Common Scenario

Let's break down some most common scenario it one by one — all using BankAccount vibe

1. Multiple Objects using Constructor

class BankAccount:
    def __init__(self, holder, balance):
        self.holder = holder
        self.balance = balance

acc1 = BankAccount("Maham", 1000)
acc2 = BankAccount("Ali", 1500)
acc3 = BankAccount("Zoya", 2000)

print(acc1.holder)  # Maham
print(acc2.balance) # 1500

Each object gets its own memory.
Means → every time constructor runs fresh for every new object.

2. 👨‍👩‍👧‍👦 Inheritance with Constructor

Suppose now we have a child class called SavingsAccount:

class BankAccount:
    def __init__(self, holder, balance):
        self.holder = holder
        self.balance = balance

class SavingsAccount(BankAccount):
    def __init__(self, holder, balance, interest_rate):
        super().__init__(holder, balance)
        self.interest_rate = interest_rate

☕ What’s happening here?

SavingsAccount is inheriting from BankAccount
Inside child’s constructor, we used super().__init__()
→ to call parent’s constructor

So now you can do:

acc4 = SavingsAccount("Maham", 5000, 0.05)
print(acc4.holder)         # Maham
print(acc4.interest_rate)  # 0.05

3. 🔁 Override Constructor in Subclass

You already saw this:

If child class defines its own constructor, it overrides the parent’s one
But… if you don’t use super(), then parent’s constructor won’t run at all

class SavingsAccount(BankAccount):
    def __init__(self, holder, interest_rate):
        self.holder = holder
        self.interest_rate = interest_rate

Now:

acc = SavingsAccount("Ali", 0.07)
print(acc.holder)          # Ali
print(acc.interest_rate)   # 0.07
print(acc.balance)         # ❌ will give error

Because we didn’t call super() → balance never got created.

4.Using constructor with classmethods

Sometimes you want to create an object in a custom way
like from a string or a dictionary or after validation etc

For this → we use @classmethod to create alternative constructors

🏦 Example 1: Create from string

class BankAccount:
    def __init__(self, holder, balance):
        self.holder = holder
        self.balance = balance

    @classmethod
    def from_string(cls, account_str):
        holder, balance = account_str.split("-")
        return cls(holder, float(balance))

acc = BankAccount.from_string("Maham-5000")
print(acc.holder)   # Maham
print(acc.balance)  # 5000.0

🧠 Wait… how this is useful?

helpful when you're getting data from APIs, files, forms etc
makes code cleaner
can hide complex logic inside

Example 2: Validating before creating

class BankAccount:
    def __init__(self, holder, balance):
        self.holder = holder
        self.balance = balance

    @classmethod
    def create_if_valid(cls, holder, balance):
        if balance < 0:
            print("Invalid balance")
            return None
        return cls(holder, balance)

acc = BankAccount.create_if_valid("Maham", -100)  # Invalid balance