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random module binomialvariate function returns wrong results #149221
Description
Bug report
Bug description:
poc:
"""
PoC: binomialvariate crash caused by random() returning 0.0.
Run: python bug_demo.py
"""
from math import log2, floor
def simulate_crash(c, n):
"""Simulate the buggy BG branch with random() hardcoded to 0.0.
Records the iteration count at which the crash occurs."""
x = y = 0
iter_count = 0
while True:
iter_count += 1
rand_val = 0.0 # simulate the rare event of hitting exactly 0.0
try:
log_rand = log2(rand_val)
except ValueError as e:
print(f"Crashed at iteration {iter_count}: {e}")
return None, iter_count
y += floor(log_rand / c) + 1
if y > n:
return x, iter_count
x += 1
if __name__ == "__main__":
c = log2(0.5) # -1.0
result, count = simulate_crash(c, n=1)
if result is None:
print(f"Program crashed after {count} iteration(s) (crashed immediately).")
else:
print(f"Result {result}, loop ran {count} iteration(s)")Actual behavior:
· When random() returns exactly 0.0 (which is within the documented range [0.0, 1.0)), _log2(0.0) raises a ValueError: math domain error, crashing the program.
Expected behavior:
· The function should never crash on a valid random value.
· The returned values should accurately follow the binomial distribution with parameters n and p.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Other, Linux
Linked PRs
- gh-149221:Fix binomialvariate Function for random module #149222
- gh-149221: Handle corner case in random.binomialvariate() #149266
- [3.14]gh-149221:Fix binomialvariate Function for random module #149276
- gh-149221: Minor comment edit #149278
- [3.13] gh-149221:Fix binomialvariate Function for random module #149279
- [3.14] gh-149221: Sync random.py with main branch #149288
- [3.14] gh-149221: Add missing
test_binomialvariate_log_zero#154879
Activity
[-]random mod `binomialvariate` function returns wrong results[/-][+]random module `binomialvariate` function returns wrong results[/+]rhettinger commentedon May 2, 2026
This looks like an LLM generated report.
Can you explain this part, "Even when random() never returns 0.0, the generated distribution is wrong – it does not match the intended binomial distribution." That doesn't make any sense.
lighting9999 commentedon May 2, 2026
I'm sorry . I'm wirte error. I solve it.
gh-149221:Fix binomialvariate Function for random module (gh-149222)
[3.14]gh-149221:Fix binomialvariate Function for random module (gh-14…
gh-149221: Minor comment edit (gh-149278)
[3.13]pythongh-149221:Fix binomialvariate Function for random module
[3.14] gh-149221: Sync random.py with main branch (#149288)
[3.14] gh-149221: Add missing `test_binomialvariate_log_zero` (#154879)
[3.13] gh-149221:Fix binomialvariate Function for random module (#149279