Steven D'Aprano wrote:
Mark Shannon wrote:
Related to the discussion on "Not a Number" can I point out a few things
that have not be explicitly addressed so far.
The IEEE standard is about hardware and bit patterns, rather than types
and values so may not be entirely appropriate for high-level language
like Python.
I would argue that the implementation of NANs is irrelevant. If NANs are
useful in hardware floats -- and I think they are -- then they're just
as equally useful as objects, or as strings in languages like REXX or
Hypertalk where all data is stored as strings, or as quantum wave
functions in some future quantum computer.
So,
Indeed, so its OK if type(NaN) != type(0.0) ?
NaN is *not* a number (the clue is in the name).
Python treats it as if it were a number:
>>> import numbers
>>> isinstance(nan, numbers.Number)
True
Can be read as "'Not a Number' is a Number" ;)
I see your wink, but what do you make of these?
class NotAnObject(object):
pass
nao = NotAnObject()
assert isinstance(nao, object)
Trying to make something not an object in a language where everything is
an object is bound to be problematic.
class NotAType(object):
pass
assert type(NotAType) is type
NaN does not have to be a float or a Decimal.
Perhaps it should have its own class.
Others have already pointed out this won't make any difference.
Fundamentally, the problem is that some containers bypass equality tests
for identity tests. There may be good reasons for that shortcut, but it
leads to problems with *any* object that does not define equality to be
reflexive, not just NANs.
>>> class Null:
... def __eq__(self, other):
... return False
...
>>> null = Null()
>>> null == null
False
>>> [null] == [null]
True
Just because you can do that, doesn't mean you should.
Equality should be reflexive, without that fundamental assumption many
non-numeric algorithms fall apart.
The default comparisons will then work as expected for collections.
(No doubt, making NaN a new class will cause a whole new set of problems)
As pointed out by Meyer:
NaN == NaN is False
is no more logical than
NaN != NaN is False
I don't agree with this argument. I think Meyer is completely mistaken
there. The question of NAN equality is that of a vacuous truth, quite
similar to the Present King of France:
http://en.wikipedia.org/wiki/Present_King_of_France
Meyer would have us accept that:
The present King of France is a talking horse
and
The present King of France is not a talking horse
are equally (pun not intended) valid. No, no they're not. I don't know
much about who the King of France would be if France had a king, but I
do know that he wouldn't be a talking horse.
Once you accept that NANs aren't equal to anything else, it becomes a
matter of *practicality beats purity* to accept that they can't be equal
Not breaking a whole bunch of collections and algorithms has a certain
practical appeal as well ;)
to themselves either. A NAN doesn't represent a specific thing. It's a
signal that your calculation has generated an indefinite, undefined,
undetermined value. NANs aren't equal to anything. The fact that a NAN
happens to have an existence as a bit-pattern at some location, or as a
distinct object, is an implementation detail that is irrelevant. If you
just happen by some fluke to compare a NAN to "itself", that shouldn't
change the result of the comparison:
The present King of France is the current male sovereign who
rules France
is still false, even if you happen to write it like this:
The present King of France is the present King of France
The problem with this argument is the present King of France does not
exist, whereas NaN (as a Python object) does exist.
The present King of France argument only applies to non-existent things.
Python objects do exist (as much as any computer language entity
exists). So the expression "The present King of France" either raises an
exception (non-existence) or evaluates to an object (existence).
In this case "the present King of France" doesn't exist and should raise
a FifthRepublicException :)
inf / inf does not raise an exception, but evaluates to NaN, so NaN
exists. For objects (that exist):
(x is x) is True.
The present President of France is the present President of France,
regardless of who he or she may be.
This might seem surprising to those who are used to reflexivity. Oh
well. Just because reflexivity holds for actual things, doesn't mean it
holds for, er, things that aren't things. NANs are things that aren't
things.
A NaN is thing that *is* a thing; it exists:
object.__repr__(float('nan'))
Of course if inf - inf, inf/inf raised exceptions,
then NaN wouldn't exist (as a Python object)
and the problem would just go away :)
After all 0.0/0.0 already raises an exception, but the
IEEE defines 0.0/0.0 as NaN.
Although both NaN == NaN and NaN != NaN could arguably be a "maybe"
value, the all important reflexivity (x == x is True) is effectively
part of the language.
All collections rely on it and Python wouldn't be much use without
dicts, tuples and lists.
Perhaps they shouldn't rely on it. Identity tests are an implementation
detail. But in any case, reflexivity is *not* a guarantee of Python.
With rich comparisons, you can define __eq__ to do anything you like.
And if you do define __eq__ to be non-reflexive then things will break.
Should an object that breaks so much (ie NaN in its current form) be in
the standard library?
Perhaps we should just get rid of it?
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
http://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
