python-peps/pep-0239.txt

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PEP: 239
Title: Adding a Rational Type to Python
Version: $Revision$
Author: pep@zadka.site.co.il (Moshe Zadka)
Status: Draft
Type: Standards Track
Created: 11-Mar-2001
Python-Version: 2.2
Post-History: 16-Mar-2001
Abstract
Python has no numeric type with the semantics of an unboundedly
precise rational number. This proposal explains the semantics of
such a type, and suggests builtin functions and literals to
support such a type. This PEP suggests no literals for rational
numbers; that is left for another PEP[1].
Rationale
While sometimes slower and more memory intensive (in general,
unboundedly so) rational arithmetic captures more closely the
mathematical ideal of numbers, and tends to have behavior which is
less surprising to newbies. Though many Python implementations of
rational numbers have been written, none of these exist in the
core, or are documented in any way. This has made them much less
accessible to people who are less Python-savvy.
RationalType
There will be a new numeric type added called RationalType. Its
unary operators will do the obvious thing. Binary operators will
coerce integers and long integers to rationals, and rationals to
floats and complexes.
The following attributes will be supported: .numerator and
.denominator. The language definition will promise that
r.denominator * r == r.numerator
that the GCD of the numerator and the denominator is 1 and that
the denominator is positive.
The method r.trim(max_denominator) will return the closest
rational s to r such that abs(s.denominator) <= max_denominator.
The rational() Builtin
This function will have the signature rational(n, d=1). n and d
must both be integers, long integers or rationals. A guarantee is
made that
rational(n, d) * d == n
References
[1] PEP 240, Adding a Rational Literal to Python, Zadka,
http://www.python.org/peps/pep-0240.html
Copyright
This document has been placed in the public domain.
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