On 2008-11-12 00:09, Antoine Pitrou wrote:
> M.-A. Lemburg <mal <at> egenix.com> writes:
>>>>> None > None
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> TypeError: unorderable types: NoneType() > NoneType()
>>>>> None != None
>> False
>>
>> Two values that compare equal to each other (and are in fact identical),
>> yet cannot be compared less-than or greater-than.
>
> The error message is clear: "unorderable types". Having some types support an
> equivalence relation (e.g. "equality") but no intuitive total order relation
> is
> hardly a surprise. As someone said, complex numbers are an example of that
> (not
> only in Python, but in real life).
The difference is that None is a singleton, so the set of all
None type instances is {None}. You always have an intuitive total order
relation on one element sets: the identity relation.
>> This would make sense if you think of None as meaning "anything
>> and/or nothing", since the left side None could stand for
>> a different None than the right one, but then you could apply the
>> same logic to inf:
>
> inf is a float instance, and as such supports ordering. I don't see how it
> invalidates None *not* supporting an order relation, since None isn't a float
> instance and doesn't pretend to be usable as a number (or as anything
> supporting
> ordering, for that matter).
Right, but you're taking the view of a CPython developer. You
need to view this as Python user.
In real (math) life, inf is a different type of number than regular floats,
ints or complex numbers and has a special meaning depending on the context
in which you use it. The relationship is much like that of None to all
other Python objects.
--
Marc-Andre Lemburg
eGenix.com
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