[Python-ideas] PEP 485: A Function for testing approximate equality (original) (raw)

David Mertz mertz at gnosis.cx
Sat Feb 7 21:56:45 CET 2015

+1 on PEP 485 as currently written.

Maybe it's not perfect in every edge case, but per Nick's standard, it is significantly "less wrong" than 'a == b'.

On Fri, Feb 6, 2015 at 5:48 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On 6 February 2015 at 23:35, Paul Moore <p.f.moore at gmail.com> wrote: > On 6 February 2015 at 12:28, Antoine Pitrou <solipsis at pitrou.net> wrote: >> Ok, more simply then: does iscloseto(0.0, 0.0) return True? > > From the formula in the PEP > > """abs(a-b) <= max( reltolerance * min(abs(a), abs(b), abstolerance_ _)"""_ _> > yes it does. More generally, iscloseto(x, x) is always true. That's > a key requirement - that "closeness" includes equality. > > I think the "weirdness" around zero is simply that there's no x for > which iscloseto(x, 0) and x != 0. Which TBH isn't really all that > weird :-)

Right, by default it degrades to exact equality when one of the operands is zero, just as it will degrade to exact equality when the relative tolerance is set to zero. This should probably be stated explicitly in the PEP, since it isn't immediately obvious from the formal definition. +1 on the PEP from me - it meets the goal I suggested of being clearly better for comparing floating point values than standard equality, and manages to hide most of the surprising complexity of floating point comparisons. Regards, Nick. -- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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