PEP: 228
Title: Reworking Python's Numeric Model
Version: $Revision$
Last-Modified: $Date$
Author: moshez@zadka.site.co.il (Moshe Zadka), guido@python.org (Guido van Rossum)
Status: Withdrawn
Type: Standards Track
Created: 4-Nov-2000
Python-Version: ??
Post-History:
Withdrawal
This PEP has been withdrawn in favor of PEP 3141.
Abstract
Today, Python's numerical model is similar to the C numeric model:
there are several unrelated numerical types, and when operations
between numerical types are requested, coercions happen. While
the C rationale for the numerical model is that it is very similar
to what happens at the hardware level, that rationale does not
apply to Python. So, while it is acceptable to C programmers that
2/3 == 0, it is surprising to many Python programmers.
NOTE: in the light of recent discussions in the newsgroup, the
motivation in this PEP (and details) need to be extended.
Rationale
In usability studies, one of the least usable aspect of Python was
the fact that integer division returns the floor of the division.
This makes it hard to program correctly, requiring casts to
float() in various parts through the code. Python's numerical
model stems from C, while an model that might be easier to work with
can be based on the mathematical understanding of numbers.
Other Numerical Models
Perl's numerical model is that there is one type of numbers --
floating point numbers. While it is consistent and superficially
non-surprising, it tends to have subtle gotchas. One of these is
that printing numbers is very tricky, and requires correct
rounding. In Perl, there is also a mode where all numbers are
integers. This mode also has its share of problems, which arise
from the fact that there is not even an approximate way of
dividing numbers and getting meaningful answers.
Suggested Interface For Python's Numerical Model
While coercion rules will remain for add-on types and classes, the
built in type system will have exactly one Python type -- a
number. There are several things which can be considered "number
methods":
1. isnatural()
2. isintegral()
3. isrational()
4. isreal()
5. iscomplex()
a. isexact()
Obviously, a number which answers true to a question from 1 to 5, will
also answer true to any following question. If "isexact()" is not true,
then any answer might be wrong.
(But not horribly wrong: it's close to the truth.)
Now, there is two thing the models promises for the field operations
(+, -, /, *):
- If both operands satisfy isexact(), the result satisfies
isexact().
- All field rules are true, except that for not-isexact() numbers,
they might be only approximately true.
One consequence of these two rules is that all exact calcutions
are done as (complex) rationals: since the field laws must hold,
then
(a/b)*b == a
must hold.
There is built-in function, inexact() which takes a number
and returns an inexact number which is a good approximation.
Inexact numbers must be as least as accurate as if they were
using IEEE-754.
Several of the classical Python functions will return exact numbers
even when given inexact numbers: e.g, int().
Coercion
The number type does not define nb_coerce
Any numeric operation slot, when receiving something other then PyNumber,
refuses to implement it.
Inexact Operations
The functions in the "math" module will be allowed to return
inexact results for exact values. However, they will never return
a non-real number. The functions in the "cmath" module are also
allowed to return an inexact result for an exact argument, and are
furthermore allowed to return a complex result for a real
argument.
Numerical Python Issues
People who use Numerical Python do so for high-performance vector
operations. Therefore, NumPy should keep its hardware based
numeric model.
Unresolved Issues
Which number literals will be exact, and which inexact?
How do we deal with IEEE 754 operations? (probably, isnan/isinf should
be methods)
On 64-bit machines, comparisons between ints and floats may be
broken when the comparison involves conversion to float. Ditto
for comparisons between longs and floats. This can be dealt with
by avoiding the conversion to float. (Due to Andrew Koenig.)
Copyright
This document has been placed in the public domain.
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