From d5ddad2a75755472f3e6a351bf98f3fb40e9d61a Mon Sep 17 00:00:00 2001
From: Taeknology <20297177+Taeknology@users.noreply.github.com>
Date: Sat, 1 Aug 2026 23:30:25 +0900
Subject: [PATCH] gh-154942: Refresh statistics.kde cache after data changes

---
 Lib/statistics.py                             | 21 +++++++++++--------
 Lib/test/test_statistics.py                   |  8 +++++++
 ...-08-01-23-30-00.gh-issue-154942.mF8QvS.rst |  2 ++
 3 files changed, 22 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-08-01-23-30-00.gh-issue-154942.mF8QvS.rst

diff --git a/Lib/statistics.py b/Lib/statistics.py
index 758b5b58848fb98..edcd7588565cc08 100644
--- a/Lib/statistics.py
+++ b/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]
diff --git a/Lib/test/test_statistics.py b/Lib/test/test_statistics.py
index 700c5ac304f7179..d700fac42a83d88 100644
--- a/Lib/test/test_statistics.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Library/2026-08-01-23-30-00.gh-issue-154942.mF8QvS.rst b/Misc/NEWS.d/next/Library/2026-08-01-23-30-00.gh-issue-154942.mF8QvS.rst
new file mode 100644
index 000000000000000..67e29f3adad83c1
--- /dev/null
+++ b/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.