[Python-ideas] Deprecate the round builtin

Steven D'Aprano steve at pearwood.info
Thu Sep 27 01:07:40 CEST 2012

On 27/09/12 06:02, Mark Dickinson wrote:
> On Wed, Sep 26, 2012 at 8:36 PM, M.-A. Lemburg<mal at egenix.com>  wrote:
>> It's actually quite common in finance and time calculations to
>> round to the nearest say basis point, cent or say micro second
>> in calculations
> That's exactly where the problems creep in, though.  Naive users
> expect rounding to give 'correct' results for decimal halfway cases,
> and then are surprised when it doesn't.
>>>> round(2.675, 2)
> 2.67
> So you end up explaining again and again that computing binary
> approximations to decimal rounds of binary approximations of decimal
> halfway cases is a bad idea.

Pretty much *everything* about binary floats is surprising to people
who expect decimal semantics. Unless we're going to make Decimal the
default floating point type, and shift binary floats to a module, I
don't see any way around that, and a particularly don't see any reason
to single round() out as more confusing than any of the other float

Speaking from my experience on the tutor@ and python-list at python.org
mailing lists, I believe that problems with round are vanishingly rare,
probably an order of magnitude fewer than "why doesn't Python add my
two floats correctly?" type questions.


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