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

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From	Devin Jeanpierre <jeanpierreda@gmail.com>
Date	Sun, 25 Dec 2011 19:06:50 -0500
Subject	Re: Test None for an object that does not implement ==
To	Larry Hudson <orgnut@yahoo.com>
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> At first glance this looked like it should be a simple boolean "and", but
> then I realized that when a and c are both unequal to None, the result would
> also be True.  This implies the logical approach would be exclusive-or (^).

Among booleans, "!=" is exclusive or and "==" is its negation. I don't
see the point of complicating the formula to use ^.

(related: <= is implication. Which is sad, because the arrow points
the wrong way!)

The issue here is that "== None" is being used instead of "is None",
but I believe that's been covered. Your response doesn't include it,
so maybe it's worth restating.

-- Devin

On Sun, Dec 25, 2011 at 6:45 PM, Larry Hudson <orgnut@yahoo.com> wrote:
> On 12/24/2011 11:09 PM, GZ wrote:
>>
>> Hi,
>>
>> I run into a weird problem. I have a piece of code that looks like the
>> following:
>>
>> f(...., a=None, c=None):
>>     assert  (a==None)==(c==None)
>>
> <...>
>
> At first glance this looked like it should be a simple boolean "and", but
> then I realized that when a and c are both unequal to None, the result would
> also be True.  This implies the logical approach would be exclusive-or (^).
>  Try this expression:
>
>     not ((a==None) ^ (c==None))
>
> OTOH, if what you really want is simply to check that both are None (my
> first impression), then it's simply:
>
>     (a==None) and (c==None)
>
> Most of the replies you're getting here seem unnecessarily complicated.
>
>> Thanks,
>> gz
>
>
>    -=- Larry -=-
> --
> http://mail.python.org/mailman/listinfo/python-list

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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