Introduction to Classes

Python has similar concept of classes as Java does, although there are many differences between both language implemenations. Let’s first see how classes are written in Python:

# Class Definition
class Car:

    # Constructor, self is like `this` of Java which is
    #the first parameter for every instance method
    def __init__(self):
        # self. is how parameters are defined, there is no separate declaration as in Java
        self.make = 'default'
        self.status = 'Neutral'

    # Instance Method
    def stop(self):
        self.status = 'Braking'
        print('Applying Brakes')

    # toString() override
    def __str__(self):
        return 'Make: ' + self.make + ' Status: ' + self.status

#This is equivalent to new Car() in Java
car = Car()

#This will invoke __str__ method on car
print(car)  # prints Make: default Status: Neutral
car.stop()  # prints Applying Brakes
print(car)  # prints Make: default Status: Braking

Instance Methods

Every instance method i.e. non static or class methods(more on this later) have self as first parameter; self acts as this parameter. When invoking instance method we don’t have to provide self parameter, it is passed implicitly.

# self is passed implicitly
def stop(self):
        self.status = 'Braking'
        print('Applying Brakes')

car = Car()
car.stop()  # prints Applying Brakes

Constructor

Constructor is defined by __init__ function. In Python classes there can be only one constructor. The first parameter is always self as told above for instance methods, apart from that constructor can take parameters which can be stored at instance level.

class Car:

    def __init__(self, make, model):
        self.make = make
        self.model = model

car = Car('Audi', 'A3')

self.make and self.model are the instance properties. All the instance properties should be initialized in constructor to avoid errors at runtime.