From fb2d736136c324f80975ac4c4e42570e9080427c Mon Sep 17 00:00:00 2001
From: Jonathan Baldie <jon@jonbaldie.com>
Date: Mon, 5 Oct 2026 15:15:03 +0100
Subject: [PATCH] gh-158856: Avoid a linear scan per member when creating an
 Enum

`_hashable_values_` is a list, so checking it for each new member made
class creation quadratic. Skip the check when the value was just added
to `_value2member_map_`, since it can't be in the list yet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 Lib/enum.py                                   |  8 ++++--
 Lib/test/test_enum.py                         | 28 +++++++++++++++++++
 ...-10-05-15-20-00.gh-issue-158856.Qe7nVb.rst |  2 ++
 3 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-05-15-20-00.gh-issue-158856.Qe7nVb.rst

diff --git a/Lib/enum.py b/Lib/enum.py
index 076aa18a02fd20a..432a7f79ac9afeb 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -316,8 +316,12 @@ def __set_name__(self, enum_class, member_name):
             # This may fail if value is not hashable. We can't add the value
             # to the map, and by-value lookups for this value will be
             # linear.
-            enum_class._value2member_map_.setdefault(value, enum_member)
-            if value not in enum_class._hashable_values_:
+            value2member_map = enum_class._value2member_map_
+            size = len(value2member_map)
+            value2member_map.setdefault(value, enum_member)
+            # A new key can't already be in the list, so skip the linear scan.
+            if (len(value2member_map) != size
+                    or value not in enum_class._hashable_values_):
                 enum_class._hashable_values_.append(value)
         except TypeError:
             # keep track of the value in a list so containment checks are quick
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 447f847f33da938..7570e0c3db31aa0 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -3530,6 +3530,34 @@ class Directions(Enum):
             UNRESTRICTED = frozenset({"sc", "cs"})
         self.assertIs(Directions({"sc"}), Directions.DOWN_ONLY)
 
+    def test_hashable_values_creation_work(self):
+        class CountedInt(int):
+            comparisons = 0
+            __hash__ = int.__hash__
+
+            def __eq__(self, other):
+                type(self).comparisons += 1
+                return super().__eq__(other)
+
+        size = 200
+        values = [CountedInt(i) for i in range(size)]
+        CountedInt.comparisons = 0
+        Big = Enum('Big', [(f'M{i}', v) for i, v in enumerate(values)])
+        self.assertLess(CountedInt.comparisons, 4 * size)
+        self.assertEqual(len(Big), size)
+        self.assertEqual(Big._hashable_values_, values)
+        self.assertIn(values[-1], Big)
+
+    def test_hashable_values_with_aliases(self):
+        class Color(Enum):
+            RED = 1
+            GREEN = 2
+            CRIMSON = 1
+            BLUE = 3
+        self.assertEqual(Color._hashable_values_, [1, 2, 3])
+        self.assertIs(Color.CRIMSON, Color.RED)
+        self.assertIn(1, Color)
+
 
 class TestOrder(unittest.TestCase):
     "test usage of the `_order_` attribute"
diff --git a/Misc/NEWS.d/next/Library/2026-10-05-15-20-00.gh-issue-158856.Qe7nVb.rst b/Misc/NEWS.d/next/Library/2026-10-05-15-20-00.gh-issue-158856.Qe7nVb.rst
new file mode 100644
index 000000000000000..f16f5141bcec347
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-05-15-20-00.gh-issue-158856.Qe7nVb.rst
@@ -0,0 +1,2 @@
+Speed up creation of :class:`~enum.Enum` classes with many members: adding a
+member no longer scans the list of every value added before it.