What is the difference between class attributes and instance attributes?

Question

In Python, what is the difference between class attributes and instance attributes? The only difference I can tell is that the instance attributes can be initialized when creating a new Class variable via the arguments passed to it. Otherwise, the behavior seems the same. For instance, the self.is_moving = False property is not being passed as an argument in def__init__(self ....), so it can be moved up to the class attribute section as .is_moving = False and it seems to get the same result. Same with self.gas = 100

class Car:
    # class attributes, NOT TO Be confused with class INSTANCE 
    wheels = 4
    doors = 2
    engine = True

    def __init__(self, model, year, make="Ford"):  # These are instance attributes created with an initializer inside the class. Initializer is a method called __init__ | #best practice to put the default arguments at the end
        self.make = make
        self.model = model
        self.year = year
        self.gas = 100
        self.is_moving = False

    def stop(self):
        if self.is_moving: 
            print('The car has stopped')
            self.is_moving = False
        else:
            print('But the car ain\'t moving..')

    def go(self, speed):
        if self.use_gas():
            if not self.is_moving:
                print(f'The car starts moving ...')
                self.is_moving = True
            print(f'The car is going {speed}')
        else:
            print('You have run out of gas')
            self.stop()

    def use_gas(self):
        self.gas -= 50
        if self.gas <= 0:
            return False
        else:
            return True 
    ```

Answer 1 · 2021-12-05T17:15:38Z

December 5, 2021 5:15pm

The main difference is that a class attribute is shared by all instances of that class, and an instance attribute is unique to each instance. So if you change a class attribute in one instance, it will also change for every other instance.

So it makes sense for "wheels" to be a class attribute, since all cars have 4 of them and it won't change. But the amount of gas would be different for each car, so that makes more sense as an instance attribute.

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JASON LEE

JASON LEE

What is the difference between class attributes and instance attributes?

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Steven Parker

Steven Parker