Re: Test None for an object that does not implement ==

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Date	Mon, 26 Dec 2011 00:35:46 +1100
Subject	Re: Test None for an object that does not implement ==
From	Chris Angelico <rosuav@gmail.com>
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On Mon, Dec 26, 2011 at 12:17 AM, Roy Smith <roy@panix.com> wrote:
> Just for fun, I tried playing around with subclassing NoneType and
> writing an __eq__ for my subclass.  Turns out, you can't do that:
>
> Traceback (most recent call last):
>  File "./none.py", line 5, in <module>
>    class Nihil(NoneType):
> TypeError: Error when calling the metaclass bases
>    type 'NoneType' is not an acceptable base type

Yes; unfortunately quite a few Python built-in classes can't be
subclassed. It's an unfortunate fact of implementation, I think,
rather than a deliberate rule.

But then, what would you ever need to subclass None for, other than
toys and testing?

ChrisA

Thread

Test None for an object that does not implement == GZ <zyzhu2000@gmail.com> - 2011-12-24 23:09 -0800
  Re: Test None for an object that does not implement == Paul Rubin <no.email@nospam.invalid> - 2011-12-24 23:28 -0800
  Re: Test None for an object that does not implement == Nobody <nobody@nowhere.com> - 2011-12-25 09:38 +0000
    Re: Test None for an object that does not implement == Lie Ryan <lie.1296@gmail.com> - 2011-12-25 22:10 +1100
      Re: Test None for an object that does not implement == Roy Smith <roy@panix.com> - 2011-12-25 08:17 -0500
        Re: Test None for an object that does not implement == Chris Angelico <rosuav@gmail.com> - 2011-12-26 00:35 +1100
          Re: Test None for an object that does not implement == Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-25 13:48 +0000
            Re: Test None for an object that does not implement == Chris Angelico <rosuav@gmail.com> - 2011-12-26 01:04 +1100
            Re: Test None for an object that does not implement == Christian Heimes <lists@cheimes.de> - 2011-12-26 01:37 +0100
          Re: Test None for an object that does not implement == Roy Smith <roy@panix.com> - 2011-12-25 09:13 -0500
            Re: Test None for an object that does not implement == Chris Angelico <rosuav@gmail.com> - 2011-12-26 01:23 +1100
            Re: Test None for an object that does not implement == Lie Ryan <lie.1296@gmail.com> - 2011-12-26 04:13 +1100
    Re: Test None for an object that does not implement == Ian Kelly <ian.g.kelly@gmail.com> - 2011-12-25 19:02 -0700
    Re: Test None for an object that does not implement == Ethan Furman <ethan@stoneleaf.us> - 2011-12-25 20:26 -0800
  Re: Test None for an object that does not implement == Larry Hudson <orgnut@yahoo.com> - 2011-12-25 15:45 -0800
    Re: Test None for an object that does not implement == Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-25 19:06 -0500
      Re: Test None for an object that does not implement == Roy Smith <roy@panix.com> - 2011-12-25 19:27 -0500
        Re: Test None for an object that does not implement == Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-25 19:58 -0500
    Re: Test None for an object that does not implement == Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-12-26 00:52 +0000
  Re: Test None for an object that does not implement == Ethan Furman <ethan@stoneleaf.us> - 2011-12-25 20:27 -0800
  Re: Test None for an object that does not implement == Devin Jeanpierre <jeanpierreda@gmail.com> - 2011-12-26 00:22 -0500
    Re: Test None for an object that does not implement == Roy Smith <roy@panix.com> - 2011-12-26 08:43 -0500
  Re: Test None for an object that does not implement == Paul Rudin <paul.nospam@rudin.co.uk> - 2011-12-26 14:33 +0000
  Re: Test None for an object that does not implement == Ethan Furman <ethan@stoneleaf.us> - 2011-12-26 09:14 -0800

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