New submission from Glyph Lefkowitz:
The purpose of 'seeding' a random number generator is usually to supply a
deterministic sequence of values starting from a known point. This works fine
if you seed random.Random with an integer. Often (for example, see Minecraft's
map generation interface) one wants to begin with a human-memorable string as
the seed, and superficially it would seem that passing a string to Random.seed
would serve exactly this use-case. In fact in its original incarnation it did.
However, since the introduction of PYTHONHASHSEED in 2.6.8, it's possible that
strings now hash to different values, and on 3.2+, they'll _always_ hash to
different values unless otherwise configured (which, as per the reason for
introducing this feature in the first place, is a security flaw).
Right now the way to work around this is to get some deterministic hash from
your string; one mechanism being a truncated SHA256 hash, for example, like
this:
Random(struct.unpack("!I", sha256(seed.encode("utf-8")).digest()[:4])[0])
but this strikes me as an obscure trick to require of someone just trying to
get their D&D character generator to produce the same values twice in a row for
unit testing.
I'm not sure what the resolution should be, but I figured I should report this
since I tripped over it.
----------
components: Library (Lib)
messages: 272137
nosy: glyph
priority: normal
severity: normal
status: open
title: Random.seed, whose purpose is purportedly determinism, behaves
non-deterministically with strings due to hash randomization
type: behavior
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue27706>
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