Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>:
>> >>> Rational(2).sqrt() * Rational(2).sqrt() == Rational(2)
> Square root of 2 is not a rational number.
Nobody said it was. It's just that even "arbitrary-precision" rational
numbers wouldn't free you from the issues of floating-point numbers. The
Decimal number class won't do it, either, of course.
On the other hand, floating-point numbers are perfect whenever you deal
with science and measurement. And when you deal with business (= money),
integers are the obvious choice.
I would venture to say that the real applications for Decimal are very
rare. In practice, I'm afraid, people with rather a weak understanding
of numbers and computation might gravitate toward Decimal unnecessarily.