Repository navigation
random.binomialvariate() can raise ZeroDivisionError when random() returns 0.0 #154001
Description
Bug report
Bug description:
random.binomialvariate() can raise ZeroDivisionError when random() returns 0.0 in one of the paths.
# Lib/random.py
u = random()
u -= 0.5
us = 0.5 - _fabs(u)
k = _floor((2.0 * a / us + b) * u + c) # <-- division by `us` which can be zeroRepro with a mock that forces random() to return zero:
import random
from unittest.mock import patch
with patch.object(random.Random, "random", side_effect=[0.0, 0.5, 0.5]):
random.binomialvariate(100, 0.25)
# k = _floor((2.0 * a / us + b) * u + c)
# ~~~~~~~~^~~~
# ZeroDivisionError: division by zeroFrom what I've seen, it looks like the chance of random returning zero is about 1 in 2^53... But still, zero is a valid result and custom generators may return it more often.
CPython versions tested on:
3.12, 3.13, 3.14, 3.15
Operating systems tested on:
macOS
Linked PRs
- gh-154001: Avoid division by zero in
binomialvariate#154004 - [3.15] gh-154001: Avoid division by zero in binomialvariate (GH-154004) #154050
- [3.14] gh-154001: Avoid division by zero in binomialvariate (GH-154004) #154051
- [3.13] gh-154001: Avoid division by zero in binomialvariate (GH-154004) #154052
Activity
pythongh-154001: Avoid division by zero in `binomialvariate`
pythongh-154001: Fix NEWS reference
pythongh-154001: Drop unnecessary NEWS attributes
pythongh-154001: Address review comments
gh-154001: Avoid division by zero in binomialvariate (GH-154004)
serhiy-storchaka commentedon Jul 19, 2026
Thank you for your contribution @lkk7.
[3.13] gh-154001: Avoid division by zero in binomialvariate (GH-154004)…
[3.14] gh-154001: Avoid division by zero in binomialvariate (GH-154004)…
[3.15] gh-154001: Avoid division by zero in binomialvariate (GH-154004)…