Result of ``a is b''

Tim Peters tim.one at comcast.net
Sat Mar 20 00:10:12 CET 2004


[Andrew Koenig]
>> I'm pretty sure that IEEE floating point requires NaN to be defined
>> in such a way that if x is NaN, then x == x must yield False.

Yes, but ... a conforming implementation of the standard requires all sorts
of comparison operators, but it's not a language standard and doesn't have
anything to say about how languages spell access to any of its facilities.
So, e.g., there must be *some* way to spell an equality operator that
doesn't consider a NaN to "be equal" to itself, but that doesn't need to be
spelled "==" (not even in a language that has an "==" comparison operator).
For example, it's fine if it needs to be spelled

    ieee.equal_considering_nans_unequal_and_unexceptional(x, y)

[Erik Max Francis]
> That's correct.  Python itself doesn't do this, though:

All Python behavior wrt IEEE gimmicks is a platform-dependent accident --
nothing is promised.

> Python 2.3.3 (#1, Dec 22 2003, 23:44:26) [GCC 3.2.3] on linux2
> >>> a = 1e3000
> >>> a
> inf
> >>> a/a
> nan
> >>> n = a/a
> >>> n == n
> True

While on Windows 2.3.3, the output looks very different, and the final
result is False.

For Python 2.4, Michael Hudson has patched things so that Python's 6 float
relationals deliver the same results the platform C compiler delivers for
the spelled-the-same-way-in-C float relationals.  There's still no
cross-platform guarantee about what those will do (because C makes no
guarantees here either), but at least NaN == NaN will be false on both gcc
and MSVC7-based Pythons (MSVC6-based Pythons will say NaN == NaN is true
then, but right now there are no plans to distribute an MSVC6-based Python
2.4).





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