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
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 then
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, round(0.98, 2)
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:
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.
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.
 PEP 239, Adding a Rational Type to Python, Zadka,
This document has been placed in the public domain.