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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: NoneType and new instances
Date: Thu, 28 Jul 2011 21:32:34 -0400
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:10496

On 7/28/2011 5:03 PM, Ethan Furman wrote:

> I'm glad you asked! I'm using dictionaries to describe fields and
> what their return values should be. There happen to be two special
> cases: empty and Null. So a portion of the dictionary looks like:
>
> fielddef = { 'empty':some_func, 'null':some_func }
>
> Depending on the needs of the user, `some_func` might be `str`,
> `int`, `custom_class`, or `NoneType`. Somewhere else in the code I
> have:
>
> if not value: # or some such cls = fielddef['empty'] return cls()
>
> This works for *everything* except NoneType. g
> ~Ethan~
>
> PS I'll use a lambda to get around it, but that's not very elegant.
> Why shouldn't NoneType be able to return the singleton None?

Possibly historical accident. You could ask on python-ideas
or put a feature request on the tracker. Start with the first in case 
there is a ready answer I do not know of.
The big question is whether there is code depending on current behavior.