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

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On Sat, 24 Dec 2011 23:09:50 -0800, GZ wrote:

> 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)
> 
> 
> The problem is that == is not implemented sometimes for values in a
> and c, causing an exception NotImplementedError.

I have no idea how that can happen. If a.__eq__(None) returns
NotImplemented, the interpreter should flip the test and perform the
equivalent of None.__eq__(a), which will return False.

> So how do I reliably test if a value is None or not?

As Paul says, use "is" to check whether a value _is_ None. Checking for
equality is almost certainly the wrong thing to do; nothing should compare
equal to None except for None itself, so "x is None" and "x == None"
shouldn't produce different results unless there's a bug in the comparison
method.

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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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