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Re: data attributes override method attributes?

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On Fri, Sep 28, 2012 at 12:02 PM, Prasad, Ramit
<ramit.prasad@jpmorgan.com> wrote:
> Just to make sure I am following, if you call
> foo.__len__() it goes to the instance code while
> if you do len(foo) it will go to class.__len__()?

Yes:

>>> class Foo(object):
...     def __len__(self):
...         return 42
...
>>> foo = Foo()
>>> foo.__len__ = lambda: 43
>>> foo.__len__()
43
>>> len(foo)
42

> If so, why?

In the first case, "foo.__len__" just does the normal attribute lookup
for the class.  Instance attributes shadow class attributes, so the
instance attribute is returned and then called.

In the second case, "len(foo)" is implemented by a method in a
prescribed location:  foo.__class__.__len__.  It only looks in the
class for efficiency and because that is what the class object is for:
to define how its instances behave.

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data attributes override method attributes? Jayden <jayden.shui@gmail.com> - 2012-09-25 06:41 -0700
  Re: data attributes override method attributes? alex23 <wuwei23@gmail.com> - 2012-09-25 06:52 -0700
  Re: data attributes override method attributes? Peter Otten <__peter__@web.de> - 2012-09-25 16:08 +0200
    Re: data attributes override method attributes? alex23 <wuwei23@gmail.com> - 2012-09-25 07:11 -0700
      Re: data attributes override method attributes? Peter Otten <__peter__@web.de> - 2012-09-25 16:54 +0200
      Re: data attributes override method attributes? Chris Angelico <rosuav@gmail.com> - 2012-09-26 01:03 +1000
      Re: data attributes override method attributes? Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-09-25 16:54 +0200
      Re: data attributes override method attributes? Terry Reedy <tjreedy@udel.edu> - 2012-09-25 15:58 -0400
      Re: data attributes override method attributes? Ian Kelly <ian.g.kelly@gmail.com> - 2012-09-25 14:07 -0600
      Re: data attributes override method attributes? Terry Reedy <tjreedy@udel.edu> - 2012-09-25 16:18 -0400
      Re: data attributes override method attributes? Terry Reedy <tjreedy@udel.edu> - 2012-09-25 16:34 -0400
      RE: data attributes override method attributes? "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-09-28 18:02 +0000
        Re: data attributes override method attributes? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-29 03:20 +0000
      Re: data attributes override method attributes? Ian Kelly <ian.g.kelly@gmail.com> - 2012-09-28 12:25 -0600
      Re: data attributes override method attributes? Terry Reedy <tjreedy@udel.edu> - 2012-09-28 14:26 -0400
    Re: data attributes override method attributes? Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-09-25 21:52 +0200
  Re: data attributes override method attributes? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-25 14:12 +0000

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