bpo-24567: Random subnormal.diff (GH-7954) (GH-7956)

Handle subnormal weights for choices()
(cherry picked from commit ddf7171911e117aa7ad4b0f9ded4f0c3a4ca0fec)

Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/Lib/test/test_random.py b/Lib/test/test_random.py
index 10f431a..877adf1 100644
--- a/Lib/test/test_random.py
+++ b/Lib/test/test_random.py
@@ -220,6 +220,14 @@ class TestBasicOps:
         with self.assertRaises(IndexError):
             choices([], cum_weights=[], k=5)
 
+    def test_choices_subnormal(self):
+        # Subnormal weights would occassionally trigger an IndexError
+        # in choices() when the value returned by random() was large
+        # enough to make `random() * total` round up to the total.
+        # See https://bugs.python.org/msg275594 for more detail.
+        choices = self.gen.choices
+        choices(population=[1, 2], weights=[1e-323, 1e-323], k=5000)
+
     def test_gauss(self):
         # Ensure that the seed() method initializes all the hidden state.  In
         # particular, through 2.2.1 it failed to reset a piece of state used