New submission from Scott Eilerman <scott.j.eiler...@gmail.com>:
I ran into a "bug" when using random.sample() in which I got some results I
didn't expect. After digging a little more, this is either a side effect of
the optimization that's made when k > 5, or I am using the function in a way
that wasn't intended. If that's the case, I would recommend calling out this
behavior in the documentation.
The crux of the issue is that, for a given seed, random.sample(choices,k) gives
the same sequence of results for k=1 to k=5, but that sequence can be different
(for the same seed) at k=6 and higher. From my initial testing this seems to
only occur when 'choices' has an even length.
Example code to reproduce this issue:
import random
seed = 199
choices = range(-10,12)
for k in range(10):
random.seed(seed)
print(random.sample(choices,k))
Example code to look at many different occurrences of this issue:
import random
choices = range(-10,12)
count = 0
for seed in range(200):
for k in range(8):
random.seed(seed)
seq1 = random.sample(choices, k)
random.seed(seed)
seq2 = random.sample(choices, k+1)
if seq1 != seq2[:-1]:
print(seed)
print(seq1)
print(seq2)
count += 1
print(f'Number of bugged results: {count}/200')
To illustrate the odd/even issue, changing choices to range(-10,11) results in
zero bugged results.
----------
assignee: docs@python
components: Documentation, Library (Lib)
messages: 314201
nosy: Scott Eilerman, docs@python
priority: normal
severity: normal
status: open
title: random.sample() behavior is unexpected/unclear from docs
type: behavior
versions: Python 3.6
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue33114>
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