[Python-Dev] NoneType(None) raises exception

Steven D'Aprano steve at pearwood.info
Fri Apr 26 02:27:59 CEST 2013

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?

But as soon as you think of it in general terms:


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)

> 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.


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