[Python-Dev] Change in Python 3's "round" behavior

Alex Walters tritium-list at sdamon.com
Sun Sep 30 06:26:49 EDT 2018

> -----Original Message-----
> From: Python-Dev <python-dev-bounces+tritium-
> list=sdamon.com at python.org> On Behalf Of Greg Ewing
> Sent: Saturday, September 29, 2018 9:50 PM
> To: python-dev at python.org
> Subject: Re: [Python-Dev] Change in Python 3's "round" behavior
> I don't really get the statistical argument. If you're doing something
> like calculating an average and care about accuracy, why are you rounding
> the values before averaging? Why not average first and then round the
> result if you need to?

Other use case is finance, where you can end up with interest calculations
that are fractional of the base unit of currency.  US$2.345 is impossible to
represent in real currency, so it has to be rounded.  With
half-towards-even, that rounds to $2.34, and $2.355 rounds to $2.36.  It
evens out in the long run.  While that is very helpful for finance
calculations, if you are doing finance with that level of precision, you
should be using decimal instead of float anyways and decimal's round has
configurable round method.

> --
> Greg
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