From 91bb7e0e3f47cdd09af742c62dc56b3e4c193b0c Mon Sep 17 00:00:00 2001
From: Joekrry <joerkerry@gmail.com>
Date: Tue, 29 Sep 2026 00:09:33 +0100
Subject: [PATCH] gh-158234: Fixed tracemalloc._compare_grouped_stas mutating
 old group. _compare_grouped_stats called old_group.pop() in a loop which
 modified the callers dict in place.

---
 Lib/test/test_tracemalloc.py                       | 14 ++++++++++++++
 Lib/tracemalloc.py                                 |  3 ++-
 ...-09-28-00-00-00.gh-issue-158234.tracemalloc.rst |  3 +++
 3 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-09-28-00-00-00.gh-issue-158234.tracemalloc.rst

diff --git a/Lib/test/test_tracemalloc.py b/Lib/test/test_tracemalloc.py
index ee02fbd16af0c4e..f55528cecfcb5f3 100644
--- a/Lib/test/test_tracemalloc.py
+++ b/Lib/test/test_tracemalloc.py
@@ -626,6 +626,20 @@ def test_snapshot_group_by_cumulative(self):
             tracemalloc.Statistic(tb_a_5, 2, 1),
         ])
 
+    def test_compare_grouped_stats_no_mutation(self):
+        tb_a = traceback_lineno('a.py', 1)
+        tb_b = traceback_lineno('b.py', 1)
+        old = {
+            tb_a: tracemalloc.Statistic(tb_a, 100, 2),
+            tb_b: tracemalloc.Statistic(tb_b, 50, 1),
+        }
+        new = {
+            tb_a: tracemalloc.Statistic(tb_a, 200, 3),
+        }
+        old_copy = dict(old)
+        tracemalloc._compare_grouped_stats(old, new)
+        self.assertEqual(old, old_copy)
+
     def test_trace_format(self):
         snapshot, snapshot2 = create_snapshots()
         trace = snapshot.traces[0]
diff --git a/Lib/tracemalloc.py b/Lib/tracemalloc.py
index cec99c59700fe05..426981ebe97b4de 100644
--- a/Lib/tracemalloc.py
+++ b/Lib/tracemalloc.py
@@ -119,6 +119,7 @@ def _sort_key(self):
 
 def _compare_grouped_stats(old_group, new_group):
     statistics = []
+    old_group = dict(old_group)
     for traceback, stat in new_group.items():
         previous = old_group.pop(traceback, None)
         if previous is not None:
@@ -478,7 +479,7 @@ def _group_by(self, key_type, cumulative):
         if key_type not in ('traceback', 'filename', 'lineno'):
             raise ValueError("unknown key_type: %r" % (key_type,))
         if cumulative and key_type not in ('lineno', 'filename'):
-            raise ValueError("cumulative mode cannot by used "
+            raise ValueError("cumulative mode cannot be used "
                              "with key type %r" % key_type)
 
         stats = {}
diff --git a/Misc/NEWS.d/next/Library/2026-09-28-00-00-00.gh-issue-158234.tracemalloc.rst b/Misc/NEWS.d/next/Library/2026-09-28-00-00-00.gh-issue-158234.tracemalloc.rst
new file mode 100644
index 000000000000000..e4f360e0154217f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-28-00-00-00.gh-issue-158234.tracemalloc.rst
@@ -0,0 +1,3 @@
+Fix :func:`tracemalloc._compare_grouped_stats` mutating its *old_group*
+argument. Also fix a typo in a :mod:`tracemalloc` error message:
+"cannot by used" → "cannot be used".