[Python-Dev] == on object tests identity in 3.x (original) (raw)

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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