Difference between 'is' and '=='

[Dave Hansen]

Re: Difference between 'is' and '=='

On 3 Apr 2006 10:37:11 -0400 in comp.lang.pytho n, roy@panix.com (Roy
Smith) wrote:
[color=blue]
>Adam DePrince <adam.deprince@ gmail.com> wrote:[color=green]
>> It just happens that the
>>logical operation
>>
>> (a is b ) -> (a == b )
>>
>>is always True.[/color]
>
>Only for small values of "always". You can always do pathological
>things with operators:
>
>class Foo:
> def __eq__ (self, other):
> return False
>
>f = Foo()
>print f is f
>print f == f
>
>frame:play$ ./is.py
>True
>False
>
>This may even be useful. What if you were trying to emulate SQL's
>NULL? NULL compares false to anything, even itself. To test for
>NULLness, you have to use the special "is NULL" operator.[/color]

Another instance where this may be useful is IEEE-754 NaN. I don't
have fpconst to verify if that's the case, but I would expect
NaN is NaN to be true, but NaN == NaN to be false.

Regards,
-=Dave

--
Change is inevitable, progress is not.

[Jon Ribbens]

Re: Difference between 'is' and '=='

In article <e0rbun$70g$1@p anix2.panix.com >, Roy Smith wrote:[color=blue]
> This may even be useful. What if you were trying to emulate SQL's
> NULL? NULL compares false to anything, even itself.[/color]

Strictly speaking, comparing NULL to anything gives NULL, not False.

[Duncan Booth]

Re: Difference between 'is' and '=='

Adam DePrince wrote:
[color=blue]
> It just happens that the
> logical operation
>
> (a is b ) -> (a == b )
>
> is always True.[/color]

That is incorrect:
[color=blue][color=green][color=darkred]
>>> inf = 1e300*1e300
>>> nan = inf-inf
>>> nan is nan, nan==nan[/color][/color][/color]
(True, False)