[Python-Dev] PyObject_RichCompareBool identity shortcut

[Steven D'Aprano]

Alexander Belopolsky wrote:

> With the status quo in Python, it may only make sense to store NaNs in
> array.array, but not in a list.


That's a bit extreme. It only gets you into trouble if you reason like this:

 >>> a = b = [1, 2, 3, float('nan')]
 >>> if a == b:
...     for x,y in zip(a,b):
...             assert x==y
...
Traceback (most recent call last):
   File "<stdin>", line 3, in <module>
AssertionError


But it's perfectly fine to do this:

 >>> sum(a)
nan


exactly as expected. Prohibiting NANs from lists is massive overkill for 
a small (alleged) problem.

I know thousands of words have been spilled on this, including many by 
myself, but I really believe this discussion is mostly bike-shedding. 
Given the vehemence of some replies, and the volume of talk, anyone 
would think that you could hardly write a line of Python code without 
badly tripping over problems caused by NANs. The truth is, I think, that 
most people will never see one in real world code, and those who are 
least likely to come across them are the most likely to be categorically 
against them.

(I grant that Alexander is an exception -- I understand that he does do 
a lot of numeric work, and does come across NANs, and still doesn't like 
them one bit.)



-- 
Steven