Add optional weights parameter to random.sample()

[rhettinger]

BPO	40541
Nosy	@tim-one, @rhettinger, @mdickinson
PRs	bpo-40541: Add optional *counts* parameter to random.sample() #19970

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-05-08.14:53:54.309>
created_at = <Date 2020-05-06.22:35:20.284>
labels = ['type-feature', 'library', '3.9']
title = 'Add optional weights parameter to random.sample()'
updated_at = <Date 2020-05-08.14:53:54.309>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2020-05-08.14:53:54.309>
actor = 'rhettinger'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-08.14:53:54.309>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2020-05-06.22:35:20.284>
creator = 'rhettinger'
dependencies = []
files = []
hgrepos = []
issue_num = 40541
keywords = ['patch']
message_count = 4.0
messages = ['368307', '368333', '368374', '368445']
nosy_count = 3.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson']
pr_nums = ['19970']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40541'
versions = ['Python 3.9']

[rhettinger]

I've seen multiple requests for this and it isn't obvious how to do it with the existing tooling.

The example currently given in the recipes section isn't scalable because it involves expanding the population into a big list with repeated elements:

      sample(['x', 'x', 'x', 'x', 'y', 'y', 'z'], k=5)

Example task: Given an urn with 8 red balls, 2 white balls, and 3 green balls, choose ten without replacement:

    >>> population = ['red', 'blue', 'green']
    >>> weights =    [  8,      5,      3   ]
    >>> sample(population, weights=weights, k=10)
    ['red', 'green', 'blue', 'red', 'green', 'blue', 'red', 'blue', 'red', 'blue']

I could also add *cum_weights* as an optional optimization but think it best to wait until someone asks for it ;-)

[mdickinson]

+1 for the functionality.

How about "counts" instead of "weights"?

I found the name "weights" misleading - my first thought was that this would be doing a weighted sampling without replacement (like NumPy's random.choice(..., replace=False, p=weights)).

Of course, now I've read the docs, I know better.

[rhettinger]

How about "counts" instead of "weights"?

That makes sense.

[rhettinger]

New changeset 81a5fc3 by Raymond Hettinger in branch 'master':
bpo-40541: Add optional *counts* parameter to random.sample() (GH-19970)
81a5fc3