From 40dfcc17e4f694960a1c4ca636d8d50f6d784ae4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C5=81ukasz?= <lukaszlapinski7@gmail.com>
Date: Sun, 19 Jul 2026 08:06:09 +0200
Subject: [PATCH] gh-154001: Avoid division by zero in binomialvariate
 (GH-154004) (cherry picked from commit
 1f1374009b681814642ca199136815b223fd90ae)
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Co-authored-by: Łukasz <lukaszlapinski7@gmail.com>
---
 Lib/random.py                                             | 6 +++++-
 Lib/test/test_random.py                                   | 8 ++++++++
 .../2026-07-18-17-09-49.gh-issue-154001.3brrUv.rst        | 2 ++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-18-17-09-49.gh-issue-154001.3brrUv.rst

diff --git a/Lib/random.py b/Lib/random.py
index 726a71e782893c8..0e6a183d179f488 100644
--- a/Lib/random.py
+++ b/Lib/random.py
@@ -861,7 +861,11 @@ def binomialvariate(self, n=1, p=0.5):
             u = random()
             u -= 0.5
             us = 0.5 - _fabs(u)
-            k = _floor((2.0 * a / us + b) * u + c)
+            try:
+                k = _floor((2.0 * a / us + b) * u + c)
+            except ZeroDivisionError:
+                # Reject case where random() returned 0.0
+                continue
             if k < 0 or k > n:
                 continue
             v = random()
diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index 0217ebd132b1104..3739fe0ac393b3c 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -1074,6 +1074,14 @@ def test_avg_std(self):
             self.assertAlmostEqual(s2/(N-1), sigmasqrd, places=2,
                                    msg='%s%r' % (variate.__name__, args))
 
+    def test_binomialvariate_btrs_random_zero(self):
+        for p, expected in ((0.25, 25), (0.75, 75)):
+            with self.subTest(p=p):
+                g = random.Random()
+                with unittest.mock.patch.object(
+                        g, 'random', side_effect=(0.0, 0.5, 0.5)):
+                    self.assertEqual(g.binomialvariate(100, p), expected)
+
     def test_constant(self):
         g = random.Random()
         N = 100
diff --git a/Misc/NEWS.d/next/Library/2026-07-18-17-09-49.gh-issue-154001.3brrUv.rst b/Misc/NEWS.d/next/Library/2026-07-18-17-09-49.gh-issue-154001.3brrUv.rst
new file mode 100644
index 000000000000000..ff019aa36188479
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-18-17-09-49.gh-issue-154001.3brrUv.rst
@@ -0,0 +1,2 @@
+Fix :func:`random.binomialvariate` raising :exc:`ZeroDivisionError`
+when :func:`random.random` returns zero.