Why not FP for Money?
Alex Martelli
aleaxit at yahoo.com
Wed Sep 22 18:35:44 CEST 2004
Christos TZOTZIOY Georgiou <tzot at sil-tec.gr> wrote:
...
> amount= tp(repr(amount))
> vat= amount * tp('0.18')
> total= amount + vat
...
> import decimal
> quote(decimal.Decimal)
> quote(float)
>
> Run it with Python 2.4. See what goes wrong? Put the numbers in an
> Excel sheet too.
The problem is pretty clear, but it shows up with decimal just as well
as with float: each of 'amount' and 'vat' (in Euro) has a fraction of a
eurocent that is less than 0.5 (about 0.4 eurocents each), and you don't
round them to eurocents before summing, so the total ends up with a
fraction of a eurocent that is _more_ than 0.5. So the final numbers
show, identically badly with both decimals and floats:
Amount: USD 2025.00 EUR 1710.30
Vat : USD 364.50 EUR 307.85
Total : USD 2389.50 EUR 2018.16
where the 1-eurocent discrepancy between 'Total' and the easily
eyeballed 'Amount'+'Vat' shows up quite nastily. (A case of "2+2 DOES
equal 5, for sufficiently large values of 2", to quote Anna [presumably
quoting somebody else...]).
> Of course, one can take extra precautions to always have correct numbers
> when working with floats and monetary amounts (eg. I did that for the
> Excel sheet the sales dpt used). Why bother, though, if one has the
> Decimal type?
It seems to me that you just showed that decimal is no panacea: you
still have to know where to round and to which precision. decimal is a
great tool but is not and cannot be a magic wand...
Alex
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