gh-154942: Refresh statistics.kde cache after same-length data changes

Lib/statistics.py

@@ -1060,24 +1060,27 @@ def cdf(x):
 
     else:
 
-        sample = sorted(data)
+        data_snapshot = tuple(data)
+        sample = sorted(data_snapshot)
         bandwidth = h * support
 
+        def refresh():
+            nonlocal n, data_snapshot, sample
+            updated_data = tuple(data)
+            if updated_data != data_snapshot:
+                data_snapshot = updated_data
+                sample = sorted(updated_data)
+                n = len(updated_data)
+
         def pdf(x):
-            nonlocal n, sample
-            if len(data) != n:
-                sample = sorted(data)
-                n = len(data)
+            refresh()
             i = bisect_left(sample, x - bandwidth)
             j = bisect_right(sample, x + bandwidth)
             supported = sample[i : j]
             return sum(K((x - x_i) / h) for x_i in supported) / (n * h)
 
         def cdf(x):
-            nonlocal n, sample
-            if len(data) != n:
-                sample = sorted(data)
-                n = len(data)
+            refresh()
             i = bisect_left(sample, x - bandwidth)
             j = bisect_right(sample, x + bandwidth)
             supported = sample[i : j]

Lib/test/test_statistics.py

@@ -2428,6 +2428,14 @@ def integrate(func, low, high, steps=10_000):
         data.append(100)
         self.assertGreater(f_hat(100), 0.0)
 
+        for cumulative in (False, True):
+            with self.subTest(cumulative=cumulative):
+                data = [1, 2, 3]
+                estimate = kde(data, 1.0, 'triangular', cumulative=cumulative)
+                data[0] = 5
+                expected = kde(data, 1.0, 'triangular', cumulative=cumulative)
+                self.assertEqual(estimate(1.5), expected(1.5))
+
     def test_kde_kernel_specs(self):
         # White-box test for the kernel formulas in isolation from
         # their downstream use in kde() and kde_random()

Misc/NEWS.d/next/Library/2026-08-01-23-30-00.gh-issue-154942.mF8QvS.rst

@@ -0,0 +1,2 @@
+Fix :func:`statistics.kde` with a bounded-support kernel to refresh its cached
+sample when the contents of the input data change without changing its length.