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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Wed, 31 Aug 2011 15:52:48 -0600
Subject: Re: Why do class methods always need 'self' as the first parameter?
To: "Prasad, Ramit" <ramit.prasad@jpmorgan.com>
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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 i=
t 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 h=
ave 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 =3D keyfunc
    def sort(self, item_list):
        item_list.sort(key=3Dself.keyfunc)

sorter =3D 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 func=
tion into a method? I.e. Is the decorator basically just the syntactic suga=
r 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