[Python-ideas] change NoneType, NotImplementedType, & ellipses to return the appropriate singleton

[Guido van Rossum]

I think it is fine if type(None)() returns None instead of raising an exception.

On Fri, Jul 29, 2011 at 9:37 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> Consider:
>
> Python 3.2 (r32:88445, Feb 20 2011, 21:29:02) [MSC v.1500 32 bit (Intel)] on
> win32
> Type "help", "copyright", "credits" or "license" for more information.
> --> for ins in ({0:'0'}, (1,), set([2, 3]), [4, 5], 6, 'seven',
> ... 8.0, True, None):
> ...   print(type(ins))
> ...   type(ins)()
> ...
> <class 'dict'>
> {}
> <class 'tuple'>
> ()
> <class 'set'>
> set()
> <class 'list'>
> []
> <class 'int'>
> 0
> <class 'str'>
> ''
> <class 'float'>
> 0.0
> <class 'bool'>
> False
> <class 'NoneType'>
> Traceback (most recent call last):
>  File "<stdin>", line 3, in <module>
> TypeError: cannot create 'NoneType' instances
>
> I hit this issue when I had a dictionary of types to instantiate, one of
> which was NoneType.
>
> Is there any reason why we shouldn't change these three to produce the
> singleton instance instead of raising an error?
>
> Pro:  They would then be consistent with every other built-in which will
> return either a new instance (int, str, float, etc), or the singleton
> instance that already exists (bool, and sometimes int & str), as well as no
> longer needing the `lambda:None` workaround (or worse, a try-except if you
> don't know what types are coming in).
>
> Con:  Might be code that relies on the exception being raised -- is anybody
> aware of any?
>
> ~Ethan~
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-- 
--Guido van Rossum (python.org/~guido)