8000 gh-130285: Fix handling of zero or empty counts in random.sample() by rhettinger · Pull Request #130291 · python/cpython · GitHub
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gh-130285: Fix handling of zero or e 8000 mpty counts in random.sample() #130291

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Merged
merged 6 commits into from
Feb 21, 2025
Merged

gh-130285: Fix handling of zero or empty counts in random.sample() #130291

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ff2db57
gh-130285: Support zero counts in random.sample()
rhettinger Feb 19, 2025
85ade39
Add test cases for zero counts or empty counts
rhettinger Feb 19, 2025
dfb8144
Fold try/except into a conditional expression
rhettinger Feb 19, 2025
76749c5
Let the user focus on the source of the negative total
rhettinger Feb 20, 2025
27f6cdb
Add blurb
rhettinger Feb 21, 2025
a2c4837
Merge branch 'main' into sample_zero_counts
rhettinger Feb 21, 2025
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6 changes: 3 additions & 3 deletions Lib/random.py
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Original file line number Diff line number Diff line change
Expand Up @@ -421,11 +421,11 @@ def sample(self, population, k, *, counts=None):
cum_counts = list(_accumulate(counts))
if len(cum_counts) != n:
raise ValueError('The number of counts does not match the population')
total = cum_counts.pop()
total = cum_counts.pop() if cum_counts else 0
if not isinstance(total, int):
raise TypeError('Counts must be integers')
if total <= 0:
raise ValueError('Total of counts must be greater than zero')
if total < 0:
raise ValueError('Counts must be non-negative')
selections = self.sample(range(total), k=k)
bisect = _bisect
return [population[bisect(cum_counts, s)] for s in selections]
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16 changes: 14 additions & 2 deletions Lib/test/test_random.py
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Expand Up @@ -225,15 +225,27 @@ def test_sample_with_counts(self):
sample(['red', 'green', 'blue'], counts=10, k=10) # counts not iterable
with self.assertRaises(ValueError):
sample(['red', 'green', 'blue'], counts=[-3, -7, -8], k=2) # counts are negative
with self.assertRaises(ValueError):
sample(['red', 'green', 'blue'], counts=[0, 0, 0], k=2) # counts are zero
with self.assertRaises(ValueError):
sample(['red', 'green'], counts=[10, 10], k=21) # population too small
with self.assertRaises(ValueError):
sample(['red', 'green', 'blue'], counts=[1, 2], k=2) # too few counts
with self.assertRaises(ValueError):
sample(['red', 'green', 'blue'], counts=[1, 2, 3, 4], k=2) # too many counts

# Cases with zero counts match equivalents without counts (see gh-130285)
self.assertEqual(
sample('abc', k=0, counts=[0, 0, 0]),
sample([], k=0),
)
self.assertEqual(
sample([], 0, counts=[]),
sample([], 0),
)
with self.assertRaises(ValueError):
sample([], 1, counts=[])
with self.assertRaises(ValueError):
sample('x', 1, counts=[0])

def test_choices(self):
choices = self.gen.choices
data = ['red', 'green', 'blue', 'yellow']
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2025-02-21-10-32-05.gh-issue-130285.C0fkh7.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Fix corner case for :func:`random.sample` allowing the *counts* parameter to
specify an empty population. So now, ``sample([], 0, counts=[])`` and
``sample('abc', k=0, counts=[0, 0, 0])`` both give the same result as
``sample([], 0)``.
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