Can I access a class variable using object of that class

Question

I have two very basic object oriented question

1) Can we modify a class variable with member function?

For example

class test:
    '''test class'''

    idd=0
    def __init__(self,mark,subject):

        self.markk=mark
        self.subjectt=subject

    def display(self):
        print "the display is",self.markk,self.subjectt;
stud1=test(30,'maths')
stud2=test(40,'english')

when i tried to modify class variable idd using the object stud1.idd=9;, the variable didnt modified. test.idd=9 modified the variable.

Can someone explain me why it is not possible to modify a class vars from a object?

2) Also in the above snippet, when I added a keyword global with the class var idd like

class test:
    '''test class'''
    global idd;
   idd=0;
print test.idd

it threw error like name class test don't have attribute idd.

But when I commented out the global idd, it displayed value.

So is global keyword not supported in class?

can someone help me to get some idea on these two question as this is my basic step to object oriented concept..

mgilson · Accepted Answer · 2016-06-29 15:53:38Z

1

I think that you're not understanding that python looks up values by looking at a "chain" of objects¹. When you do value = self.foo, python will first look for foo on the instance. Then it will look on the class. Then it will look on the super-classes (in their "Method Resolution Order").

When you write:

self.foo = 'bar'

Python simply puts a foo on the instance. So now subsequent lookups on that instance will give you 'bar' even if foo is also defined on the class. Also note that since foo gets put on the instance, no changes are visible on the class.

If you want to update the class in a particular method, you might be able to use a classmethod:

class Foo(object):
    idd = 0
    @classmethod
    def increment_idd(cls):
        cls.idd += 1

f = Foo()
f.increment_idd()
print(Foo.idd)
print(f.idd)

This doesn't work if you need access to self however. In that case, you'll need to get a reference to the class from the instance:

class Foo(object):
    idd = 0
    def increment_idd(self):
        cls = type(self)
        cls.idd += 1

f = Foo()
f.increment_idd()
print(Foo.idd)
print(f.idd)

^{¹If you know javascript, it's actually not too much different than javascript's prototypical inheritance}

edited Jun 29, 2016 at 15:53

answered Jun 29, 2016 at 15:47

mgilson

312k70 gold badges656 silver badges722 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

syam Over a year ago

any idea on second question?@mgilson

mgilson Over a year ago

@syam -- Correct, global is not supported on a class. I'm not really sure what that would do exactly ...

syam Over a year ago

I have a question when i executed this.In the same code i used classmethod and i am able to modify the class var from object. But in my case i have two objects.. so when i use the 1st object to display its attr, the 2nd obj attr is displaying. class test: idd=0 @classmethod def __init__(self,mark,subject): self.markk=mark self.subjectt=subject self.idd=67; def display(self): print "the display is",self.markk,self.subjectt; stud1=test(30,'maths') stud2=test(40,'english') stud1.display();-------->will display stud2's values

syam Over a year ago

If i comment the @classmaker, it works as expected. Any idea why this happens so?

syam Over a year ago

When i used the @classmethod and created two objs like ob1=class(arg) obj2=class(arg) then when i used the obj1 for displaying the attrs, i got obj2's attr. I meant to say attrs of obj1 got overwritten? when i commented classmaker, each objects attr displayed correctly.. example i used is same as of parent question. i used stud1.display() to display stud1's attr. Here it displayed stud2's attr when i used classmaker [stud2 obj created after stud1]. I am sorry if i am confusing you.. let me know, i can explain more

|

rojobo · Accepted Answer · 2016-06-29 15:43:59Z

0

As long as it is a public variable you should be able to.

answered Jun 29, 2016 at 15:43

rojobo

4864 silver badges17 bronze badges

1 Comment

syam Over a year ago

thanks for quick replay. Are you replying for first question?. there i can modify the class var from inside any member function outside using classname.var, but not with the object

Collectives™ on Stack Overflow

Can I access a class variable using object of that class

2 Answers 2

6 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

6 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related