Tim Peters <t...@python.org> added the comment:
I agree: we "shouldn't have" documented anything about hash codes beyond the invariants needed to guarantee they work for their intended purpose, chiefly that x == y implies hash(x) == hash(y). Which leads to your other question ;-) That invariant is tricky to maintain across a pile of distinct but comparable numeric types! Whatever we return for an int needs also to be what's returned for a float that happens to have the same value, and whatever we return for a float needs also to be what's returned for a fraction.Fraction with the same value, and so on. So Python uses a single numeric hashing algorithm defined on mathematical rationals, described here: https://docs.python.org/3.8/library/stdtypes.html#hashing-of-numeric-types Presumably that's documented so that people defining their own numeric types have a clue about how to implement compatible hash codes. Anyway, that algorithm uses a large fixed prime P (different on 32- and 64-bit boxes), and uses operations modulo P. That's why the full bit width isn't used. And not just any prime P, it picks one for which computing the remainder mod P is especially efficient. 2**61 - 1 is as good as it gets on 64 bit boxes. I didn't pick this algorithm (I suspect Mark did), but I certainly approve of it. It's clever and general enough to apply to any plausible subset-of-real numeric type short of the constructive reals (where equality isn't even theoretically decidable, so "x == y implies ..." can't get off the ground ;-) ). ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue37807> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
