gh-158232: Fix _compare_grouped_stats mutating old_group and a typo

Author: Tony123-tech

Lib/test/test_tracemalloc.py

@@ -598,6 +598,23 @@ def test_snapshot_group_by_traceback(self):
         self.assertRaises(ValueError,
                           snapshot.statistics, 'traceback', cumulative=True)
 
+    def test_compare_to_does_not_mutate_group(self):
+        # gh-158232: _compare_grouped_stats() used to pop from old_group,
+        # silently draining the caller's dict.
+        tb_a = traceback(('f.py', 1))
+        tb_b = traceback(('f.py', 2))
+        tb_c = traceback(('f.py', 3))
+
+        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),
+               tb_c: tracemalloc.Statistic(tb_c, 10, 1)}
+        old_copy = dict(old)
+
+        tracemalloc._compare_grouped_stats(old, new)
+
+        self.assertEqual(old, old_copy)
+
     def test_snapshot_group_by_cumulative(self):
         snapshot, snapshot2 = create_snapshots()
         tb_0 = traceback_filename('<unknown>')

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 = {}