Classes & Objects

An object bundles state (data) and behavior (methods that act on that data) together — you met the mechanics of this in the Python course's Classes module. This course goes a level further: designing a class around a real responsibility, with multiple methods that collaborate through shared state.

class Inventory:
    def __init__(self):
        self.items = {}   # shared state every method reads/writes

    def add_item(self, name, qty):
        self.items[name] = self.items.get(name, 0) + qty
        return self   # returning self enables method chaining

Every method that touches self.items is really just a different VIEW or OPERATION on the same underlying state — that's the point of grouping them into one class instead of writing free-floating functions that each take an items dict as an argument. The object owns its data, and every interaction with that data goes through a method, giving you one place to look when you need to understand (or change) how inventory tracking actually works.

def total_items(self):
    return sum(self.items.values())