John,
I agree with you and I also think the definition given on the official
python site is somewhat confusing, at least for an engineer like myself.
But I'll take a stab at explaning it using what I know thus far.
I think to understand what a class method is you have to first understand
what a class variable is. Take this code snippet for example.
class foo(object):
x=10
def __init__(self):
self.x=20
In this example if you create an instance of foo and call it f. You can
access the instance attribute x by using f.x or you can access the class
variable using the notation foo.x. These two will give you two different
results. Now lets do something a bit more interesting.
Say you have the following snippet.
class foo(object):
x=0
def __init__(self):
self.x=10
foo.x+=1
>>>f=foo()
>>>g=goo()
>>>f.x
10
>>>g.x
10
>>>foo.x
2
What happened here is that the class variable foo.x is used to keep a count
of the total number of instances created. So we see that a class variable
can be looked at as what "connects" the two instances in a way, so that data
can be shared between instances of the same class. This defintion may very
well only apply to this case, but I think the mechanics is fundamentally the
same.
Keeping this in mind, lets make the jump to class methods. When an
instance of a class is created, the methods are just functions that "work
on" the attributes (the variables). Similarly, a class method is a method
that works on a class variable. For example,
class foo(object):
x=10
def __init__(self):
self.x=20
@classmethod
def change(self):
self.x=15
>>>f=foo()
>>>f.x
20
>>>foo.x
10
>>>f.change()
>>>f.x
20
>>>foo.x
15
So this is my explanation for what a classmethod is. Hope it helps. Good
luck.
Denis
On Mon, Aug 23, 2010 at 2:24 AM, John Nagle <na...@animats.com> wrote:
> On 8/22/2010 9:16 PM, James Mills wrote:
>
>> On Mon, Aug 23, 2010 at 1:53 PM, Paulo da Silva
>>
>> <psdasilva.nos...@netcabonospam.pt> wrote:
>>
>>> I did it before posting ...
>>>
>>> The "explanation" is not very clear. It is more like "how to use it".
>>>
>>
>> Without going into the semantics of languages basically the
>> differences are quite clear:
>>
>> @classmethod is a decorator that warps a function with
>> passes the class as it's first argument.
>>
>> @staticmethod (much like C++/Java) is also a decorator that
>> wraps a function but does not pass a class or instance as
>> it's first argument.
>>
>
> That reads like something the C++ standards revision committee
> would dream up as they add unnecessary template gimmicks to the
> language.
>
> John Nagle
>
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> http://mail.python.org/mailman/listinfo/python-list
>
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