Best way to access class-method into instance method

Question

class Test(object):

    def __init__(self):
        pass

    def testmethod(self):
        # instance method
        self.task(10)      # type-1 access class method

        cls = self.__class__ 
        cls.task(20)        # type-2 access class method 

    @classmethod 
    def task(cls,val)
        print(val)

I have two way to access class method into instance method.

self.task(10)

or

cls = self.__class__
cls.task(20)

My question is which one is the best and why??

If both ways are not same, then which one I use in which condition?

You can also just directly call Test.task(20) from within that method, instead of your two-liner. — xgord
– xgord, Commented Jul 18, 2017 at 17:39
@xgord: yes but the two are not equivalent. Task.task(20) will always call the task defined in Task whereas a subclass can override the method. In that case self.task(20) in a SubTask class will access SubTask.task(20). — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jul 18, 2017 at 17:42
That depends on the intended use - self will always refer to the current instances' methods and will persist not only through the inheritance chain (compared to calling by a class name, i.e. Test.task()), picking up the latest override, even a dynamic one, while referring by class type will always point to the actual class method. Nothing stops you from setting your_instance.task = some_dynamic_override and then self.task() will be calling that function. — zwer
– zwer, Commented Jul 18, 2017 at 17:44
wouldnt it be the first one considering that self.__class__ would create an object while the first one is a direct call? — paws
– paws, Commented Jul 18, 2017 at 17:46
@xgord I want to know that both ways are same or not, if not then what is differences and when I use first type or second method — Kallz
– Kallz, Commented Jul 19, 2017 at 7:01

Serge Ballesta · Accepted Answer · 2017-07-19 12:44:03Z

9

self.task(10) is definitely the best.

First, both will ultimately end in same operation for class instances:

__class__ is a special attribute that is guaranteed to exist for an class instance object and is is the class of the object (Ref: Python reference manual / Data model / The standard type hierarchy)

Class instances
...
Special attributes: __dict__ is the attribute dictionary; __class__ is the instance’s class

calling a classmethod with a class instance object actually pass the class of the object to the method (Ref: same chapter of ref. manual):

...When an instance method object is created by retrieving a class method object from a class or instance, its __self__ attribute is the class itself

But the first is simpler and does not require usage of a special attribute.

answered Jul 19, 2017 at 12:44

Serge Ballesta

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