[Python-checkins] python/nondist/peps pep-0326.txt, NONE,
1.1 pep-0000.txt, 1.260, 1.261
goodger at users.sourceforge.net
goodger at users.sourceforge.net
Sun Jan 4 12:30:49 EST 2004
Update of /cvsroot/python/python/nondist/peps
In directory sc8-pr-cvs1:/tmp/cvs-serv9714
Modified Files:
pep-0000.txt
Added Files:
pep-0326.txt
Log Message:
added PEP 326, A Case for All, by Josiah Carlson
--- NEW FILE: pep-0326.txt ---
PEP: 326
Title: A Case for All
Version: $Revision: 1.1 $
Last-Modified: $Date: 2004/01/04 17:30:47 $
Author: Josiah Carlson <jcarlson at uci.edu>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 20-Dec-2003
Python-Version: 2.4
Post-History: 20-Dec-2003, 03-Jan-2004
Abstract
========
This PEP proposes a new named constant or built-in: ``All``.
Users of Python have had the constant None, which represents a lack of
value, for quite some time (possibly from the beginning, this is
unknown to the author at the current time). The author believes that
``All`` should be introduced in order to represent a functionally
infinite value, with similar behavior corresponding to None.
Rationale
=========
While None can be used as an absolute minimum that any value can
attain [1]_, there does not exist an equivalent absolute maximum. For
example::
>>> print min(None, -2**1000)
None
All comparisons including None and another object results in None
being the smaller of the two.
However, there does not exist a value such that the comparison of any
other object results in the constant being the larger of the two.
What is commonly done to deal with such cases, is to set a value in a
script that is larger than the author ever expects the input to reach,
and hope that it isn't reached.
Guido has brought up [2]_ the fact that there exists two constants
that can be used in the interim: sys.maxint and floating point
positive infinity (1e309 will evaluate to positive infinity).
However, each has their drawbacks. On most architectures sys.maxint
is arbitrarily small (2**31-1 or 2**63-1), and can be easily eclipsed
by large 'long' integers or floating point numbers. Furthermore,
comparing long integers larger than the largest floating point number
representable against any float will result in an exception being
raised::
>>> cmp(1.0, 10**309)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to float
Even when large integers are compared against positive infinity::
>>> cmp(1e309, 10**309)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to float
Introducing an ``All`` built-in that works as described does not take
much effort. A sample Python implementation is included.
Motivation
==========
There are hundreds of algorithms that begin by initializing some set
of values to a logical (or numeric) infinity. Python lacks a positive
infinity that works consistently or really is the largest value that
can be attained. By adding the ``All`` constant (or built-in), Python
would have a real maximum value, and such algorithms can become
clearer due to the reduction of special cases.
Take for example, finding the minimum in a sequence::
def findmin_Num(seq):
BIG = 0
cur = BIG
for obj in seq:
if cur == BIG:
cur = obj
BIG = max(cur, BIG) + 1
else:
cur = min(cur, obj)
return cur
def findmin_None(seq):
cur = None
for obj in seq:
if obj < cur or (cur is None):
cur = obj
if cur is None:
return cur
return cur
def findmin_All(seq):
cur = All
for obj in seq:
cur = min(obj, cur)
return cur
Guido brought up the idea of just negating everything and comparing
[2]_. Certainly this does work when using numbers, but it does not
remove the special case (actually adds one) and results in the code
being less readable. ::
#we have All available
a = min(a, b)
#we don't have All available
if a is not None:
if b is None:
a = b
else:
a = -max(-a, -b)
Implementation
==============
::
class _AllType(object):
def __cmp__(self, other):
if isinstance(other, self.__class__):
return 0
return 1
def __repr__(self):
return 'All'
import sys
sys.modules['__builtin__'].All = _AllType()
Results of Test Run in Python 2.3.3::
>>> max(All, 2**65536)
All
>>> min(All, 2**65536)
20035299304068464649790...
(lines removed for brevity)
...72339445587895905719156736L
>>> max(All, None)
All
>>> min(All, None)
>>> print min(All, None)
None
>>> bool(All)
True
>>> not None
1
>>> not All
0
Open Issues
===========
``All`` seemed to be an awkward object to various Python developers on
the python-dev mailing list. The author hopes that with this updated
PEP, developers will no longer find ``All`` awkward.
References
==========
.. [1] RE: [Python-Dev] Re: Got None. Maybe Some?, Peters, Tim
(http://mail.python.org/pipermail/python-dev/2003-December/041374.html)
.. [2] Re: [Python-Dev] Got None. Maybe Some?, von Rossum, Guido
(http://mail.python.org/pipermail/python-dev/2003-December/041352.html)
.. [3] [Python-Dev] Re: Got None. Maybe Some?, Reedy, Terry
(http://mail.python.org/pipermail/python-dev/2003-December/041337.html)
Changes
=======
Added this section.
Renamed ``Some`` to ``All``: Some was an arbitrary name that suffered
from being unclear. [3]_
Made ``All`` a subclass of object in order for it to become a new
style class.
Removed mathematical negation and casting to float in Open Issues.
None is not a number and is not treated as one, ``All`` shouldn't be
either.
Added Motivation section.
Changed markup to reStructuredText.
Copyright
=========
This document has been placed in the public domain.
..
Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 74
End:
Index: pep-0000.txt
===================================================================
RCS file: /cvsroot/python/python/nondist/peps/pep-0000.txt,v
retrieving revision 1.260
retrieving revision 1.261
diff -C2 -d -r1.260 -r1.261
*** pep-0000.txt 3 Jan 2004 16:23:15 -0000 1.260
--- pep-0000.txt 4 Jan 2004 17:30:47 -0000 1.261
***************
*** 122,125 ****
--- 122,126 ----
S 324 popen5 - New POSIX process module Astrand
S 325 Resource-Release Support for Generators Pedroni
+ S 326 A Case for All Carlson
S 754 IEEE 754 Floating Point Special Values Warnes
***************
*** 344,347 ****
--- 345,349 ----
S 324 popen5 - New POSIX process module Astrand
S 325 Resource-Release Support for Generators Pedroni
+ S 326 A Case for All Carlson
SR 666 Reject Foolish Indentation Creighton
S 754 IEEE 754 Floating Point Special Values Warnes
***************
*** 371,374 ****
--- 373,377 ----
Baxter, Anthony anthony at interlink.com.au
Bellman, Thomas bellman+pep-divmod at lysator.liu.se
+ Carlson, Josiah jcarlson at uci.edu
Carroll, W Isaac icarroll at pobox.com
Cole, Dave djc at object-craft.com.au
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