added PEP 326, A Case for All, by Josiah Carlson
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David Goodger
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@ -121,6 +121,7 @@ Index by Category
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S 323 Copyable Iterators Martelli
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S 324 popen5 - New POSIX process module Astrand
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S 325 Resource-Release Support for Generators Pedroni
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S 326 A Case for All Carlson
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S 754 IEEE 754 Floating Point Special Values Warnes
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Finished PEPs (done, implemented in CVS)
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@ -343,6 +344,7 @@ Numerical Index
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S 323 Copyable Iterators Martelli
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S 324 popen5 - New POSIX process module Astrand
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S 325 Resource-Release Support for Generators Pedroni
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S 326 A Case for All Carlson
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SR 666 Reject Foolish Indentation Creighton
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S 754 IEEE 754 Floating Point Special Values Warnes
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@ -370,6 +372,7 @@ Owners
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Barrett, Paul barrett@stsci.edu
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Baxter, Anthony anthony@interlink.com.au
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Bellman, Thomas bellman+pep-divmod@lysator.liu.se
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Carlson, Josiah jcarlson@uci.edu
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Carroll, W Isaac icarroll@pobox.com
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Cole, Dave djc@object-craft.com.au
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Craig, Christopher python-pep@ccraig.org
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@ -0,0 +1,217 @@
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PEP: 326
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Title: A Case for All
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Version: $Revision$
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Last-Modified: $Date$
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Author: Josiah Carlson <jcarlson@uci.edu>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 20-Dec-2003
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Python-Version: 2.4
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Post-History: 20-Dec-2003, 03-Jan-2004
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Abstract
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========
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This PEP proposes a new named constant or built-in: ``All``.
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Users of Python have had the constant None, which represents a lack of
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value, for quite some time (possibly from the beginning, this is
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unknown to the author at the current time). The author believes that
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``All`` should be introduced in order to represent a functionally
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infinite value, with similar behavior corresponding to None.
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Rationale
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=========
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While None can be used as an absolute minimum that any value can
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attain [1]_, there does not exist an equivalent absolute maximum. For
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example::
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>>> print min(None, -2**1000)
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None
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All comparisons including None and another object results in None
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being the smaller of the two.
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However, there does not exist a value such that the comparison of any
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other object results in the constant being the larger of the two.
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What is commonly done to deal with such cases, is to set a value in a
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script that is larger than the author ever expects the input to reach,
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and hope that it isn't reached.
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Guido has brought up [2]_ the fact that there exists two constants
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that can be used in the interim: sys.maxint and floating point
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positive infinity (1e309 will evaluate to positive infinity).
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However, each has their drawbacks. On most architectures sys.maxint
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is arbitrarily small (2**31-1 or 2**63-1), and can be easily eclipsed
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by large 'long' integers or floating point numbers. Furthermore,
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comparing long integers larger than the largest floating point number
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representable against any float will result in an exception being
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raised::
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>>> cmp(1.0, 10**309)
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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OverflowError: long int too large to convert to float
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Even when large integers are compared against positive infinity::
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>>> cmp(1e309, 10**309)
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Traceback (most recent call last):
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File "<stdin>", line 1, in ?
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OverflowError: long int too large to convert to float
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Introducing an ``All`` built-in that works as described does not take
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much effort. A sample Python implementation is included.
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Motivation
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==========
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There are hundreds of algorithms that begin by initializing some set
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of values to a logical (or numeric) infinity. Python lacks a positive
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infinity that works consistently or really is the largest value that
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can be attained. By adding the ``All`` constant (or built-in), Python
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would have a real maximum value, and such algorithms can become
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clearer due to the reduction of special cases.
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Take for example, finding the minimum in a sequence::
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def findmin_Num(seq):
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BIG = 0
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cur = BIG
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for obj in seq:
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if cur == BIG:
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cur = obj
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BIG = max(cur, BIG) + 1
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else:
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cur = min(cur, obj)
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return cur
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def findmin_None(seq):
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cur = None
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for obj in seq:
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if obj < cur or (cur is None):
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cur = obj
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if cur is None:
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return cur
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return cur
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def findmin_All(seq):
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cur = All
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for obj in seq:
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cur = min(obj, cur)
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return cur
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Guido brought up the idea of just negating everything and comparing
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[2]_. Certainly this does work when using numbers, but it does not
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remove the special case (actually adds one) and results in the code
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being less readable. ::
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#we have All available
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a = min(a, b)
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#we don't have All available
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if a is not None:
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if b is None:
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a = b
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else:
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a = -max(-a, -b)
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Implementation
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==============
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::
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class _AllType(object):
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def __cmp__(self, other):
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if isinstance(other, self.__class__):
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return 0
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return 1
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def __repr__(self):
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return 'All'
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import sys
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sys.modules['__builtin__'].All = _AllType()
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Results of Test Run in Python 2.3.3::
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>>> max(All, 2**65536)
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All
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>>> min(All, 2**65536)
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20035299304068464649790...
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(lines removed for brevity)
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...72339445587895905719156736L
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>>> max(All, None)
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All
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>>> min(All, None)
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>>> print min(All, None)
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None
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>>> bool(All)
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True
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>>> not None
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1
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>>> not All
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0
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Open Issues
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===========
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``All`` seemed to be an awkward object to various Python developers on
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the python-dev mailing list. The author hopes that with this updated
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PEP, developers will no longer find ``All`` awkward.
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References
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==========
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.. [1] RE: [Python-Dev] Re: Got None. Maybe Some?, Peters, Tim
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(http://mail.python.org/pipermail/python-dev/2003-December/041374.html)
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.. [2] Re: [Python-Dev] Got None. Maybe Some?, von Rossum, Guido
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(http://mail.python.org/pipermail/python-dev/2003-December/041352.html)
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.. [3] [Python-Dev] Re: Got None. Maybe Some?, Reedy, Terry
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(http://mail.python.org/pipermail/python-dev/2003-December/041337.html)
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Changes
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=======
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Added this section.
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Renamed ``Some`` to ``All``: Some was an arbitrary name that suffered
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from being unclear. [3]_
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Made ``All`` a subclass of object in order for it to become a new
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style class.
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Removed mathematical negation and casting to float in Open Issues.
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None is not a number and is not treated as one, ``All`` shouldn't be
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either.
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Added Motivation section.
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Changed markup to reStructuredText.
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 74
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End:
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