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[Python-Dev] == on object tests identity in 3.x

Chris Angelico rosuav at gmail.com
Tue Jul 8 19:13:00 CEST 2014 
On Wed, Jul 9, 2014 at 3:00 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Tue, Jul 08, 2014 at 04:58:33PM +0200, Anders J. Munch wrote:
>
>> For two NaNs computed differently to compare equal is no worse than 2+2
>> comparing equal to 1+3.  You're comparing values, not their history.
>
> a = -23
> b = -42
> if log(a) == log(b):
>     print "a == b"

That could also happen from rounding error, though.

>>> a = 2.0**52
>>> b = a+1.0
>>> a == b
False
>>> log(a) == log(b)
True

Any time you do any operation on numbers that are close together but
not equal, you run the risk of getting results that, in
finite-precision floating point, are deemed equal, even though
mathematically they shouldn't be (two unequal numbers MUST have
unequal logarithms).

ChrisA
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