[Python-Dev] Decimal data type issues

Michael Chermside mcherm at mcherm.com
Tue Apr 13 15:31:16 EDT 2004

Facundo Batista writes:
> Exponent Maximum
> ----------------
> The exponent is an integer, and in the actual implementation exists a
> maximum
> value::
> ABSOLUTE_MAX_EXP = 999999999
> ABSOLUTE_MIN_EXP = -999999999
> The issue is that this limit is artificial:
> So, should we impose an artificial limit to the exponent?
> This is important, as there're several cases where this 
> maximums are checked
> and exceptions raised and/or the numbers get changed.

I strongly prefer for my programming language not to impose
aribtrary limits, however, I can't see myself using decimal
to represent numbers with 1 billion digits. So IMHO, I would
avoid the limit if possible, but if imposing the limit
permitted substantial code simplification or performance
improvements then I wouldn't complain. Much.

> New operations
> --------------
> Tim Peters found I missed three operations required by the 
> standard.

Sure. Your proposal sounds good.

> 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.from_float(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.

-- Michael Chermside

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