[3.13] gh-154001: Avoid division by zero in binomialvariate (GH-154004) (GH-154052)

(cherry picked from commit 1f13740)

Co-authored-by: Łukasz <lukaszlapinski7@gmail.com>

Author: miss-islington

Lib/random.py

@@ -849,7 +849,11 @@ 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
 

Lib/test/test_random.py

@@ -1101,6 +1101,14 @@ 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

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.