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Fri Apr 26 03:12:56 CEST 2013 · https://mail.python.org/pipermail/python-dev/2013-April/125570.html

On 26/04/2013 01:27, Steven D'Aprano wrote:
> On 26/04/13 09:56, MRAB wrote:
>
>> On the one hand, NoneType(None) seems a strange thing to do.
>
> Only when you write it out like that as constants. It's no more,
> or less, strange than str('spam') or int(1) or list([]). Why
> would you do that?
>
None is a singleton, but instances of str, int, list, etc aren't. Why
can it take an argument when there's only ever one of them?

That's why it seems strange to me.

> But as soon as you think of it in general terms:
>
> some_type(some_instance)
>
> that's a pretty normal thing to do. And if it just so happened
> that some_instance were an instance of some_type, it would be
> surprising if the call failed.
>
> (I initially wrote "astonishing", but then I realised that some
> types take more than one argument, e.g. FunctionType. So it's
> merely surprising.)
>
>> On the other hand:
>>
>>      type(value)(value) == value
>>
>> would return True for the built-in types (will certain exceptions, such
>> as when value is float("NaN")).
>
> Not an exception, that works fine in 3.3:
>
>>>> value = float('nan')
>>>> type(value)(value)
> nan
>
But:

 >>> value = float('NaN')
 >>> type(value)(value) == value
False

That's what I mean by it being an "exception".

>> Let's ask the Zen:
>>
>> Special cases aren't special enough to break the rules.
>>
>> Although practicality beats purity.
>
> I cannot think of any use-case where I would actively want
> NoneType(None) to fail. That would be like having bool(True)
> raise an exception.
>
> On the other hand, NoneType(x) for any other x ought to fail.
>
OK, so practicality (or pragmatism) wins.

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[Python-Dev] NoneType(None) raises exception