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

MRAB python at mrabarnett.plus.com
Tue Jul 8 19:33:31 CEST 2014 
On 2014-07-08 17:57, Steven D'Aprano wrote:
[snip]
>
> In particular, reflexivity for NANs was dropped for a number of reasons,
> some stronger than others:
>
> - One of the weaker reasons for NAN non-reflexivity is that it preserved
>    the identity x == y <=> x - y == 0. Although that is the cornerstone
>    of real arithmetic, it's violated by IEEE-754 INFs, so violating it
>    for NANs is not a big deal either.
>
> - Dropping reflexivity preserves the useful property that NANs compare
>    unequal to everything.
>
> - Practicality beats purity: dropping reflexivity allowed programmers
>    to identify NANs without waiting years or decades for programming
>    languages to implement isnan() functions. E.g. before Python had
>    math.isnan(), I made my own:
>
>    def isnan(x):
>        return isinstance(x, float) and x != x
>
> - Keeping reflexivity for NANs would have implied some pretty nasty
>    things, e.g. if log(-3) == log(-5), then -3 == -5.
>
The log of a negative number is a complex number.
>
> Basically, and I realise that many people disagree with their decision
> (notably Bertrand Meyer of Eiffel fame, and our own Mark Dickenson), the
> IEEE-754 committee led by William Kahan decided that the problems caused
> by having NANs compare unequal to themselves were much less than the
> problems that would have been caused without it.
>
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