27c9ef9d2190..01fecca3e5aac0 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -734,9 +734,7 @@ def __contains__(cls, value):
         if issubclass(cls, Flag):
             try:
                 result = cls._missing_(value)
-                if isinstance(result, cls):
-                    return True
-                return False
+                return isinstance(result, cls)
             except ValueError:
                 pass
         return (