[issue11986] Min/max not symmetric in presence of NaN
Marco Sulla added the comment:
marco@buzz:~$ python3.9
Python 3.9.0a0 (heads/master-dirty:d8ca2354ed, Oct 30 2019, 20:25:01)
[GCC 9.2.1 20190909] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import Decimal as Dec, BasicContext as Bctx
>>> a = Dec("1981", Bctx)
>>> b = Dec("nan", Bctx)
>>> a.max(b)
Decimal('1981')
>>> b.max(a)
Decimal('1981')
>>> Bctx.max(a, b)
Decimal('1981')
>>> Bctx.max(b, a)
Decimal('1981')
`Decimal` completely adheres to IEEE 754 standard.
There's a very, very simple and generic solution for builtin min and max:
_sentinel = object()
def max(*args, key=None, default=_sentinel):
args_len = len(args)
if args_len == 0:
if default is _sentinel:
fname = max.__name__
raise ValueError(f"{fname}() expected 1 argument, got 0")
return default
elif args_len == 1:
seq = args[0]
else:
seq = args
it = iter(seq)
vmax = next(it, _sentinel)
if vmax is _sentinel:
if default is _sentinel:
fname = max.__name__
raise ValueError(f"{fname}() arg is an empty sequence")
return default
first_comparable = False
if key is None:
for val in it:
if vmax < val:
vmax = val
first_comparable = True
elif not first_comparable and not val < vmax :
# equal, or not comparable object, like NaN
vmax = val
else:
fmax = key(vmax)
for val in it:
fval = key(val)
if fmax < fval :
fmax = fval
vmax = val
first_comparable = True
elif not first_comparable and not fval < fmax:
fmax = fval
vmax = val
return vmax
This function continues to give undefined behavior with sets... but who
calculates the "maximum" or "minimum" of sets?
--
nosy: +Marco Sulla
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[issue11986] Min/max not symmetric in presence of NaN
Mark Dickinson dicki...@gmail.com added the comment: Prof. Kahan states that nan x must signal. Would that be the sentence that starts In the syntax of C ... ? -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Mark Dickinson dicki...@gmail.com added the comment: keep naive implementation of builtin max() Agreed. provide symmetric float.max such that nan.max(x) = x.max(nan) = x (nan result would be a valid but less useful alternative.) That might be viable (a math module function might also make sense here), though it feels a bit YAGNI to me. If we were going to add such a method, it should follow IEEE 754: nan.max(x) == x.max(n) == x, but also nan.min(x) == x.min(nan) == x, for finite x. (See section 5.3.1.) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Alexander Belopolsky belopol...@users.sourceforge.net added the comment: On Sat, May 21, 2011 at 3:22 PM, Mark Dickinson rep...@bugs.python.org wrote: Mark Dickinson dicki...@gmail.com added the comment: Prof. Kahan states that nan x must signal. Would that be the sentence that starts In the syntax of C ... ? This is just sophistry. If Python was more popular than C at the time Prof. Kahan wrote this, he would write in the syntax of Python. (Not directly on-topic, but Python 3 seems to be moving towards C spelling of operators. I, for one, miss the removal of easy to type '' in favor of finger-twisting '!='.) -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Mark Dickinson dicki...@gmail.com added the comment: This is just sophistry. If Python was more popular than C at the time Prof. Kahan wrote this, he would write in the syntax of Python. I doubt it. C has a standard that explicitly states that must signal on comparison with NaNs. Python doesn't. Alexander, I've read both these documents (Kahan's lecture notes and IEEE 754-2008) many many times. I've looked hard in the past for language that would give this exact connection, about signalling. It just isn't there in either document, and it's dishonest to claim it is. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Mark Dickinson dicki...@gmail.com added the comment: and it's dishonest to claim it is. This language was going too far, and I apologise for it. I think I need one of those 'wait 5 minutes before allowing you to post' controls. -- keywords: +gsoc ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Alexander Belopolsky belopol...@users.sourceforge.net added the comment: On Sat, May 21, 2011 at 3:32 PM, Mark Dickinson rep...