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Date: Wed, 9 Jul 2014 03:57:25 +1000
Subject: Re: NaN comparisons - Call For Anecdotes
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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On Wed, Jul 9, 2014 at 3:54 AM, Chris Angelico <rosuav@gmail.com> wrote:
>>>> class X:
>     def __eq__(self, other):
>         if self is other:
>             print("Comparing against self - I am me!")
>             return True
>         print("Comparing against",other,"-",id(other))
>         return False
>     def __hash__(self):
>         return 0
>
>>>> d[X()]
> Comparing against nan - 18777952
> Comparing against nan - 19624864
> Comparing against nan - 18776272
> Traceback (most recent call last):
>   File "<pyshell#20>", line 1, in <module>
>     d[X()]
> KeyError: <__main__.X object at 0x016B40D0>

Better example: Subclass float, then it can actually *be* a nan.

>>> class NoisyFloat(float):
    def __eq__(self, other):
        print("Comparing",id(self),"against",id(other))
        return super().__eq__(other)
    def __hash__(self):
        return super().__hash__()

>>> d[NoisyFloat("nan")]
Comparing 23777152 against 18777952
Comparing 23777152 against 19624864
Comparing 23777152 against 18776272
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
    d[NoisyFloat("nan")]
KeyError: nan

That's comparing nan==nan three types with four different nans.

ChrisA