PEP 240 – Adding a Rational Literal to Python
- Christopher A. Craig <python-pep at ccraig.org>, Moshe Zadka <moshez at zadka.site.co.il>
- Standards Track
A different PEP 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.
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 .
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 firstname.lastname@example.org have shown, people often get bit by strange
semantics of floating point numbers: for example,
still gives 0.97999999999999998.
Literals conforming to the regular expression
'\d*.\d*' will be
The only backwards compatible issue is the type of literals mentioned above. The following migration is suggested:
- The next Python after approval will allow
from __future__ import rational_literalsto cause all such literals to be treated as rational numbers.
- 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”.
- 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.
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
'1/2'). This means that many rationals must be truncated at some
point, which gives us a new loss of precision.
This document has been placed in the public domain.
Last modified: 2022-01-21 11:03:51 GMT