Oscar Benjamin <oscar.j.benja...@gmail.com> added the comment:
I've never found numbers.Real/Complex to be useful. The purpose of the ABCs
should be that they enable you to write code that works for instances of any
subclass but in practice writing good floating point code requires knowing
something e.g. the base, precision, max exponent etc of the type. Also many
implementations like Decimal have contexts and rounding control etc that need
to be used and the ABC gives no way to know that or to do anything with it.
The main thing that is useful about the Rational/Integer ABCs is that they
define the numerator and denominator attributes which makes different
implementations interoperable by providing exact conversion. If Real was
presumed to represent some kind of floating point type then an analogous
property/method would be something that can deconstruct the object in an exact
way like:
mantissa, base, exponent = deconstruct(real)
You would also need a way to handle nan, inf etc. Note that as_integer_ratio()
is not suitable because it could generate enormous integers unnecessarily e.g.
Decimal('1E+100000000').as_integer_ratio().
Instead the Real ABC only defines conversion to float. That's useful in the
sense that you can write code for float and pass in some other floating point
type and have everything reduce to float. You don't need an ABC for that though
because __float__ does everything. In practice most alternate "real" number
implementations exist precisely to be better than float in some way by either
having greater range/precision or a different base but anything written for the
Real ABC is essentially reduced to float as a lowest common (inexact)
denominator.
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue43602>
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