Python Class (2)

25 Mar 2019

Class Variables & Instance Variables

Class variables: are shared by all the instances

Instance variables: contain data that is unique to each instance

class Person:
    no = 0            # class variable
    raise_amt = 1.05  # class variable

    def __init__(self, name, age, salary):
        self.name = name      # instance variable
        self.age = age        # instance variable
        self.salary = salary  # instance variable
        Person.no += 1

    def raise_sal(self):
        self.salary = int(self.salary*self.raise_amt)

# we are going to create two instances p1 and p2 and print the class variable 'Person.no' 
print('# of instances before creating them:', Person.no)
p1 = Person('kim', 25, 100)
p2 = Person('lee', 30, 50)
print('# of instances after creating them: ', Person.no)

# of instances before creating them: 0
# of instances after creating them:  2

Note: in a method ‘raise_sal()’ we have accessed class variable raise_amt by self.raise_amt instead of Person.raise_amt. WHY?

it is possible to access class variable from class itself as well as from instances

Then why self.raise_amt?
since it can change the value for each instance seperately

# class variable is accessible by 'class name.variable name' or 'instance name.variable name'
print(Person.raise_amt) # accessed by class 
print(p1.raise_amt)     # accessed by instance 
print(p2.raise_amt)

1.05
1.05
1.05

#Access by class itself will change the value all
Person.raise_amt = 1.10
print(Person.raise_amt)
print(p1.raise_amt)
print(p2.raise_amt)

1.1
1.1
1.1

#Access by each instance (p1) will only change p1.raise_amt
p1.raise_amt = 1.03   #creating attribute('raise_amount') within emp_1
print(Person.raise_amt)
print(p1.raise_amt)
print(p2.raise_amt)

1.1
1.03
1.1