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Global Attributes vs. Instance Attributes

Without knowing it, you’ve been using both globally accessible attributes and instance accessible attributes. Global attributes can be referenced by the class directly and all its instances, whereas instance attributes (which are defined within the init method) can only be accessed by the class instances. If an attribute is declared inside of a class, but not within the init method, then it is known as a global attribute. Any attributes declared within the init method using the self keyword are instance attributes. Let’s see an example:

1| # using and accessing global class attributes 3| class Car( ):
4| sound = "beep" # global attribute, accessible through the class itself 6| def __init__(self, color):
7| self.color = "blue" # instance specific attribute, not accessible through the class itself
9| print(Car.sound)
11| # print(Car.color) won't work, as color is only available to instances of the Car class, not the class itself
13| ford = Car("blue")
15| print(ford.sound, ford.color) # color will work as this is an instance

Go ahead and run the cell. On the 6th line we print out the sound “beep” by directly accessing it through the class blueprint with dot syntax. You do this by using the name of the class, instead of the name of an instance. We’re able to do this because the sound attribute is set up as a globally accessible attribute. The entire 7th line is commented out because it would produce an error since the color attribute is declared within the init method and is only accessible to instances, not the class itself. Lastly, on the 9th line, after we instantiate the ford instance, we print out both the sound and the color attribute. All class instances have access to global and instance level attributes, which is why we’re able to output the sound. What you must keep in mind, however, is that we weren’t able to give the ford instance a personalized value for the sound attribute. Only when attributes are declared in the init method are we able to give instances personal values upon instantiation. Currently, in order to give ford a different value for the sound attribute, we would have to change it after its instantiation.