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Nov 23, 2019
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bpo-38881: choices() raises ValueError when all weights are zero #17362

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bb8d496
bpo-38881: choices() raises ValueError when all weights are zero
rhettinger Nov 23, 2019
17c553d
Update docs
rhettinger Nov 23, 2019
9091dba
Focus on the case where all weights are zero.
rhettinger Nov 23, 2019
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8 changes: 6 additions & 2 deletions Doc/library/random.rst
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Expand Up @@ -165,8 +165,9 @@ Functions for sequences

The *weights* or *cum_weights* can use any numeric type that interoperates
with the :class:`float` values returned by :func:`random` (that includes
integers, floats, and fractions but excludes decimals). Weights are
assumed to be non-negative.
integers, floats, and fractions but excludes decimals). Behavior is
undefined if any weight is negative. A :exc:`ValueError` is raised if all
weights are zero.

For a given seed, the :func:`choices` function with equal weighting
typically produces a different sequence than repeated calls to
Expand All @@ -177,6 +178,9 @@ Functions for sequences

.. versionadded:: 3.6

.. versionchanged:: 3.9
Raises a :exc:`ValueError` if all weights are zero.


.. function:: shuffle(x[, random])

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4 changes: 3 additions & 1 deletion Lib/random.py
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Expand Up @@ -413,8 +413,10 @@ def choices(self, population, weights=None, *, cum_weights=None, k=1):
raise TypeError('Cannot specify both weights and cumulative weights')
if len(cum_weights) != n:
raise ValueError('The number of weights does not match the population')
bisect = _bisect
total = cum_weights[-1] + 0.0 # convert to float
if total <= 0.0:
raise ValueError('Total of weights must be greater than zero')
bisect = _bisect
hi = n - 1
return [population[bisect(cum_weights, random() * total, 0, hi)]
for i in _repeat(None, k)]
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5 changes: 5 additions & 0 deletions Lib/test/test_random.py
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Expand Up @@ -241,6 +241,11 @@ def test_choices_subnormal(self):
choices = self.gen.choices
choices(population=[1, 2], weights=[1e-323, 1e-323], k=5000)

def test_choices_with_all_zero_weights(self):
# See issue #38881
with self.assertRaises(ValueError):
self.gen.choices('AB', [0.0, 0.0])

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
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2019-11-22-20-03-46.bpo-38881.7HV1Q0.rst
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@@ -0,0 +1 @@
random.choices() now raises a ValueError when all the weights are zero.