How should I choose between using instance vs. class attributes?

Question

I tried this example code:

class testclass:
    classvar = 'its classvariable LITERAL'
    def __init__(self,x,y):
        self.z = x
        self.classvar = 'its initvariable LITERAL'
        
        self.test()
    
    def test(self):
        print('class var',testclass.classvar)
        print('instance var',self.classvar)
        
if __name__ == '__main__':
    x = testclass(2,3)

I need some clarification. In both cases, I'm able to access the class attribute and instance in the test method.

So, suppose if I have to define a literal that needs to be used across all function, which would be the better way to define it: an instance attribute or a class attribute?

score 11 · Answer 1 · edited Sep 26 '22 at 09:43

I found this in an old presentation made by Guido van Rossum in 1999 ( http://legacy.python.org/doc/essays/ppt/acm-ws/sld001.htm ) and I think it explains the topic beautifully:

Instance variable rules

On use via instance (self.x), search order:

(1) instance, (2) class, (3) base classes
this also works for method lookup

On assigment via instance (self.x = ...):

always makes an instance variable
Class variables "default" for instance variables

But...!

mutable class variable: one copy shared by all
mutable instance variable: each instance its own

score 6 · Accepted Answer · answered Sep 30 '12 at 01:56

Class variables are quite good for "constants" used by all the instances (that's all methods are technically). You could use module globals, but using a class variable makes it more clearly associated with the class.

There are often uses for class variables that you actually change, too, but it's usually best to stay away from them for the same reason you stay away from having different parts of your program communicate by altering global variables.

Instance variables are for data that is actually part of the instance. They could be different for each particular instance, and they often change over the lifetime of a single particular instance. It's best to use instance variables for data that is conceptually part of an instance, even if in your program you happen to only have one instance, or you have a few instances that in practice always have the same value.

score 5 · Answer 3 · answered Sep 30 '12 at 02:01

5

It's good practice to only use class attributes if they are going to remain fixed, and one great thing about them is that they can be accessed outside of an instance:

class MyClass():
    var1 = 1
    def __init__(self):
        self.var2 = 2

MyClass.var1 # 1 (you can reference var1 without instantiating)
MyClass.var2 # AttributeError: class MyClass has no attribute 'var2'

If MyClass.var is defined, it should be the same in every instance of MyClass, otherwise you get the following behaviour which is considered confusing.

a = MyClass()
b = MyClass()

a.var1, a.var2 # (1,2)
a.var1, a.var2 = (3,4) # you can change these variables
a.var1, a.var2 # (3,4)
b.var1, b.var2 # (1,2) # but they don't change in b
MyClass.var1 # 1         nor in MyClass

answered Sep 30 '12 at 02:01

Andy Hayden

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b gets its own "instance" of the class variables? – Reb.Cabin Oct 12 '16 at 21:23
@Reb.Cabin no they are global. – Andy Hayden Oct 12 '16 at 21:56
`a.var1` is different from `b.var1`, though, as your example shows, and I think `b.var1` is the same as `MyClass.var1` (at least my experiments are consistent with that idea). So, somehow assigning `3` to `a.var1` made something specific to instance `a`. Would you be so kind as to explain a bit more? – Reb.Cabin Oct 12 '16 at 22:00
@Reb.Cabin it's the difference between immutable and mutable, 3 is an int which is immutable. So replacing becomes per instance. If you make it a list and append on different instances you'll see they share the mutable state. – Andy Hayden Oct 12 '16 at 22:18

score 4 · Answer 4 · answered Sep 30 '12 at 01:13

You should define it as a class attribute if you want it to be shared among all instances. You should define it as an instance variable if you want a separate one for each instance (e.g., if different instances might have different values for the variable).

How should I choose between using instance vs. class attributes?

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