Passing a function as an argument from within the same class?

[Steven D'Aprano]

On Fri, 01 May 2009 08:11:01 -0700, zealalot wrote:

> On May 1, 10:50 am, CTO <debat... at gmail.com> wrote:
>> Make doNothing a classmethod.
>>
>> class SomeClass:
>>
>>     @classmethod
>>     def doNothing(cls):
>>         pass
>>
>>     def function1(self):
>>         print "Running function 1"
>>
>>     def function2(self, passedFunction=SomeClass.doNothing):
>>         print "Running passed function"
>>         passedFunction()
>>
>> someObject = SomeClass()
>> someObject.function2()
>> someObject.function2(someObject.function1)
> 
> It's not surprising, but I've never heard of a classmethod before.
> Basically, I read that it basically removes the need for the 'self'
> argument.  Very cool!

Not so.

When you call an ordinary method (an "instance method"), Python 
automatically inserts the object itself as the first argument. So you 
need to define the method with one extra parameter, self.

(Using the name self is just a convention. You can call it anything you 
like.)

For classmethods, Python automatically inserts not the object itself, but 
the object's *class* as the first argument. So you still need to define 
the method with an extra parameter, only now the convention is to call is 
cls rather than self.

Here's an example:

class Test(object):
    x = 0  # class attribute
    def __init__(self, x):
        self.x = x  # instance attribute
    def spam(self):
        print self, self.x
    @classmethod
    def ham(cls):
        print cls, cls.x


And in use:

>>> t = Test(42)
>>> t.spam()
<__main__.Test object at 0xb7cccc6c> 42
>>> t.ham()
<class '__main__.Test'> 0


There is one other related built-in decorator, staticmethod(). 
staticmethod() tells Python not to pass any extra argument to the method. 
That means the inside a static method, you can't refer to either the 
instance (self) or the class! Needless to say, there aren't very many 
uses for staticmethod(). 



-- 
Steven