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Oct 24, 2009, 11:15 AM
Post #6 of 22
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[issue7117] Backport py3k float repr to trunk
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Mark Dickinson <dickinsm [at] gmail> added the comment:
One issue occurs to me: should the backport change the behaviour of the
round function?
In py3k, round consistently uses round-half-to-even for halfway cases.
In trunk, round semi-consistently uses round-half-away-from-zero (and
this is documented). E.g., round(1.25, 1) will give 1.2 in py3k and
(usually) 1.3 in trunk.
I definitely want to use Gay's code for round in 2.7, since having round
work sensibly is part of the motivation for the backport in the first
place. But this naturally leads to a round-half-to-even version of
round, since the Python-adapted version of Gay's code isn't capable of
doing anything other than round-half-to-even at the moment.
Options:
(1) change round in 2.7 to do round-half-to-even. This is easy,
natural, and means that round will agree with float formatting
(which does round-half-to-even in both py3k and trunk). But it
may break existing applications. However: (a) those applications
would need fixing anyway to work with py3k, and (b) I have little
sympathy for people depending on behaviour of rounding of
*binary* floats for halfway *decimal* cases. (Decimal is another
matter, of course: there it's perfectly reasonable to expect
guaranteed rounding behaviour.)
It's more complicated than that, though, since if rounding
becomes round-half-to-even for floats, it should also change
for integers, Fractions, etc.
(2) have round stick with round-half-away-from-zero. This may be
awkward to implement (though I have some half-formed ideas about
how to make it work), and would lead to round occasionally not
agreeing with float formatting. For example:
>>> '{0:.1f}'.format(1.25)
'1.2'
>>> round(1.25, 1)
1.3
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