gh-144127: Counter.most_common() Optimization for Small n + Test Cases

Lib/collections/__init__.py

@@ -634,6 +634,13 @@ def most_common(self, n=None):
         if n is None:
             return sorted(self.items(), key=_itemgetter(1), reverse=True)
 
+        if n == 1:
+            # Optimization: use max() instead of heapq for single element
+            # max() raises ValueError on empty sequence, so check first
+            if not self:
+                return []
+            return [max(self.items(), key=_itemgetter(1))]
+
         # Lazy import to speedup Python startup time
         global heapq
         if heapq is None:

Lib/test/test_collections.py

@@ -2279,6 +2279,24 @@ def test_invariant_for_the_in_operator(self):
             self.assertTrue(elem in c)
             self.assertIn(elem, c)
 
+    def test_most_common(self):
+        c = Counter(a=5, b=3, c=5, d=2, e=0, f=-1)
+
+        self.assertEqual(c.most_common(), [('a', 5), ('c', 5), ('b', 3), ('d', 2), ('e', 0), ('f', -1)])
+        self.assertEqual(c.most_common(3), [('a', 5), ('c', 5), ('b', 3)])
+        self.assertEqual(c.most_common(0), [])
+        self.assertEqual(c.most_common(-2), [])
+        self.assertEqual(c.most_common(1), [('a', 5)])
+
+        # Test empty counter
+        empty_c = Counter()
+
+        self.assertEqual(empty_c.most_common(), [])
+        self.assertEqual(empty_c.most_common(3), [])
+        self.assertEqual(empty_c.most_common(0), [])
+        self.assertEqual(empty_c.most_common(-2), [])
+        self.assertEqual(empty_c.most_common(1), [])
+
     def test_multiset_operations(self):
         # Verify that adding a zero counter will strip zeros and negatives
         c = Counter(a=10, b=-2, c=0) + Counter()

Misc/NEWS.d/next/Library/2026-01-21-15-46-53.gh-issue-144127.aBc123.rst

@@ -0,0 +1,3 @@
+Optimize :meth:`collections.Counter.most_common` for ``n=1`` to use
+:func:`max` instead of :func:`heapq.nlargest`, improving performance for
+the common case of finding the single most common element.