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gh-149221: Fix binomialvariate() raising ValueError when random() returns 0.0 #2

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gh-149221: Fix binomialvariate() raising ValueError when random() returns 0.0 #2

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6 changes: 5 additions & 1 deletion Lib/random.py
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Original file line number Diff line number Diff line change
Expand Up @@ -836,7 +836,11 @@ def binomialvariate(self, n=1, p=0.5):
if not c:
return x
while True:
y += _floor(_log2(random()) / c) + 1
try:
y += _floor(_log2(random()) / c) + 1
except ValueError:
# Reject case where random() returned 0.0
continue
Comment on lines +839 to +843

@chatgpt-codex-connector chatgpt-codex-connector Bot Jun 6, 2026

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P2 Badge Limit suppression to the zero draw

When a Random subclass's random() raises ValueError (for example, because its entropy source fails), this handler catches that exception as though the method returned zero and retries forever instead of propagating the failure. It also loops on invalid negative or NaN return values. Call random() outside the try and explicitly retry only when the returned value is 0.0, so unrelated errors retain their previous behavior.

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if y > n:
return x
x += 1
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11 changes: 11 additions & 0 deletions Lib/test/test_random.py
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Expand Up @@ -1154,6 +1154,17 @@ def test_binomialvariate(self):
self.assertTrue(89_000_000 <= B(100_000_000, 0.9) <= 91_000_000)


@unittest.mock.patch('random.Random.random')
def test_binomialvariate_zero_random(self, random_mock):
# gh-149221: random() can legitimately return 0.0 (its documented
# range is [0.0, 1.0)). The BG geometric branch must reject that
# draw instead of raising "math domain error" from log2(0.0).
seq = iter([0.0])
random_mock.side_effect = lambda: next(seq, 0.5)
# n*p == 2.0 < 10.0 selects the BG geometric method.
self.assertIn(random.binomialvariate(20, 0.1), range(21))


def test_von_mises_range(self):
# Issue 17149: von mises variates were not consistently in the
# range [0, 2*PI].
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-06-18-57-00.gh-issue-149221.QVCmhr.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
:meth:`random.Random.binomialvariate` no longer raises :exc:`ValueError`
when the underlying :meth:`~random.Random.random` returns ``0.0`` while sampling
with ``n * p < 10``.
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