SS 3 First Term

Lesson objectives

On completion, candidates should be able to:

1. Define class, object, attribute and method.
2. Write a two-attribute constructor.
3. Create objects and call methods.
4. Reject an overdrawn withdrawal.
5. Explain encapsulation's benefit.

Key vocabulary

• Class: a template for objects.
• Object: one instance of a class.
• Attribute: a named value on an object.
• Method: a function inside a class.
• Constructor: the init method.
• Encapsulation: data kept with its methods.

Common misconceptions

• "A class is an object." The class is the template; the object is an instance.
• "self stores the class name." self refers to the current object.

Teacher activities

1. Starter: sort class and attribute nouns.
2. Teach the class-object link.
3. Model a two-attribute constructor.
4. Add deposit and withdraw guards.
5. Run one rejected withdrawal.
6. Ask why the balance survives.

Student activities

1. Complete the class sort.
2. Copy the Account skeleton.
3. Write deposit with a guard.
4. Write withdraw with a funds guard.
5. Test one rejected withdrawal.
6. Add a balance report.

class Account:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance

    def deposit(self, amount):
        if amount > 0:
            self.balance = self.balance + amount

    def withdraw(self, amount):
        if 0 < amount <= self.balance:
            self.balance = self.balance - amount
            return True
        return False

a = Account("Ngozi", 5000)
a.deposit(2500)
print(a.withdraw(9000))
print(a.owner, a.balance)

Expected output:

False
Ngozi 7500

Practical / learning evidence

ss3_account_musa.py defines Account with a constructor, deposit and withdraw, creates two objects and records three tests. Success needs correct self references and a balance of 7500.

Resources and preparation

Python 3, projector and a printed skeleton. Offline route: trace calls on an object diagram and write each attribute value.

Assessment for learning

Look for self on every attribute and a Boolean return from withdraw. Watch for a balance changed before the guard.

Inclusion and homework

Give the skeleton with method signatures; extend others with a transfer method. Homework: add a 4% interest method on 7500.