[Python-Dev] Decimal data type issues (original) (raw)

Bob Ippolito bob at redivi.com
Tue Apr 13 16:01:22 EDT 2004

On Apr 13, 2004, at 3:31 PM, Michael Chermside wrote:

Facundo Batista writes:

Hash behaviour -------------- [...] In the PEP I wrote that Decimal must be hashable. But what hash should it give?

Should the following be true?:: hash(Decimal(25) == hash(25) hash(Decimal.fromfloat(25.35) == hash(25.35) hash(Decimal('-33.8')) == hash(-33.8) I don't think so. I think that hash(Decimal(...)) just should return a different value in each case, but no the same value that other data types. I disagree. If x == y, then that should imply that hash(x) == hash(y). So hash(Decimal(25)) == hash(25) had better be true. On the other hand, hash(Decimal('-33')) CANNOT equal hash(Decimal('-33')), since the latter must equal hash(-33). And I'm not particularly concerned about floats... if it's easy to make the hashes equal for those few numbers which can be expressed EXACTLY as both a (binary) float and a (decimal) Decimal, then that's be "kinda nice", but since there are so few values which can be expressed exactly as both a float and a Decimal, I don't see it as a big issue.

hash(float(1.0)) == hash(1) == hash(1L) True

Since this is already true, hash(Decimal.from_float(1.0)), hash(Decimal(1)), hash(Decimal(1L)), hash(Decimal('1.0')), hash(Decimal('1')), hash(Decimal(u'1.0')), etc. better be equal as well, whatever that takes.

As far as arbitrary floats hashing equally, I don't really care, but the simple ones definitely should. Hopefully at some point, "float constants" (as in coming from source code or user input) in Python will actually be Decimal representations, so you don't lose precision until you ask for it.

-bob

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