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Question about floating point


On Sat, 1 Sep 2018 at 12:31, Frank Millman <frank at chagford.com> wrote:
>
> "Frank Millman"  wrote in message news:pm3l2m$kv4$1 at blaine.gmane.org...
> >
> > I know about this gotcha -
> >
> > >>> x = 1.1 + 2.2
> > >>> x
> > 3.3000000000000003
> >
> [...]
>
> I have enjoyed the discussion, and I have learnt a lot about floating point.
> Thanks to all.
>
> I have just noticed one oddity which I thought worth a mention.
>
> >>> from decimal import Decimal as D
> >>> f"{D('1.1')+D('2.2'):.60f}"
> '3.300000000000000000000000000000000000000000000000000000000000'
> >>> '{:.60f}'.format(D('1.1') + D('2.2'))
> '3.300000000000000000000000000000000000000000000000000000000000'
> >>> '%.60f' % (D('1.1') + D('2.2'))
> '3.299999999999999822364316059974953532218933105468750000000000'
> >>>
>
> The first two format methods behave as expected. The old-style '%' operator
> does not.
>
> Frank

Presumably, Decimal has a custom formatting method. The old-style %
formatting doesn't support custom per-class formatting, so %.60f
converts its argument to float and then prints it.

Paul