Author rhettinger
Recipients facundobatista, gvanrossum, jyasskin, mark.dickinson, rhettinger
Date 2008-01-10.01:58:43
SpamBayes Score 0.209008
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Message-id <1199930327.74.0.944846044889.issue1682@psf.upfronthosting.co.za>
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> Decimal.from_rational(Rat(1, 3)) wouldn't be lossless

It should be implemented as Decimal(1)/Decimal(3) and then let the
context handle any inexactness.

> Rational.from_decimal is easier than from_float.

Yes.  Just use Decimal.as_tuple() to get the integer components.

> Then Decimal.from_rational() could rely on just numbers.
> Rational, so it would be independent of this module. 
>Is that a method you'd want on Decimal anyway? 

Instead, just expand the decimal constructor to accept a rational input.

> Regarding float->rational, I propose to refuse Rational(1.1)
> for the same reasons as Decimal(1.1) is refused,

+1 

> but to add a separate constructor (a class method?) that 
> converts a float to a rational exactly (as long as it 
> isn't nan or inf), large denominators be damned. This 
> can help people who are interested in taking floating
> point numbers apart. 

Here's how to pick the integer components out of a float:

   mant, exp = math.frexp(x)
   mant, exp = int(mant * 2.0 ** 53), exp-53


>> * Likewise, follow decimal's lead in avoiding all 
>> automatic coercions from floats:  
>>    Rational(3,10) + 0.3 for example.  The two don't mix.

> I'm reluctant to remove the fallback to float,

You will need some plan to scale-down long integers than exceed the
range of floats (right shifting the numerator and denominator until they
fit).
History
Date User Action Args
2008-01-10 01:58:48rhettingersetspambayes_score: 0.209008 -> 0.209008
recipients: + rhettinger, gvanrossum, facundobatista, mark.dickinson, jyasskin
2008-01-10 01:58:47rhettingersetspambayes_score: 0.209008 -> 0.209008
messageid: <1199930327.74.0.944846044889.issue1682@psf.upfronthosting.co.za>
2008-01-10 01:58:46rhettingerlinkissue1682 messages
2008-01-10 01:58:43rhettingercreate