Difference between Class variables and Instance variables

Question

I have already read many answers here on Stack Exchange like Python - why use "self" in a class? After reading these answers, I understand that instance variables are unique to each instance of the class while class variables are shared across all instances.

While playing around, I found that this code which gives the output [1]:

class A:
    x = []
    def add(self):
        self.x.append(1)

x = A()
y = A()
x.add()

print "Y's x: ", y.x

However, this code gives 10 as the output, when in my opinion it should be 11:

class A:
    x = 10
    def add(self):
        self.x += 1

x = A()
y = A()
x.add()

print "Y's x: ", y.x

Why A class variable is not updated when I run x.add()? I am not very experienced in programming, so please excuse me.

Answer 1

Class variables are shadowed by instance attribute. This means that when looking up an attribute, Python first looks in the instance, then in the class. Furthermore, setting a variable on an object (e.g. self) always creates an instance variable - it never changes the class variable.

This means that when, in your second example you do:

self.x += 1

which is (in this case, see footnote) equivalent to:

self.x = self.x + 1

what Python does is:

1. Look up self.x. At that point, self doesn't have the instance attribute x, so the class attribute A.x is found, with the value 10.
2. The RHS is evaluated, giving the result 11.
3. This result is assigned to a new instance attribute x of self.

So below that, when you look up x.x, you get this new instance attribute that was created in add(). When looking up y.x, you still get the class attribute. To change the class attribute, you'd have to use A.x += 1 explicitly – the lookup only happens when reading the value of an attribute.

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Your first example is a classical gotcha and the reason you shouldn't use class attributes as "default" values for instance attributes. When you call:

self.x.append(1)

there is no assignment to self.x taking place. (Changing the contents of a mutable object, like a list, is not the same as assignment.) Thus, no new instance attribute is added to x that would shadow it, and looking up x.x and y.x later on gives you the same list from the class attribute.

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Note: In Python, x += y is not always equivalent to x = x + y. Python allows you to override the in-place operators separately from the normal ones for a type. This mostly makes sense for mutable objects, where the in-place version will directly change the contents without a reassignment of the LHS of the expression. However, immutable objects (such as numbers in your second example) do not override in-place operators. In that case, the statement does get evaluated as a regular addition and a reassignment, explaining the behaviour you see.

(I lifted the above from this SO answer, see there for more details.)