Access Class method and variable using self

Question

In below example Test class has two instance method and one classmethod

In set_cls_var_1 method I set class variable using self.

In set_cls_var_2 method I call class method using self.

   class Test():

        #class variable
        cls_var = 10

        def __init__(self):
           obj_var=20

        def set_cls_var_1(self,val):
            #second method to access class variable
            print "first "
            self.cls_var = val

        def set_cls_var_2(self):
            print "second"
            self.task(200)

        @classmethod
        def task(cls,val):
            cls.cls_var = val


t=Test()

#set class variable by first method
t.set_cls_var_1(100)

print Test.cls_var

#set class variable by second method
t.set_cls_var_2()

print Test.cls_var

Output

first 
10
second
200

Expected Output

first 
100
second
200

My question is: why only classmethod can call by self, Why not class variable

Answer 1

When you attempt to access an object's attribute using self, Python first searches the object's attributes. If it cannot find it there, then is searches the object's class's attributes. That is what's happening in your case;

Python first searches t's attributes. It doesn't find cls_var, so it then searches the T class's attributes. It finds cls_var so it stops, and returns cls_var's value.

However, when assigning attributes to self, Python always assigns them directly to the object, and never the object's class unless explicitly told to do so. That's why assinging self.cls_var to 100 didn't affect Test's cls_var attrbiute.