NaN comparisons - Call For Anecdotes

[Chris Angelico]

On Wed, Jul 9, 2014 at 3:31 AM, Marko Rauhamaa <marko at pacujo.net> wrote:
> Chris Angelico <rosuav at gmail.com>:
>
>> I'd say it would surprise people rather a lot if operations like dict
>> insertion/lookup could trigger arithmetic exceptions. :)
>
> That wouldn't trigger exceptions.
>
> Dict operations do an "is" test before an "==" test. In fact, you
> couldn't even use NaN as a dict key otherwise. Thus, dict operations
> never test NaN == NaN.

Check out the example I posted early in this thread of a dict with
three keys, all of them NaN. And note that hash(float("nan"))==0. Now
try looking up d[0]. Before it raises KeyError, it has to compare that
0 for equality with each of the nans, because it can't shortcut it
based on the hash. In fact, I can prove it thus:

>>> 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>

Any lookup of anything with a hash of 0 will do this. 0 itself (as any
type of number), another NaN, or anything at all. For the dict to work
sanely, these comparisons have to work and be False.

ChrisA