[Python-Dev] Re: Rich comparisons

[Delaney, Timothy C (Timothy)]

> From: Edward Loper
> 
> Tim Delaney's suggestion [1] seemed pretty reasonable.  In particular:
> 
>    - Ensure that NaN is a singleton (like True and False).  I.e., the
>      float constructor checks if the float is NaN, and if so returns a
>      singleton.

Did I say that? I know I thought about the possibility of NaN being a singleton subclass of float, but I don't remember posting it ...

It would be nice, but I don't think it's overly feasible.

> Advantages:
>    - We have an easy way to test if a variable is nan: "x is NaN"
>    - nan will work "properly" as a dictionary key

This second statement is not correct. Since NaN would always compare non-equal with itself, it most definitely would not work as a dictionary key. My thoughts were that by having NaN as a separate class, it could raise if __hash__ were called i.e. explicitly preventing it from being used as a dictionary key.

> The important question is whether this would slow down other 
> operations. 
>   As for cmp(), my understanding was that it should just return -1 if 
> two unordered objects are not equal.

Nah - negative for <, 0 for ==, positive for >. And an exception if it's non-comparable (it used to be that __cmp__ wasn't allowed to raise an exception, but this restriction was relaxed).

Tim Delaney