python-peps / pep-0240.txt

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PEP: 240
Title: Adding a Rational Literal to Python
Version: $Revision$
Last-Modified: $Date$
Author: Christopher A. Craig <python-pep@ccraig.org>,
    Moshe Zadka <moshez@zadka.site.co.il>
Status: Rejected
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.

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.

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. 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] PEP 239, Adding a Rational Type to Python, Zadka,
        http://www.python.org/dev/peps/pep-0239/


Copyright

    This document has been placed in the public domain.



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