PEP: 240 Title: Adding a Rational Literal 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 A different PEP[1] 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. 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 then 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. "from __future__ import rational_literals" will cause all such literals to be treated as rational numbers. 2. Python 2.2 will have a warning, turned off 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 2.3 will have the warning turned on 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) References [1] PEP 239, Adding a Rational Type to Python, Zadka, http://www.python.org/peps/pep-0239.html Copyright This document has been placed in the public domain. Local Variables: mode: indented-text indent-tabs-mode: nil End: