2001-03-15 23:27:13 -05:00
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PEP: 240
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Title: Adding a Rational Literal to Python
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Version: $Revision$
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Author: pep@zadka.site.co.il (Moshe Zadka)
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Status: Draft
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Type: Standards Track
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Created: 11-Mar-2001
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Python-Version: 2.2
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2001-03-16 11:02:24 -05:00
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Post-History: 16-Mar-2001
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2001-03-15 23:27:13 -05:00
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Abstract
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A different PEP[1] suggests adding a builtin rational type to
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Python. This PEP suggests changing the ddd.ddd float literal to a
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rational in Python, and modifying non-integer division to return
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it.
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Rationale
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2001-03-21 13:40:28 -05:00
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Rational numbers are useful for exact and unsurprising arithmetic.
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They give the correct results people have been taught in various
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math classes. Making the "obvious" non-integer type one with more
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2001-03-15 23:27:13 -05:00
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predictable semantics will surprise new programmers less then
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2001-03-21 13:40:28 -05:00
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using floating point numbers. As quite a few posts on c.l.py and
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on tutor@python.org have shown, people often get bit by strange
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semantics of floating point numbers: for example, round(0.98, 2)
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still gives 0.97999999999999998.
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2001-03-15 23:27:13 -05:00
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Proposal
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Literals conforming to the regular expression '\d*.\d*' will be
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rational numbers.
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Backwards Compatibility
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The only backwards compatible issue is the type of literals
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mentioned above. The following migration is suggested:
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1. "from __future__ import rational_literals" will cause all such
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literals to be treated as rational numbers.
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2. Python 2.2 will have a warning, turned off by default, about
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such literals in the absence of a __future__ statement. The
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warning message will contain information about the __future__
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statement, and indicate that to get floating point literals,
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they should be suffixed with "e0".
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3. Python 2.3 will have the warning turned on by default. This
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warning will stay in place for 24 months, at which time the
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literals will be rationals and the warning will be removed.
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2001-03-21 06:25:51 -05:00
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Common Objections
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Rationals are slow and memory intensive!
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(Relax, I'm not taking floats away, I'm just adding two more characters.
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1e0 will still be a float)
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2001-03-15 23:27:13 -05:00
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References
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[1] PEP 239, Adding a Rational Type to Python, Zadka,
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http://python.sourceforge.net/peps/pep-0239.html
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Copyright
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This document has been placed in the public domain.
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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