12. Classes, Instances, Class Attributes & @classmethod vs @staticmethod • Python 0-to-Hero Mastery (Big Tech SDE & ML Standard) • Rishabh Ki Kaksha

*   **Class Attribute:** Single shared variable stored on class object (`User.min_password_length = 8`).
*   **Instance Attribute:** Unique per object instance (`self.username = "rishabh"`).

---

### 2. `@classmethod` vs `@staticmethod` vs Instance Method

```python
class PaymentGateway:
    commission_rate = 0.02 # Class Attribute

def __init__(self, merchant_id: str):
        self.merchant_id = merchant_id # Instance Attribute

# 1. Instance Method (Accesses self):
    def process_charge(self, amount: float):
        fee = amount * self.commission_rate
        return amount + fee

# 2. Class Method (Factory Constructor, Accesses cls):
    @classmethod
    def from_environment(cls):
        import os
        return cls(os.getenv("MERCHANT_ID", "default_merchant"))

# 3. Static Method (Utility Function, No self or cls):
    @staticmethod
    def validate_currency(currency: str) -> bool:
        return currency.upper() in ["INR", "USD", "EUR"]
```

---

### 3. Interview Ke 3 Sawal Is Topic Se

1. **`@classmethod` factory pattern me standard `__init__` se kyu preferred hai?** -> Subclassing ke sath polymorphic instantiation ensure karta hai (`cls()` instantiates derived class correctly).
2. **Class attribute ko `self.attr = new_val` se modify karne par kya hota hai?** -> Class attribute modify nahi hota; instance par naya shadowed instance attribute create ho jata hai.
3. **`__dict__` attribute Python objects me memory overhead kaise create karta hai?** -> Har instance apna dynamic dictionary maintain karta hai (Mitigated via `__slots__`).

---
**Next Topic:** 13. Dunder / Magic Methods Masterclass (__repr__, __call__, __getitem__)

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1. Custom Matrix Data Model with add & getitem

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PHP 8+ & Laravel 11 Master Track

🎯 Big Tech Coding & Interview Challenges

1. Custom Matrix Data Model with __add__ & __getitem__

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CERTIFICATE OF COMPLETION

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PHP 8+ & Laravel 11 Master Track

1. Custom Matrix Data Model with add & getitem