@bugs.python.org wrote: .. That might be viable (a math module function might also make sense here), though it feels a bit YAGNI to me. I have to admit that it would be YAGNI for most of my code because it uses numpy for numeric calculations. Still, for consistency with decimal, it may be a good addition. Going a bit off-topic, I would like to mention the feature that may actually be quite useful: float.sorting_key() that will return an integer for each float in such a way that keys are ordered in IEEE 754 total ordering. Note that decimal has compare_total() that can be used for sorting, but a cmp-style method is less useful than a key since in py3k sort does not take cmp function anymore. Nice thing about IEEE 754 is that float.sorting_key() can be implemented very efficiently because one can simply use float's binary representation interpreted as an integer for the key. If we were going to add such a method, it should follow IEEE 754: nan.max(x) == x.max(n) == x, but also nan.min(x) == x.min(nan) == x, for finite x. (See section 5.3.1.) Agree. Unfortunately, numpy does not do it that way: nan maximum(1.0, nan) nan I am not sure whether this is an argument for or against float.max/min: if numpy had properly defined maximum, I would just recommend to use that. -- ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Alexander Belopolsky belopol...@users.sourceforge.net added the comment:
rhettinger Your other tracker item correctly focused on the behavior of
rhettinger float('NaN') itself,
You closed issue11949 as well, so it won't help. I disagree that this issue
would be resolved by resolving issue11949. Defining max(nan, x) and nan x
are two different issues. Quoting Kahan,
Some familiar functions have yet to be defined for NaN . For instance max{x, y}
should deliver the same result as max{y, x} but almost no implementations do
that when x is NaN . There are good reasons to define max{NaN, 5} := max{5,
NaN} := 5 though many would disagree.
-- Lecture Notes on the Status of IEEE Standard 754 for Binary
Floating-Point Arithmetic by Prof. W. Kahan
http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps
In the same lecture, Prof. Kahan states that nan x must signal.
My ideal solution would be to make nan x signal, keep naive implementation of
builtin max() and provide symmetric float.max such that nan.max(x) = x.max(nan)
= x (nan result would be a valid but less useful alternative.)
This will make float behavior closer to that of Decimal.
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[issue11986] Min/max not symmetric in presence of NaN
Changes by Raymond Hettinger raymond.hettin...@gmail.com: -- assignee: - rhettinger ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
New submission from Alexander Belopolsky belopol...@users.sourceforge.net:
nan = float('nan')
min(nan, 5)
nan
min(5, nan)
5
Good arguments can be made in favor of either result, but different value for
min(x, y) depending on the order of arguments can hardly be justified.
In the face of ambiguity, refuse the temptation to guess suggests that
min/max with NaN should be an error. See also issue 11949.
--
messages: 135055
nosy: alex, belopolsky, durban, mark.dickinson
priority: normal
severity: normal
status: open
title: Min/max not symmetric in presence of NaN
type: behavior
versions: Python 3.3
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[issue11986] Min/max not symmetric in presence of NaN
Antoine Pitrou pit...@free.fr added the comment:
Not specific to NaNs:
min({1}, {2})
{1}
min({2}, {1})
{2}
--
nosy: +pitrou
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[issue11986] Min/max not symmetric in presence of NaN
Raymond Hettinger raymond.hettin...@gmail.com added the comment: Undefined ordering means just that. -- nosy: +rhettinger resolution: - invalid status: open - closed ___ Python tracker rep...@bugs.python.org http://bugs.python.org/issue11986 ___ ___ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue11986] Min/max not symmetric in presence of NaN
Alexander Belopolsky belopol...@users.sourceforge.net added the comment:
On Tue, May 3, 2011 at 2:41 PM, Raymond Hettinger
rep...@bugs.python.org wrote:
..
Undefined ordering means just that.
Means what? Compare float behavior to
Decimal('1')
Decimal(1).max(Decimal('nan'))
Decimal('1')
max(Decimal('1'), Decimal('nan'))
Traceback (most recent call last):
..
decimal.InvalidOperation: comparison involving NaN
Raymond, you don't really need to stop the debate 4 minutes after the
bug has been reported.
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[issue11986] Min/max not symmetric in presence of NaN
Raymond Hettinger raymond.hettin...@gmail.com added the comment:
The report is invalid because min/max make no guarantees about values without a
total ordering.
Your other tracker item correctly focused on the behavior of float('NaN')
itself, rather than on the behavior of everything else in the Python world that
compares two values.
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