Python OOP Checkpoint: Design, Test, and Refactor a Small Model

Check your ability to model, validate and test a small object-oriented system.

What You Will Learn

This checkpoint puts several design choices together: one class owns a value and guards its invariants, while another class uses its public methods. Favor a short, clear interface over a large inheritance hierarchy. Verify both a successful operation and a rejected operation so the object cannot enter an invalid state.

Class interfaces should preserve their stated invariants. Type annotations describe intent but do not enforce runtime types on their own.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

from dataclasses import dataclass

@dataclass
class Account:
    owner: str
    balance: int = 0

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("Deposit must be positive")
        self.balance += amount

account = Account("Sam")
account.deposit(25)
print(account.balance)
try:
    account.deposit(-2)
except ValueError as error:
    print(type(error).__name__)

How the Example Works

The imports provide the standard-library tools used in the demonstration: dataclasses.
The class Account groups data and behavior; its methods run when called on an instance.
The variable account holds the result of Account('Sam'). Later lines use that value or inspect how it changes.
The control-flow block demonstrates when each value is processed or when cleanup and exception handling take effect.

Expected output from the example:

25
ValueError

When You Would Use This

A class becomes useful when several operations must maintain rules around related data. Think about which state belongs to each instance, which operations should be public, and how errors should be reported. Avoid making the design more complex than the underlying task.

Practice: Modify the Code

Add a withdrawal method that rejects negative amounts and insufficient funds; test both cases.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Putting all validation in the user interface and leaving Account methods unprotected.
Adding inheritance merely to share a tiny utility function.

Key Takeaways

Check your ability to model, validate and test a small object-oriented system.
This checkpoint puts several design choices together: one class owns a value and guards its invariants, while another class uses its public methods.
Apply this operation only after checking the assumptions and input conditions shown in the example.