Classes, Objects & Instances

Core path: 25 minutes

When does grouping data into an object make a program easier to reason about?

Start with repeated structure

Without objects, several players might be represented as dictionaries:

player1 = {"name": "Nova", "score": 850}
player2 = {"name": "Maya", "score": 950}

That works. Classes are not a magic upgrade. They become useful when a repeated kind of thing has related state and behavior that belong together.

Define a class

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

A class defines the structure and behavior for a kind of object. An instance is one actual object created from that class.

Interactive model

One class, independent instance state

Pause motionReset layout

The class defines the shared structure. Each constructor call creates a separate instance whose attributes can change independently.

CLASS Player

CONSTRUCTOR CALLS Player(name, score)

INSTANCE: nova name = Nova score = 850

INSTANCE: maya name = Maya score = 950

nova.score = 900 Maya remains 950

React Flow

Press enter or space to select a node. You can then use the arrow keys to move the node around. Press delete to remove it and escape to cancel.

Press enter or space to select an edge. You can then press delete to remove it or escape to cancel.

Drag nodes to inspect the relationships. Motion shows the active path; the plain background keeps attention on the relationships instead of graph-paper decoration.

View static diagram

self refers to the current instance

nova = Player("Nova", 850)
maya = Player("Maya", 950)

Each call creates a different instance. self.name does not mean one global Player name. It means the name attribute belonging to whichever instance is currently being initialized or used.

Diagrams open at a readable shape-aware scale. Zoom or expand when you need more detail.

Guided example: prove the state is independent

Run:

nova = Player("Nova", 850)
maya = Player("Maya", 950)

nova.score = 900

print(nova.score)
print(maya.score)

Predict before running:

900
950

Why did Maya's score remain 950? Both objects follow the same class definition, but each instance carries its own state.

Now imagine a bug where changing one player unexpectedly changes every player. That failure would tell you the state is being shared somewhere you did not intend, which is a design problem worth tracing rather than blaming on "objects being weird."

Attributes and behavior

name and score are attributes. In the next lesson, methods will place behavior next to the state it is responsible for changing.

When not to use a class

If a program only needs one tiny record and no meaningful behavior, a dictionary or simple function may be clearer. Architecture is a tradeoff, not a contest to use the most advanced syntax.