[Python-Dev] PyObject_RichCompareBool identity shortcut

[Łukasz Langa]

Wiadomość napisana przez Hrvoje Niksic w dniu 2011-04-27, o godz. 11:37:

> The other day I was surprised to learn this:
> 
> >>> nan = float('nan')
> >>> nan == nan
> False
> >>> [nan] == [nan]
> True                  # also True in tuples, dicts, etc.
> 
> # also:
> >>> l = [nan]
> >>> nan in l
> True
> >>> l.index(nan)
> 0
> >>> l[0] == nan
> False
> 

This surprises me as well. I guess this is all related to the fact that:
>>> nan is nan
True

Have a look at this as well:

>>> inf = float('inf')
>>> inf == inf
True
>>> [inf] == [inf]
True
>>> l = [inf]
>>> inf in l
True
>>> l.index(inf)
0
>>> l[0] == inf
True

# Or even:
>>> inf+1 == inf-1
True

For the infinity part, I believe this is related to the funky IEEE 754 standard. I found
some discussion about this here: http://compilers.iecc.com/comparch/article/98-07-134

-- 
Best regards,
Łukasz Langa