[Python-Dev] builtin round 2.7 (original) (raw)

Mark Dickinson dickinsm at gmail.com
Sat Aug 7 12:58:39 CEST 2010

2010/8/7 Kristján Valur Jónsson <kristjan at ccpgames.com>:

Hi there. [...] But it appears that the builtin round() method also changed.  Whereas I see the changing of floating point representation in string formatting as not being very serious, why did the arithmetic function round() have to change?

This was part of the short float repr changes that were backported from 3.1. The round function in 2.7 (on most platforms; barring bugs) always gives correctly rounded results; in 2.6 it was a bit less predictable in halfway cases.

One reason to want round to be correctly rounded is to ensure that the repr of the result is always the 'short' one; this means that repr(round(x, 2)) will never produce results like '0.23000000000000001'. If the round function hadn't been updated to be correctly rounded then this wouldn't be true.

Another reason is to make sure that string formatting and the round function finally agree with each other: both are doing the same job of rounding to some nearest decimal representation (except that one returns a float, while the other returns a string), so the results should be comparable; the discrepancy between these two operations has confused users in the past. Unfortunately, the agreement isn't quite complete, since round in 2.7 continues to use round-half-away-from-zero for actual exact halfway cases, while string formatting uses round-half-to-even (so e.g. round(0.125, 2) gives (a binary approximation to) 0.13, while format(0.125, '.2f') gives '0.12'). In 3.x they both use round-half-to-even.

The only place where people are likely to notice that the round result has changed is in halfway cases, for example round(12.385, 2) (which, of course, thanks to the usual binary floating-point issues, is only actually an approximation to a halfway case). In general, if you're expecting predictable results from decimal rounding of binary approximations to decimal halfway cases then you're asking for trouble. For predictable rounding, use the decimal module.

I recently added some text to the floating-point section of the 2.7 tutorial to help explain these round problems.

I don‘t see this mentioned in the release notes and was initially a bit puzzled by it.

True; I don't see it in the whatsnew document either. It's in Misc/NEWS, though. (Search for Issue 7117; there are several entries).

Mark

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