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Re: type(None)()

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On Thu, Aug 16, 2012 at 10:47 PM, Hans Mulder <hansmu@xs4all.nl> wrote:
> Why doesn't it just return an existing instance of the type,
> like bool, int, str and other built-in non-mutable types do?
>
>> py> type(False)() is False
>> True

With int and str, it's only an optimization, and not guaranteed to happen.

>>> a=int("1234")
>>> a is int("1234")
False

>>> a=str(1234)
>>> a is str(1234)
False

But with bool, it's required, as a means of "casting to boolean". With
True/False/None, it's normal to compare them with is:

>>> a=bool("1")
>>> a is bool("2")
True

So bool() has to return one of those two actual objects, and not an equivalent.

(Note: All examples done in CPython 3.2's IDLE on Windows. Other
environments, Pythons, versions, etc, may affect exactly what these
show.)

ChrisA

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type(None)() Hans Mulder <hansmu@xs4all.nl> - 2012-08-16 14:47 +0200
  Re: type(None)() Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-16 14:54 +0200
  Re: type(None)() Chris Angelico <rosuav@gmail.com> - 2012-08-16 23:37 +1000
  Re: type(None)() Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 07:56 -0600
  Re: type(None)() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-16 13:58 +0000
    Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:31 +0200
    Re: type(None)() Stefan Behnel <stefan_ml@behnel.de> - 2012-08-16 17:44 +0200
    Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:06 -0700
  Re: type(None)() Robert Kern <robert.kern@gmail.com> - 2012-08-16 15:56 +0100
  Re: type(None)() MRAB <python@mrabarnett.plus.com> - 2012-08-16 16:13 +0100
  Re: type(None)() Ethan Furman <ethan@stoneleaf.us> - 2012-08-16 09:07 -0700

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