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Date: Thu, 28 Jul 2011 15:26:21 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Subject: Re: NoneType and new instances
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:10487

Ben Finney wrote:
> Ethan Furman <ethan@stoneleaf.us> writes:
> 
>> Why is NoneType unable to produce a None instance? I realise that None
>> is a singleton
> 
> That answers your question. Because None is a singleton, the job of its
> type is to make sure there are no other instances.

Which it can do quite easily by returning the single instance of None -- 
it is not necessary to raise an exception to fulfill its duty.


>> but so are True and False, and bool is able to handle returning them:
> 
> Well, they don't meet the definition of a singleton, because there are
> two instances of ‘bool’ :-)

Okay, a slightly relaxed definition of singleton, since there is only 
ever one instance with the value of True, or the value of False; 
likewise, there is only ever one instance with the value of None.


>> This feels like a violation of 'Special cases aren't special enough to
>> break the rules.'
> 
> In the case of ‘bool’, the rule was broken before being introduced.

I think we disagree on what the rule is.  I see it as "Return an 
instance if you can."  Nobody has yet pointed out a good reason on why 
NoneType, NotImplementedType, and ellipsis (to be thorough ;) cannot or 
should not return the single instance that exists, when every other 
built-in will return either a new object, or the single object that 
exists for that value.

~Ethan~