[Python-Dev] math.hypot, complex.__abs__, and documentation

[Steven D'Aprano]

On Wed, 17 Feb 2010 08:19:00 am David DiCato wrote:
> I have a minor concern about certain corner cases with math.hypot and
> complex.__abs__, namely when one component is infinite and one is not
> a number. If we execute the following code:
>
> import math
> inf = float('inf')
> nan = float('nan')
> print math.hypot(inf, nan)
> print abs(complex(nan, inf))
>
> ... then we see that 'inf' is printed in both cases. The standard
> library tests (for example, test_cmath.py:test_abs()) seem to test
> for this behavior as well, and FWIW, I personally agree with this
> convention.

What's the justification for that convention? It seems wrong to me. 

If you expand out hypot and substitute a=inf and b=nan, you get:

>>> math.sqrt(inf*inf + nan*nan)
nan

which agrees with my pencil-and-paper calculation:

sqrt(inf*inf + nan*nan)
= sqrt(inf + nan)
= sqrt(nan)
= nan



-- 
Steven D'Aprano