Re: getattr Confusion

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From	Peter Otten <__peter__@web.de>
Subject	Re: __getattr__ Confusion
Date	Mon, 04 Feb 2013 15:15:47 +0100
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Saul Spatz wrote:

> Now I have another question.  If dunder methods are looked up only in the
> class, not the instance, why did defining __nonzero__ the way I did work? 
> Shouldn't I have had to define it with a def?  Is __nonzero__ a special
> case?

Unfortunately the situation is a bit more complex. Classic classes (like 
Tkinter.Frame) behave differently from newstyle classes (subclasses of 
object):

>>> def nz():
...     print "nonzero"
...     return 0
... 
>>> class Classic: pass
... 
>>> c = Classic()
>>> c.__nonzero__ = nz
>>> not c
nonzero
True
>>> class New(object): pass
... 
>>> n = New()
>>> n.__nonzero__ = nz
>>> not n
False

So Steven is wrong here.

> Shouldn't I have had to define it with a def?

If you mean as opposed to a lambda, there is no difference between

f = lambda ...

and

def f(...): ...

other than that the last one gives you a nice name in a traceback.

Thread

__getattr__ Confusion Saul Spatz <saul.spatz@gmail.com> - 2013-02-03 17:08 -0800
  Re: __getattr__ Confusion Chris Angelico <rosuav@gmail.com> - 2013-02-04 12:28 +1100
  Re: __getattr__ Confusion Terry Reedy <tjreedy@udel.edu> - 2013-02-03 21:47 -0500
  Re: __getattr__ Confusion Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-04 04:35 +0000
    Re: __getattr__ Confusion Saul Spatz <saul.spatz@gmail.com> - 2013-02-04 05:44 -0800
      Re: __getattr__ Confusion Peter Otten <__peter__@web.de> - 2013-02-04 15:15 +0100
        Re: __getattr__ Confusion Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-05 01:46 +1100
        Re: __getattr__ Confusion Saul Spatz <saul.spatz@gmail.com> - 2013-02-04 09:29 -0800
          Re: __getattr__ Confusion Peter Otten <__peter__@web.de> - 2013-02-04 19:23 +0100
          Re: __getattr__ Confusion Chris Angelico <rosuav@gmail.com> - 2013-02-05 12:00 +1100
        Re: __getattr__ Confusion Saul Spatz <saul.spatz@gmail.com> - 2013-02-04 09:29 -0800

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