难以理解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?

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

方法increase_speed只是返回self.speed + 1，如果你想将self.speed = self.speed + 1所需的速度更新为方法增加速度。

Answer 3

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)