Stephen Evans <step...@recombinant.co.uk> added the comment: As suggested by Mark following my post on comp.lang.python I am adding further comments to the discussion on this (closed) issue.
For a more mathematical consideration of the issue: Stepanov, Alexander and Paul McJones. 2009. Elements of Programming. Addison Wesley. Pages 52-53 The problem with the builtin max() is with weak comparisons. Consider two python objects a and b that are equivalent and where the following are True: a is not b repr([a, b]) == repr(sorted([a, b])) repr([a, b]) == repr(sorted([a, b], reverse=True)) repr([b, a]) == repr(sorted([b, a])) repr([b, a]) == repr(sorted([b, a], reverse=True)) Assuming repr() implemented correctly for a and b. The only Python rich comparison required is (weak) __lt__. If (weak) __eq__ is implemented then the following are True: a == b b == a In bltinmodule.c builtin_max() uses Py_GT. For correctness this should use the converse of builtin_min() i.e. the boolean negation of PyObject_RichCompare using Py_LT (for valid results). If using Python rich comparisions then only __lt__ would be required for both min() and max() as with list.sort(). The following will then be True: min([a, b]) is a max([a, b]) is b min([b, a]) is b max([b, a]) is a min([a, b]) is max([b, a]) min([a, b]) is not min([b, a]) max([a, b]) is min([b, a]) max([a, b]) is not max([b, a]) The above will work if Py_GE is subtituted for Py_GT in builtin_max(), though this will require the implementation of __ge__ which is inconsistent with list.sort() and is a point of potential failure if the implementation of __ge__ is not the converse of the implementation __lt__. To reiterate - builtin max() should be the converse of builtin min(). ---------- components: +None -Documentation nosy: +Stephen.Evans versions: +Python 2.7, Python 3.3 Added file: http://bugs.python.org/file20996/min_max.py _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue9802> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
