OOP - lesson 2 | HackIndia Skills

OOP - lesson 2 | HackIndia Skills | HackIndia

class Trip:
    def __init__(self, name):
        self.name = name
        self.expenses = []

    def add_expense(self, paid_by, amount, note):
        self.expenses.append({
            "paid_by": paid_by,
            "amount": amount,
            "note": note
        })

    def total_amount(self):
        total = 0
        for expense in self.expenses:
            total += expense["amount"]
        return total

manali_trip = Trip("Manali")
goa_trip = Trip("Goa")

manali_trip.add_expense("Asha", 1200, "bus booking")
goa_trip.add_expense("Ravi", 800, "scooter rent")

print(manali_trip.total_amount())  # 1200
print(goa_trip.total_amount())     # 800

Common mistake

Shared state bug

A common bug is placing changeable state on the class instead of the object. Then all objects may accidentally share the same list.

If a list or dictionary should be different per object, create it inside the constructor.

class BadTrip:
    expenses = []  # shared by every BadTrip object

    def __init__(self, name):
        self.name = name

class GoodTrip:
    def __init__(self, name):
        self.name = name
        self.expenses = []  # separate list for each object

Classes, objects, and state

One idea

Feature first

Class to object

What state means

Shared state bug

Model a small feature

Use AI carefully

Do this now

Quick check