Instance variables are unique for each instance, while class variables are shared by all instances.
class C:
x = 2 # class variable
def __init__(self, y):
self.y = y # instance variable
C.x
# 2
C.y
# AttributeError: type object 'C' has no attribute 'y'
c1 = C(3)
c1.x
# 2
c1.y
# 3
c2 = C(4)
c2.x
# 2
c2.y
# 4Class variables can be accessed on instances of this class, but assigning to the class attribute will create an instance variable which shadows the class variable
c2.x = 4
c2.x
# 4
C.x
# 2Note that mutating class variables from instances can lead to some unexpected consequences.
class D:
x = []
def __init__(self, item):
self.x.append(item) # note that this is not an assigment!
d1 = D(1)
d2 = D(2)
d1.x
# [1, 2]
d2.x
# [1, 2]
D.x
# [1, 2]