Class vs object — blueprint vs instance — Hinglish notes

2.1.1 · Coding › OOP Fundamentals

HUM kis baare mein baat kar rahe hain?

YEH distinction kyun exist karti hai?

Blueprint ko instance se alag kyun kiya jaata hai?

Reuse — description ek baar likho, bahut saare objects stamp karo.
Independent state — har object apna data rakhta hai; ek ko change karne se doosre corrupt nahi hote.
Shared behaviour — methods ek baar (class par) store hoti hain, isliye 1000 objects mein se har ek code ki 1000 copies store nahi karta.

Inhe kaise banate aur use karte hain?

class Dog:                 # blueprint begins
    species = "Canis"      # CLASS attribute (shared by all dogs)
 
    def __init__(self, name, age):   # the constructor
        self.name = name             # INSTANCE attribute (per object)
        self.age  = age
 
    def bark(self):                  # behaviour
        return f"{self.name} says Woof!"
 
d1 = Dog("Rex", 3)         # instantiation -> an OBJECT
d2 = Dog("Fido", 5)        # another OBJECT, independent state

Instantiation step by step kaise kaam karta hai:

Dog("Rex", 3) Python se request karta hai ki Dog type ka ek blank object create karo.
Python __init__ ko call karta hai jahan self = naya blank object hota hai.
self.name = name data ko us object mein likhta hai.
Taiyaar object ka reference wapas hand over ho jaata hai aur d1 mein store ho jaata hai.

Figure — Class vs object — blueprint vs instance

Worked Examples

Worked example Example 1 — Independent state

d1 = Dog("Rex", 3)
d2 = Dog("Fido", 5)
d1.age = 4
print(d1.age, d2.age)   # 4 5

Yeh step kyun? d1.age = 4 sirf d1 ki apni memory mein likhta hai. d2 apna age rakhta hai. Yeh prove karta hai ki objects alag instance attributes rakhte hain chahe woh ek class share karte hon.

Worked example Example 2 — Shared class attribute

print(d1.species, d2.species)   # Canis Canis
Dog.species = "Lupus"
print(d1.species, d2.species)   # Lupus Lupus

Yeh step kyun? species class par rehta hai, har object par nahi. Dono objects same value lookup karte hain, isliye class par change karne se sabke liye change ho jaata hai — instance attributes ki tarah nahi.

Worked example Example 3 — Identity vs equality

a = Dog("Rex", 3)
b = Dog("Rex", 3)
print(a is b)        # False  -> different objects in memory
print(type(a) is Dog) # True  -> both built from same class

Yeh step kyun? Same blueprint aur same data hone se woh same object nahi ban jaate. is identity check karta hai (same memory cell); yeh do alag instances hain.

Common Mistakes

Recall Feynman: 12-saal ke bacche ko samjhao

Ek class cookie cutter ki tarah hai. Cutter khud cookie nahi hai — woh shape hai. Jab bhi tum usse dough mein press karte ho tum ek cookie (object) paate ho. Saari cookies ka same shape hota hai (same class), lekin tum har ek par alag toppings daal sakte ho (alag data). Ek cookie phenkne se doosri ko kuch nahi hota, aur tumhare paas zyada cookies banane ke liye cutter abhi bhi hai.

Flashcards

Class kya hoti hai?

Ek blueprint/template jo data (attributes) aur behaviour (methods) bundle karta hai; yeh kisi cheez ka description deta hai lekin khud runtime instance nahi hota.

Object kya hota hai?

Ek concrete instance jo class se bana ho, memory occupy karta ho, aur instance attributes ki apni copy rakhta ho.

Instantiation kya hai?

Class se object create karne ki act, jaise Dog("Rex", 3), jo __init__ trigger karta hai.

Instance attributes kahan rehte hain?

Individual object par (via self.x = ... likhe jaate hain), har object ke liye alag.

Class attributes kahan rehte hain?

Class par khud, saare instances ke beech shared, jab tak kisi instance par shadow na ho jaayein.

d1.age change karne se d2.age kyun affect nahi hota?

Har object ke paas instance attributes ke liye apni independent memory hoti hai.

Do equal-dikhte objects ke liye a is b kya test karta hai?

Identity (same memory location) — yeh False hoga do alag se instantiate kiye objects ke liye chahe data equal ho.

Class likhne se koi object create hota hai kya?

Nahi — yeh sirf ek blueprint define karta hai; objects tab aate hain jab tum class ko call karte ho.

Methods class par kyun store hoti hain, har object par kyun nahi?

Code ki ek copy saare instances ke beech share karne ke liye, memory bachane ke liye.

Connections

Constructors and __init__ — instantiation par object kaise initialise hota hai.
self and instance methods — self methods ko ek specific object se kaise bind karta hai.
Class vs instance attributes — shared-vs-own data ka distinction.
Encapsulation — class mein data+behaviour bundle karna kyun matter karta hai.
Identity vs Equality (is vs ==) — objects ko distinguish karna.
Memory model and references — objects actually kahan rehte hain.