Having difficulty understanding Python OOP

Question

I'm fairly green to OOP and I was just playing around with it in Python and came across something I can't explain so hopefully you guys will be able to help.

I was playing with the code below:

class Car():
 def __init__(self, brand, model, speed):
 self.brand = brand
 self.model = model
 self.speed = speed
 def increase_speed(self):
 return self.speed + 1
 def decrease_speed(self, decrease_by):
 return self.speed - decrease_by
car1 = Car("tesla","x",30)
print(car1.brand)
print(car1.speed)
print(car1.increase_speed())
print(car1.speed)
print(car1.decrease_speed(10))

My question is, I am expecting after increasing the speed, car1's speed will be 31 but instead it prints out 30. Why is it that way and how should the code be written for the speed to be 31 instead?

• Chris: You haven't assigned the increased speed back to the origial self.speed. Thus increase_speed didn't really increased the speed; it just showed 1 unit higher speed and gone.
• mikuszefski: ....and BTW that's not a problem of OOP, but just the "P" in OOP
• mikuszefski: ...a = 5 now print( a + 7 ). What is the value of a?

Answer 1

def increase_speed(self):
 self.speed += 1
 return self.speed

Previously you did not increase your speed, rather you just return a value that is equal to the speed plus 1. Similarly, change your decrease_speed function.

Answer 2

Instead of returning the values, set them on the attributes:

class Car():
 def __init__(self, brand, model, speed):
 self.brand = brand
 self.model = model
 self.speed = speed
 def increase_speed(self):
 self.speed = self.speed + 1
 def decrease_speed(self, decrease_by):
 self.speed = self.speed - decrease_by

I deliberately don't return the changed speed anymore, as it's good style (at least with methods mainly setting attributes) to either return something or change state:

car1 = Car("tesla","x",30)
print(car1.brand)
print(car1.speed)
car1.increase_speed()
print(car1.speed)
car1.decrease_speed(10)
print(car1.speed)

Answer 3

The method increase_speed is just returning self.speed + 1, if you wish to update the speed you need to self.speed = self.speed + 1 into the method increase speed.