float("nan") in set or as key

[Chris Angelico]

On Sat, Jun 4, 2011 at 9:29 AM, Nobody <nobody at nowhere.com> wrote:
> Floats are supposed to approximate reals. They're also a Python
> data type, and should make some effort to fit in with the rest of
> the language.
>

That's what I thought a week ago. But that's not really true. Floats
are supposed to hold non-integral values, but the data type is "IEEE
754 floating point", not "real number". There's several ways to store
real numbers, and not one of them is (a) perfectly accurate, or (b)
plausibly fast to calculate. Using rationals (fractions) with infinite
range leads to exponential performance costs, and still doesn't
properly handle irrationals like pi. And if you cap the denominator to
a power of 2 and cap the length of the mantissa, err I mean numerator,
then you have IEEE 754 floating point. Python offers you a way to
store and manipulate floating point numbers, not real numbers.

Chris Angelico