Sharing class variables in Python inheritance

Question

I am confused with variable sharing concept in Python in inheritance.

Consider the following code:-

class a(object):
    var1 = 0
    var2 = {}
    def print_var(self):
        print(self.var1)
        print(self.var2)

class b(a):
    @classmethod
    def modify_var(cls):
        cls.var1 = 1
        cls.var2['temp']="something"

o1 = a()
o2 = b()
print("Initial Values")
o1.print_var()
o2.print_var()
print("Changing Values")
o2.modify_var()
print("Print values after change")
o1.print_var()
o2.print_var()

After running above code I can see the dictionary is being shared between child and parent class, but integer variable isn't.

Can anyone please explain this, or what I am doing wrong here?

Output of above code:

Initial Values
0
{}
0
{}
Changing Values
Print values after change
0    # <- this zero should be one according to my understanding
{'temp': 'something'}
1
{'temp': 'something'}

The dictionary seems shared because you are not reassigning it. Try assigning it to a different dictionary. Inheritance of static/class variables will work as long as you don't reassign. — mshsayem
– mshsayem, Commented Apr 30, 2018 at 11:51
Sorry if i ask anything very basic. I am relatively new to python. Here I am modifying the variables. Reassigning ... I am not getting whats that. — Sourabh Jaiswal
– Sourabh Jaiswal, Commented Apr 30, 2018 at 11:59
Don't be. It is a good question, and I don't think it is very basic. — mshsayem
– mshsayem, Commented Apr 30, 2018 at 12:13

James · Accepted Answer · 2018-04-30 12:10:05Z

6

In general, you do not want to use mutable class variables. A class variable is shared by object reference to all instances of the class. But it is not shared across inherited classes.

So all instances of a share object references to a.var1 and a.var2. Similarly, all instances of b share object references to b.var1 and b.var2. Upon creating the inheritance, b also get the references to var1 and var2, but they are not shared back to a or any instantiations of a.

Because you use a mutable object for var2, the object's reference is never changed after the inheritance. So you can modify var2 and it will be modified for all references to it, because the object reference is the same.

If you reassign var2 instead of modifying it, you get the expected behavior.

Try this:

class a(object):
    var1 = 0
    var2 = {}
    def print_var(self):
        print(self.var1, end=', ')
        print(self.var2)

class b(a):
    @classmethod
    def modify_var(cls):
        cls.var1 = 1
        cls.var2 = {'temp': 'something'}  # reassign to a new dict

Here are some tests:

ob1 = a()
ob2 = b()

o1.print_var()
o2.print_var()

o2.modify_var()

o1.print_var()
o2.print_var()

# prints
0, {}
0, {}
0, {}
1, {'temp': 'something'}

edited Apr 30, 2018 at 12:10

answered Apr 30, 2018 at 12:04

James

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2 Comments

Sourabh Jaiswal Over a year ago

Thanks a lot for clarification. So basically I should use class B's objects as I want variables to be shared among objects.

James Over a year ago

Correct. If you need to share variables between objects of the same class, then a class variable is the right choice. If you need to share variable between objects of multiple types, you make an additional class that is just a container for the instances of the objects you are using.

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