From b16e4e8211bb3221942b15d6ee86f8dfaf634c08 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bern=C3=A1t=20G=C3=A1bor?= <gaborjbernat@gmail.com>
Date: Mon, 1 Jun 2026 17:26:23 -0700
Subject: [PATCH] gh-150817: Speed up Flag bitwise operations

Flag.__or__, __and__ and __xor__ walked both operands on every call to reject
None values. Run that scan only when one of the operand values is actually
None, so valid combinations skip it. The TypeError and its message are
unchanged for the invalid cases.
---
 Lib/enum.py                                   | 24 ++++++++++++-------
 ...-06-02-15-44-57.gh-issue-150817.lpFCh0.rst |  2 ++
 2 files changed, 17 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-02-15-44-57.gh-issue-150817.lpFCh0.rst

diff --git a/Lib/enum.py b/Lib/enum.py
index f97a5193492e602..7aff36c94ce1dcf 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -1620,10 +1620,14 @@ def __or__(self, other):
         if other_value is NotImplemented:
             return NotImplemented
 
-        for flag in self, other:
-            if self._get_value(flag) is None:
-                raise TypeError(f"'{flag}' cannot be combined with other flags with |")
         value = self._value_
+        # _get_value(self) is self._value_ and _get_value(other) is
+        # other_value, so only walk the operands (to raise on the offending
+        # flag) when one of those values is actually None.
+        if value is None or other_value is None:
+            for flag in self, other:
+                if self._get_value(flag) is None:
+                    raise TypeError(f"'{flag}' cannot be combined with other flags with |")
         return self.__class__(value | other_value)
 
     def __and__(self, other):
@@ -1631,10 +1635,11 @@ def __and__(self, other):
         if other_value is NotImplemented:
             return NotImplemented
 
-        for flag in self, other:
-            if self._get_value(flag) is None:
-                raise TypeError(f"'{flag}' cannot be combined with other flags with &")
         value = self._value_
+        if value is None or other_value is None:
+            for flag in self, other:
+                if self._get_value(flag) is None:
+                    raise TypeError(f"'{flag}' cannot be combined with other flags with &")
         return self.__class__(value & other_value)
 
     def __xor__(self, other):
@@ -1642,10 +1647,11 @@ def __xor__(self, other):
         if other_value is NotImplemented:
             return NotImplemented
 
-        for flag in self, other:
-            if self._get_value(flag) is None:
-                raise TypeError(f"'{flag}' cannot be combined with other flags with ^")
         value = self._value_
+        if value is None or other_value is None:
+            for flag in self, other:
+                if self._get_value(flag) is None:
+                    raise TypeError(f"'{flag}' cannot be combined with other flags with ^")
         return self.__class__(value ^ other_value)
 
     def __invert__(self):
diff --git a/Misc/NEWS.d/next/Library/2026-06-02-15-44-57.gh-issue-150817.lpFCh0.rst b/Misc/NEWS.d/next/Library/2026-06-02-15-44-57.gh-issue-150817.lpFCh0.rst
new file mode 100644
index 000000000000000..42532bb0222cf4e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-02-15-44-57.gh-issue-150817.lpFCh0.rst
@@ -0,0 +1,2 @@
+Speed up the ``|``, ``&`` and ``^`` operations on :class:`enum.Flag` members.
+Patch by Bernát Gábor.