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Re: Why do class methods always need 'self' as the first parameter?

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On Wed, Aug 31, 2011 at 11:12 AM, Prasad, Ramit
<ramit.prasad@jpmorgan.com> wrote:
> It seems to me that if I add a function to the list of class attributes it will automatically wrap with "self" but adding it to the object directly will not wrap the function as a method. Can somebody explain why? I would have thought that any function added to an object would be a method (unless decorated as a class method).

Because things stored on the class are generally viewed as part of the
class definition, whereas things stored on an instance are generally
viewed as data -- a function stored on an object instance is usually
just meant to be a function.  Consider the following code:

class Sorter(object):
    def __init__(self, keyfunc):
        self.keyfunc = keyfunc
    def sort(self, item_list):
        item_list.sort(key=self.keyfunc)

sorter = Sorter(lambda x: x.id)
sorter.sort(some_list_of_items)

If adding keyfunc as an attribute to the object wrapped it up as a
method, it would break, since the function is not expecting a "self"
argument.

More technically, because descriptors are only invoked when they're
stored on the class.

> Hmm, or does the decoration just tell Python not to turn an object's function into a method? I.e. Is the decorator basically just the syntactic sugar for doing the above?

If you mean the staticmethod decorator, yes, it pretty much just wraps
the function as a "staticmethod" instance to prevent it from being
wrapped into an ordinary method when it's accessed.

Cheers,
Ian

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Thread

Why do class methods always need 'self' as the first parameter? "T. Goodchild" <tgoodchild@gmail.com> - 2011-08-31 07:35 -0700
  Re: Why do class methods always need 'self' as the first parameter? John Gordon <gordon@panix.com> - 2011-08-31 14:54 +0000
    Re: Why do class methods always need 'self' as the first parameter? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-09-01 02:06 +1000
      Re: Why do class methods always need 'self' as the first parameter? Grant Edwards <invalid@invalid.invalid> - 2011-08-31 16:20 +0000
  Re: Why do class methods always need 'self' as the first parameter? Emile van Sebille <emile@fenx.com> - 2011-08-31 08:01 -0700
  Re: Why do class methods always need 'self' as the first parameter? Neil Cerutti <neilc@norwich.edu> - 2011-08-31 15:05 +0000
  Re: Why do class methods always need 'self' as the first parameter? Javier Collado <javier.collado@gmail.com> - 2011-08-31 17:10 +0200
  Re: Why do class methods always need 'self' as the first parameter? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-09-01 01:41 +1000
    Re: Why do class methods always need 'self' as the first parameter? Chris Torek <nospam@torek.net> - 2011-08-31 20:15 +0000
      Re: Why do class methods always need 'self' as the first parameter? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-09-01 10:48 +1000
        Re: Why do class methods always need 'self' as the first parameter? Chris Angelico <rosuav@gmail.com> - 2011-09-01 11:47 +1000
        Re: Why do class methods always need 'self' as the first parameter? Eric Snow <ericsnowcurrently@gmail.com> - 2011-08-31 20:07 -0600
        Re: Why do class methods always need 'self' as the first parameter? Chris Torek <nospam@torek.net> - 2011-09-01 02:20 +0000
      Re: Why do class methods always need 'self' as the first parameter? Piet van Oostrum <piet@vanoostrum.org> - 2011-09-05 16:15 +0200
        Re: Why do class methods always need 'self' as the first parameter? Chris Torek <nospam@torek.net> - 2011-09-06 01:10 +0000
          Re: Why do class methods always need 'self' as the first parameter? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-09-06 14:31 +1000
  Re: Why do class methods always need 'self' as the first parameter? Terry Reedy <tjreedy@udel.edu> - 2011-08-31 12:30 -0400
  RE: Why do class methods always need 'self' as the first parameter? "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2011-08-31 13:12 -0400
    Re: Why do class methods always need 'self' as the first parameter? Piet van Oostrum <piet@vanoostrum.org> - 2011-09-08 01:01 +0200
  Re: Why do class methods always need 'self' as the first parameter? Chris Rebert <clp2@rebertia.com> - 2011-08-31 11:31 -0700
  Re: Why do class methods always need 'self' as the first parameter? Ian Kelly <ian.g.kelly@gmail.com> - 2011-08-31 15:52 -0600
  Re: Why do class methods always need 'self' as the first parameter? Terry Reedy <tjreedy@udel.edu> - 2011-08-31 18:40 -0400
  Re: Why do class methods always need 'self' as the first parameter? UncleLaz <andrei.lisnic@gmail.com> - 2011-09-01 03:40 -0700
    Re: Why do class methods always need 'self' as the first parameter? Michiel Overtoom <motoom@xs4all.nl> - 2011-09-01 13:23 +0200
  Re: Why do class methods always need 'self' as the first parameter? John Roth <johnroth1@gmail.com> - 2011-09-01 05:45 -0700
    Re: Why do class methods always need 'self' as the first parameter? Ian Kelly <ian.g.kelly@gmail.com> - 2011-09-01 08:26 -0600
      Re: Why do class methods always need 'self' as the first parameter? John Roth <johnroth1@gmail.com> - 2011-09-02 10:51 -0700
        Re: Why do class methods always need 'self' as the first parameter? Ian Kelly <ian.g.kelly@gmail.com> - 2011-09-02 14:30 -0600
          Re: Why do class methods always need 'self' as the first parameter? John Roth <johnroth1@gmail.com> - 2011-09-02 15:30 -0700
  Re: Why do class methods always need 'self' as the first parameter? rantingrick <rantingrick@gmail.com> - 2011-09-05 20:58 -0700

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