Class variables and instance variables are often utilized at the same time, but for different purposes. As mentioned before, class variables can refer to some meta information or common information shared amongst instances of the same class.
The example below demonstrates both topics: each class owns a counter variable that holds the number of class instances created. Moreover, each class owns information that helps identify the class instance origins. Similar functionality could be achieved with the isinstance() function, but we want to check if class variables can be helpful in this domain.
Both the Duck and Chicken classes own a class variable named species, which holds a value unique to each class. When we iterate over all objects, we can examine the value of this variable to take appropriate action.
So many ducks were born: 3
duck quacks
duck quacks
duck quacks
chicken clucks
output
Code
class Duck:counter = 0
species = 'duck'
def __init__(self, height, weight, sex):
self.height = height
self.weight = weight
self.sex = sex
Duck.counter +=1
def walk(self):
pass
def quack(self):
print('quacks')
class Chicken:
species = 'chicken'
def walk(self):
pass
def cluck(self):
print('clucks')
duckling = Duck(height=10, weight=3.4, sex="male")
drake = Duck(height=25, weight=3.7, sex="male")
hen = Duck(height=20, weight=3.4, sex="female")
chicken = Chicken()
print('So many ducks were born:', Duck.counter)
for poultry in duckling, drake, hen, chicken:
print(poultry.species, end=' ')
if poultry.species == 'duck':
poultry.quack()
elif poultry.species == 'chicken':
poultry.cluck()