Hi Jeff,
The decimal module docstring starts with:
"""
This is an implementation of decimal floating point arithmetic based on
the General Decimal Arithmetic Specification:
http://speleotrove.com/decimal/decarith.html
and IEEE standard 854-1987:
http://en.wikipedia.org/wiki/IEEE_854-1987
Decimal floating point has finite precision with arbitrarily large bounds.
"""
I suggest you to look into these standards.
Victor
On Thu, Sep 30, 2021 at 9:13 AM Jeff Allen <ja...@farowl.co.uk> wrote:
>
> Is an implementation of Python free to make up its own answers to division
> and modulus operatons in the case of inf and nan arguments?
>
> The standard library documentation says only that // is "rounded towards
> minus infinity". The language reference says that :
>
> x == (x//y)*y + (x%y),
> the modulus has the same sign as y, and
> division by (either kind of) zero raises ZeroDivisionError .
>
> It's consistent, but it doesn't define the operator over the full range of
> potential arguments. In Python 3.8 and 3.9:
>
> >>> from decimal import Decimal
> >>> Decimal('-3.14') // Decimal('Infinity')
> Decimal('-0')
> >>> -3.14 // float("inf")
> -1.0
> >>> import math
> >>> math.floor(-3.14 / float("inf"))
> 0
>
> I can see sense in all three answers, as possible interpretations of "rounded
> towards minus infinity", but I quite like decimal's. There seem to be no
> regression tests for floor division of floats, and for modulus only with
> finite arguments, perhaps intentionally.
>
> --
>
> Jeff Allen
>
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