Message 59639 - Python tracker

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Author	rhettinger
Recipients	facundobatista, gvanrossum, jyasskin, mark.dickinson, rhettinger
Date	2008-01-10.00:29:16
SpamBayes Score	0.10358839
Marked as misclassified	No
Message-id	<[email protected]>
In-reply-to

Content
> If it's lossless, why not just allow >Decimal(Rational(...)) and Rational(Decimal(...))? Those conversions are fine. It's the float<-->rational conversions that are problematic. There are exact float to rational conversions using math.frexp() but I don't think the results tend to match what users expect (since 1.1 isn't exactly represented). Also, there may be overflow issues with trying to go from rationals to floats when the denomintor is very large. I think the history of rational modules is that simplistic implementations get built and then the writers find that exploding denominators limit their usefulness for anything other than trivial problems. The solution is to limit denominators but that involves less trivial algorithms and a more sophisticated API.

> If it's lossless, why not just allow 
>Decimal(Rational(...)) and Rational(Decimal(...))?

Those conversions are fine.  

It's the float<-->rational conversions that are problematic.  There are
exact float to rational conversions using math.frexp() but I don't think
the results tend to match what users expect (since 1.1 isn't exactly
represented).  Also, there may be overflow issues with trying to go from
rationals to floats when the denomintor is very large.

I think the history of rational modules is that simplistic
implementations get built and then the writers find that exploding
denominators limit their usefulness for anything other than trivial
problems.  The solution is to limit denominators but that involves less
trivial algorithms and a more sophisticated API.

History
Date	User	Action	Args
2008-01-10 00:29:17	rhettinger	set	spambayes_score: 0.103588 -> 0.10358839 recipients: + rhettinger, gvanrossum, facundobatista, mark.dickinson, jyasskin
2008-01-10 00:29:17	rhettinger	set	spambayes_score: 0.103588 -> 0.103588 messageid: <[email protected]>
2008-01-10 00:29:16	rhettinger	link	issue1682 messages
2008-01-10 00:29:16	rhettinger	create