[issue31689] random.choices does not work with negative weights

[Aldwin Pollefeyt]

Aldwin Pollefeyt  added the comment:

issue37624: not adding an extra O(n) step to check for unusual inputs with 
undefined meaning -- that would just impair the normal use cases for near zero 
benefit.

--
nosy: +aldwinaldwin

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[issue31689] random.choices does not work with negative weights

[Ted Whalen]

Ted Whalen  added the comment:

I think this should be reopened, as the behavior doesn't always raise an error, 
and, in fact, does something very unexpected:

Python 3.7.2 (default, Jan 13 2019, 12:50:01)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import Counter
>>> from random import choices
>>> Counter(choices("abcdefg", weights=(1,1,-1,1,1,0,1), k=1))
Counter({'a': 2569, 'b': 2514, 'e': 2487, 'g': 2430})

It's really not clear to me why supplying a negative weight for "c" should have 
any effect on "d".

--
nosy: +Ted Whalen

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[issue31689] random.choices does not work with negative weights

[David Kopec]

David Kopec  added the comment:

It's not a bug, but I agree with Allen that it could use a much more clear 
error message. I think his proposed ValueError makes a lot more sense than just 
raising an IndexError as currently occurs. This will help people debug their 
programs who don't even realize they're accidentally using negative weights to 
begin with.

--
nosy: +davecom
versions: +Python 3.7 -Python 3.6

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[issue31689] random.choices does not work with negative weights

[Raymond Hettinger]

Change by Raymond Hettinger :


--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue31689] random.choices does not work with negative weights

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I'm content with the current exception.

--
assignee:  -> rhettinger

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[issue31689] random.choices does not work with negative weights

[Allen Riddell]

Allen Riddell  added the comment:

Upon some reflection, I think raising a ValueError is the right thing to do. 
Negative weights don't have an obvious interpretation.

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[issue31689] random.choices does not work with negative weights

[Mark Dickinson]

Mark Dickinson  added the comment:

@ariddell: What behaviour did you want to see here?

It wouldn't have occurred to me to even try using `random.choices` with 
negative weights; forcing the weights to be nonnegative (with strictly positive 
sum) sounds like a natural restriction.

--
nosy: +mark.dickinson, rhettinger

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[issue31689] random.choices does not work with negative weights

[Allen Riddell]

New submission from Allen Riddell :

Code to reproduce problem:

population = list(range(10))
weights = list(-1 * w for w in range(10))
[random.choices(population, weights) for _ in range(1000)]

will raise IndexError:

358 bisect = _bisect.bisect
359 total = cum_weights[-1]
--> 360 return [population[bisect(cum_weights, random() * total)] for i 
in range(k)]
361 
362 ##  real-valued distributions  ---

IndexError: list index out of range

--
components: Library (Lib)
messages: 303683
nosy: ariddell
priority: normal
severity: normal
status: open
title: random.choices does not work with negative weights
versions: Python 3.6

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