[Python-Dev] Return type of round, floor, and ceil in 2.6

[Guido van Rossum]

On Jan 4, 2008 11:31 AM, Tim Peters <tim.peters at gmail.com> wrote:
> [skip at pobox.com]
> > Thanks for the pointer.  Given that it's [round-to-even[ been an ASTM
> > standard since 1940 and apparently in fairly common use since the
> > early 1900s, I wonder why it's not been more widely used in the past
> > in programming languages.
>
> Because "add a half and chop" was also in wide use for even longer, is
> also (Wikipedia notwithstanding) part of many standards (for example,
> the US IRS requires it if you do your taxes under the "round to whole
> dollars" option), and-- probably the real driver --is a little cheaper
> to implement for "normal sized" floats.  Curiously, round-to-nearest
> can be unboundedly more expensive to implement in some obscure
> contexts when floats can have very large exponents (as they can in
> Python's "decimal" module -- this is why the proposed decimal standard
> allows operations like "remainder-near" to fail if applied to inputs
> that are "too far apart":
>
>     http://www2.hursley.ibm.com/decimal/daops.html#footnote.8
>
> The "secret reason" is that it can be unboundedly more expensive to
> determine the last bit of the quotient (to see whether it's even or
> odd) than to determine an exact remainder).

Wow. Do you have an opinion as to whether we should adopt
round-to-even at all (as a default)?

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)