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random.sample raises "IndexError: pop from empty list" when both "population" and "counts" are empty #130285
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Neither of these cases was tested or intended behavior, so it would be reasonable to fix them both. I'll work on a PR soonish. Thanks for the report.
Because of the possibility of breaking code, I'm -0 on backporting the edit.
Reacted by Stan UlbrychSomewhat related is the situation when
k=0and allcountsare zero:>>> random.sample('abc', k=0, counts=[0,0,0]) Traceback (most recent call last): File "<python-input-1>", line 1, in <module> random.sample('abc', k=0, counts=[0,0,0]) ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/path/to/lib/python3.14/random.py", line 428, in sample raise ValueError('Total of counts must be greater than zero') ValueError: Total of counts must be greater than zero
According to the docs:
Repeated elements can be specified one at a time or with the optional keyword-only counts parameter. For example,
sample(['red', 'blue'], counts=[4, 2], k=5)is equivalent tosample(['red', 'red', 'red', 'red', 'blue', 'blue'], k=5).So, extrapolating this to zero counts, this would be
random.sample('abc', k=0, counts=[0,0,0])is equivalent torandom.sample([], k=0)which, however, returns[].I propose we do
Line 421 in 8207454
if not cum_counts: if k == 0: return [] else: raise ValueError("Sample larger than population or is negative")Line 422 in 8207454
And add tests, what do you think @rhettinger ? I see you are planning to do thisWhat about (random.py#L424)
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 non-negative')
That would also address the
random.sample('abc', k=0, counts=[0,0,0]) == random.sample([], k=0)case.If
total == 0 and k > 0this also wouldn't change the type of exception as it would becomeValueError("Sample larger than population or is negative")instead.- added a commit that references this issue
on Feb 19, 2025 - added a commit that references this issue
on Feb 21, 2025
Bug report
Bug description:
I just encountered the situation where I used
random.samplebut both thepopulationandcountsarguments were empty (my algorithm had nothing left to choose from). So, basically this situation:Instead of the
IndexError, I expected aValueError, similar to the following situations:The docs mention that
In addition, I would expect the following to work:
similar to how it works when
countsis not specified:Not sure though what CPython's backwards-compatibility policy has to say here, since changing the exception type – or, in the second case, removing the exception altogether – might actually break someone's code...
Tested with:
CPython versions tested on:
3.14
Operating systems tested on:
Linux
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