python-peps/peps/pep-0240.rst

PEP: 240
Title: Adding a Rational Literal to Python
Author: Christopher A. Craig <python-pep@ccraig.org>, Moshe Zadka <moshez@zadka.site.co.il>
Status: Rejected
Type: Standards Track
Content-Type: text/x-rst
Created: 11-Mar-2001
Python-Version: 2.2
Post-History: 16-Mar-2001


Abstract
========

A :pep:`different PEP <239>` suggests adding a builtin rational type to
Python.  This PEP suggests changing the ddd.ddd float literal to a
rational in Python, and modifying non-integer division to return
it.


BDFL Pronouncement
==================

This PEP is rejected.  The needs outlined in the rationale section
have been addressed to some extent by the acceptance of :pep:`327`
for decimal arithmetic.  Guido also noted, "Rational arithmetic
was the default 'exact' arithmetic in ABC and it did not work out as
expected".  See the python-dev discussion on 17 June 2005 [1]_.


Rationale
=========

Rational numbers are useful for exact and unsurprising arithmetic.
They give the correct results people have been taught in various
math classes.  Making the "obvious" non-integer type one with more
predictable semantics will surprise new programmers less than
using floating point numbers. As quite a few posts on c.l.py and
on tutor@python.org have shown, people often get bit by strange
semantics of floating point numbers: for example, ``round(0.98, 2)``
still gives 0.97999999999999998.


Proposal
========

Literals conforming to the regular expression ``'\d*.\d*'`` will be
rational numbers.


Backwards Compatibility
=======================

The only backwards compatible issue is the type of literals
mentioned above.  The following migration is suggested:

1. The next Python after approval will allow
   ``from __future__ import rational_literals``
   to cause all such literals to be treated as rational numbers.

2. Python 3.0 will have a warning, turned on by default, about
   such literals in the absence of a ``__future__`` statement.  The
   warning message will contain information about the ``__future__``
   statement, and indicate that to get floating point literals,
   they should be suffixed with "e0".

3. Python 3.1 will have the warning turned off by default.  This
   warning will stay in place for 24 months, at which time the
   literals will be rationals and the warning will be removed.


Common Objections
=================

Rationals are slow and memory intensive!
(Relax, I'm not taking floats away, I'm just adding two more characters.
``1e0`` will still be a float)

Rationals must present themselves as a decimal float or they will be
horrible for users expecting decimals (i.e. ``str(.5)`` should return ``'.5'`` and
not ``'1/2'``).  This means that many rationals must be truncated at some
point, which gives us a new loss of precision.



References
==========

.. [1] Raymond Hettinger, Propose rejection of PEPs 239 and 240 -- a builtin
       rational type and rational literals
       https://mail.python.org/pipermail/python-dev/2005-June/054281.html


Copyright
=========

This document has been placed in the public domain.