Lesson — OOP

DOUBLE-UNDERSCORE
KA JAADU.

print(obj), len(obj), a + b — apni class ke liye ye sab tum sikha sakte ho. Magic methods se object smart banta hai.

WHY: print(Bill) me kachra kyun

Apni class ka object print karo to <__main__.Bill object at 0x...> aata hai — bekaar. Magic methods (double-underscore wale) Python ko sikhate hain ki tumhare object ke saath print, len, + kya karein.

class Bill:
    def __init__(self, items):
        self.items = items
    def __str__(self):
        return f"Bill: {len(self.items)} items"
    def __len__(self):
        return len(self.items)

b = Bill(["doodh", "bread"])
print(b)  # Bill: 2 items
print(len(b))  # 2

HOW: dunder likhna

Naam fix hain — __str__, __len__, __add__(self, other), __eq__. Python khud bulata hai jab tum print/len/+ use karte ho. Tumhe sirf method likhna hai.

class Wallet:
    def __init__(self, cash):
        self.cash = cash
    def __add__(self, other):
        return Wallet(self.cash + other.cash)

w = Wallet(100) + Wallet(50)
print(w.cash)  # 150

Common mistakes

• __str__ me print karna return ki jagah — return string chahiye.
• Naam galat likhna (_str_) — double underscore dono side must.
• Har method me magic dhoondhna — normal methods normal naam se.