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

Richard Damon Richard at Damon-Family.org
Sun Sep 30 18:55:20 EDT 2018

On 9/30/18 6:15 PM, Greg Ewing wrote:
> Chris Angelico wrote:
>> ]That
>> means that any representable number actually has to indicate a range
>> of values centered on that value.
> That's not always true -- it depends on the source of the
> information. For example, a reading of 5 seconds on a clock
> with 1 second resolution actually represents a value between
> 5 and 6 seconds.
> So if you're fussy about rounding, you might want to round
> clock readings differently from measurements on a ruler.
Actually it could be from 4+ to 6- seconds, say the first reading is 1,
that could be anything from 1.000 to 1.999 and the second reading be 6,
that could be from 6.000 to 6.999, thus the interval be from 6.000 -
1.999 = 4.001 tp 6.999 - 1.000 = 5.999 seconds. Now if you waited for
the start time to roll over so you knew you were near 1.000, that would
be different, but from just sampling you get ranges.

Now if it was a stop watch that started at the beginning it depends on
how it presents the time, it might respond 5 for 5.000 to 5.999 seconds,
or it might intentionally round the data and say 5 from about 4.5 to 5.5.

Now, one case where there is an intentional bias to the bottom is Map
Grid Coordinate system, where you specify 1 meter resolution within a
grid with 5 digits, but if you want to specify to less precision, the
specification it to ALWAYS truncate so map coordinate 1234 represent the
range from 12340.0000 to 12349.9999

Richard Damon

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