Methods Put Behavior With the Data

Core path: 25 minutes

How can an object do something with its own data?

Currently, our Player class only stores data. But players also have behavior.

Create a method

A method is a function that belongs to a class.

class Player:
    def __init__(self, name, score):
        self.name = name
        self.score = score

    def display(self):
        print(f"{self.name}: {self.score}")

Call the method using dot notation:

player = Player("Nova", 850)
player.display() # Outputs: Nova: 850

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Add behavior

Objects can change their own state:

    def add_score(self, points):
        self.score += points

When you call player.add_score(100), self refers to player, and points is 100.

Keep related state synchronized

Suppose rank is based on score. If we add score but forget to update rank, the object's state becomes inconsistent.

    def add_score(self, points):
        self.score += points
        self.rank = self.determine_rank()

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The object owns the rule connecting score and rank, protecting its own internal consistency.

Dictionary vs object

Dictionaries are great for simple data records (like API responses). Objects are great when data and rules belong strongly together. Neither automatically wins.

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Protecting rules (Invariants)

Methods can prevent bad data:

    def drain_battery(self, amount):
        self.battery -= amount
        if self.battery < 0:
            self.battery = 0

This protects a rule: battery cannot be negative.