If you change an attribute for one instance, how do you get it to also change when you edit the entire class?

Question

I understand that we've edited the .doors of car_one to be 6 while the Car class still has doors = 4, it's now stored somewhere else on the computer's memory and won't be edited with the rest of the class. I then went in and manually changed car_one.doors = 2 and it showed that it was now stored on the same location as car_two and the Car class. But then when I changed Car.doors = 4, car_one didn't update. Is there a reason it didn't change even though at one point it was again stored on the same location? And, probably more importantly, is there a way to re-group it or whatever?

class Car:
    wheels = 4
    doors = 2
    engine = True

car_one = Car()
car_two = Car()

car_one.doors = 6
id(car_one.doors) #139921268173264
id(car_two.doors) #139921268173168
id(Car.doors) #139921268173168

car_one.doors = 2
id(car_one.doors) #139921268173168
id(car_two.doors) #139921268173168
id(Car.doors) #139921268173168

Car.doors = 4
car_one.doors #6
car_two.doors #4
Car.doors #4

Answer 1 · 2024-03-06T16:16:52Z

March 6, 2024 4:16pm

An instance variable will take precedence over a class variable of the same name. So initially, when you create the two instances, they only have the class variable "doors" which they share.

But when you assign car_one.doors, that creates a new instance variable for car_one but it has no effect on car_two. When you later check the value of car_one.doors you will see the instance variable instead of the class variable.

But when you check the value of car_two.doors, you still see the value of the class variable.

If you want car_one to go back to using the shared class variable, you can delete the instance variable.

car_one.doors = 2    # creates instance variable unique to car_one
Car.doors = 4        # updates the shared class variable

print(car_one.doors) # 2 (instance variable)
print(car_two.doors) # 4 (class variable)
print(Car.doors)     # 4 (class variable)

del car_one.doors    # deletes the instance variable
print(car_one.doors) # 4 (class variable)

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Gracie Jensen

Gracie Jensen

If you change an attribute for one instance, how do you get it to also change when you edit the entire class?

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Steven Parker

Steven Parker