Issue 8786: Add support for IEEE 754 contexts to decimal module. (original) (raw)
Created on 2010-05-22 10:28 by mark.dickinson, last changed 2022-04-11 14:57 by admin.
Messages (8)
Author: Mark Dickinson (mark.dickinson) * 
Date: 2010-05-22 10:28
Discussion migrated from issue 8540 into its own issue.
For ease of communication with other libraries, it would be good to be able to easily create contexts corresponding to the IEEE 754 (2008) decimal interchange formats.
Author: Stefan Krah (skrah) *
Date: 2010-05-22 11:24
Some context from issue 8540:
[Stefan Krah]
I'm busy implementing the IEEE754 contexts for cdecimal. To keep things in sync, it would be nice to agree how they should be created.
Suggestions:
c = Decimal64Context
c = Context(Decimal64)
[Mark Dickinson]
Rather that complicating the Context constructor, I'd prefer a separate factory function. I was thinking of something like:
def IEEEContext(n): """Return the decimal IEEE 754 context. n should be a multiple of 32.""" ...
Again, it's clear with this that you get a new context object (I agree that there are problems with (1) and the mutability of Contexts).
I like that, too. Where do you find the "multiple of 32" in the standard? In a draft of IEEE 754, I only see:
Table 2—Interchange format parameters defining floating-point numbers:
storage format: decimal32
basic format: decimal64 and decimal128
This is what Java and decNumber offer.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-22 11:29
It's Table 3.6 ("Decimal interchange format parameters") in the final version of IEEE 754; I'm not sure what that corresponds to in the various drafts. It has column headings: "decimal32", "decimal64", "decimal128" and "decimal{k} (k >= 32)".
Parameters for decimal{k}: k must be a multiple of 32. precision is 9k/32-2. emax is 32**(k/16+3). I think these formulas all work for the specific cases k in {32, 64, 128} too, so it should be easy to check that they make sense.
They give an example below the table, too:
"For example, decimal256 would have p = 70 and emax = 1572864."
Author: Stefan Krah (skrah) *
Date: 2010-05-22 12:07
Mark Dickinson <report@bugs.python.org> wrote:
It's Table 3.6 ("Decimal interchange format parameters") in the final version of IEEE 754;
Thanks! I think this is not in the draft I have.
+1 for IEEEContext(n). Could we have module constants Decimal32, Decimal64 and Decimal128 so that people coming from Java or DecNumber can write:
c = IEEEContext(Decimal64)
It is somewhat redundant, but 99% of people will use only those.
Author: Mark Dickinson (mark.dickinson) *
Date: 2010-05-22 22:21
Thinking ahead a bit: at some point we might well also want functions to pack and unpack these IEEE formats into byte sequences, using the bit representations described in the standard.
A natural place for those functions would be as methods on a Context object; perhaps IEEEContext should be a subclass of Context? (OTOH, the struct module is another possible place for such functionality.)
I'll think about this.
Author: Michele Orrù (maker) *
Date: 2012-09-18 07:53
Something like this? That's a pretty trivial draft for the patch.
About "byte sequences", those features should be available using builtins bin(), oct() and hex(), hacking on index, or with internal methods?
I am lacking imagination, what else there should be to test?
Author: Terry J. Reedy (terry.reedy) *
Date: 2012-10-14 18:51
I do not think IEEEContext should not be made a class rather than a function until it really is a subclass with altered or added behavior. This means at least its own str method and in this case, a setattr that enforces the invariants. Here that means refusing to modify the context, or at least not prec and Emax (assuming that changes to any other context attributes (are there any?) are harmless. Something like
def setattr(self, key, value): if key in ('prec', 'Emax'): raise AttributeError("cannot change {} of IEEEContext".format(key)) else: super(IEEEContext, self).setattr(key, name)
Author: Stefan Krah (skrah) *
Date: 2013-08-30 16:08
BTW, in _decimal the feature can already be enabled with:
./configure CFLAGS=-DEXTRA_FUNCTIONALITY
IEEEContext(DECIMAL128) Context(prec=34, rounding=ROUND_HALF_EVEN, Emin=-6143, Emax=6144, capitals=1, clamp=1, flags=[], traps=[]) IEEEContext(DECIMAL64) Context(prec=16, rounding=ROUND_HALF_EVEN, Emin=-383, Emax=384, capitals=1, clamp=1, flags=[], traps=[]) IEEEContext(DECIMAL32) Context(prec=7, rounding=ROUND_HALF_EVEN, Emin=-95, Emax=96, capitals=1, clamp=1, flags=[], traps=[])
IEEEContext(512) Context(prec=142, rounding=ROUND_HALF_EVEN, Emin=-103079215103, Emax=103079215104, capitals=1, clamp=1, flags=[], traps=[]) IEEEContext(1024) Traceback (most recent call last): File "", line 1, in ValueError: argument must be a multiple of 32, with a maximum of 512
Of course this isn't the official API yet, but I think it's reasonable.
History
Date
User
Action
Args
2022-04-11 14:57:01
admin
set
github: 53032
2013-08-30 16:08:54
skrah
set
messages: +
2013-07-07 14:51:26
christian.heimes
set
nosy: + christian.heimes
2012-10-14 18:51:46
terry.reedy
set
nosy: + terry.reedy
messages: +
2012-10-14 18:16:06
terry.reedy
set
versions: + Python 3.4, - Python 3.2
2012-09-18 07:53:43
maker
set
files: + issue8786.patch
nosy: + ezio.melotti, maker
messages: +
keywords: + patch
2010-05-22 22:21:39
mark.dickinson
set
messages: +
2010-05-22 12:09:12
mark.dickinson
set
nosy:rhettinger, facundobatista, mark.dickinson, skrah
type: enhancement
components: + Library (Lib)
stage: needs patch
2010-05-22 12:07:53
skrah
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messages: +
2010-05-22 11:29:40
mark.dickinson
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messages: +
2010-05-22 11:24:03
skrah
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messages: +
2010-05-22 10:28:31
mark.dickinson
create