[Python-ideas] checking for identity before comparing built-in objects

[Steven D'Aprano]

On 11/10/12 09:05, Joshua Landau wrote:

> After re-re-reading this thread, it turns out one *(1)* post and two
> *(2)* answers
> to that post have covered a topic very similar to the one I have raised.
> All of the others, to my understanding, do not dwell over the fact
> that *float("nan") is not float("nan")* .

That's no different from any other float.

py> float('nan') is float('nan')
False
py> float('1.5') is float('1.5')
False

Floats are not interned or cached, although of course interning is
implementation dependent and this is subject to change without notice.

For that matter, it's true of *nearly all builtins* in Python. The
exceptions being bool(obj) which returns one of two fixed instances,
and int() and str(), where *some* but not all instances are cached.


> Response 1:
> This implies that you want to differentiate between -0.0 and +0.0. That is
> bad.
>
> My response:
> Why would I want to do that?

If you are doing numeric work, you *should* differentiate between -0.0
and 0.0. That's why the IEEE 754 standard mandates a -0.0.

Both -0.0 and 0.0 compare equal, but they can be distinguished (although
doing so is tricky in Python). The reason for distinguishing them is to
distinguish between underflow to zero from positive or negative values.
E.g. log(x) should return -infinity if x underflows from a positive value,
and a NaN if x underflows from a negative.



-- 
Steven