Encapsulation — hiding internal state, name mangling (`__name`)

2.1.6 · Coding › OOP Fundamentals

Encapsulation HAI kya?

Python "private" signal kaise karta hai

Python mein Java jaisa koi hard private keyword nahi hai. Iske bajaye yeh conventions use karta hai, plus ek mild enforcement mechanism (name mangling).

Name Mangling — woh ek asli mechanism

YE exist kyun karta hai? (Motivation scratch se derive karo)

Iska purpose security nahi hai. Yeh inheritance mein accidental name clashes avoid karne ke liye hai.

Socho ek Base class hai jisme ek internal __id hai, aur ek Child bhi __id naam ka attribute invent karta hai lekin bilkul alag purpose ke liye. Mangling ke bina, Child.__id silently Base.__id ko overwrite kar deta aur Base ke methods tod deta.

Mangling yeh kaise solve karta hai: interpreter har ek ko apni class ka naam deke rename kar deta hai:

• Base.__id → _Base__id
• Child.__id → _Child__id

Ab woh collide nahi karte — har class ke methods quietly apne version ko refer karte hain.

Worked Example 1 — Ek self-guarding bank account

class Account:
    def __init__(self, balance):
        self.__balance = 0          # private internal state
        self.deposit(balance)       # go through the guard
 
    def deposit(self, amount):
        if amount < 0:                       # invariant check
            raise ValueError("no negatives")
        self.__balance += amount
 
    def get_balance(self):
        return self.__balance

a = Account(100)
print(a.get_balance())   # 100
a.__balance              # AttributeError!
print(a._Account__balance)  # 100  <- the mangled name

• self.__balance __init__ mein kyun? Yeh step kyun: __balance likhne se mangling trigger hoti hai, toh yeh actually _Account__balance store karta hai. Bahar se a.__balance type karne wale ek non-existent attribute dhundhte hain → AttributeError. Invariant (balance >= 0) sirf deposit ke through hi change ho sakta hai.
• a._Account__balance kyun kaam karta hai? Yeh step kyun: mangling sirf class body ke andar ke code ko rewrite karti hai. Bahar se tum mangled naam literally type kar sakte ho — yeh sabit karta hai ki yeh security nahi hai, bas clash-avoidance hai.

Worked Example 2 — Mangling inheritance collision rokta hai

class Base:
    def __init__(self):
        self.__data = "base secret"     # -> _Base__data
    def reveal(self):
        return self.__data              # compiled as self._Base__data
 
class Child(Base):
    def __init__(self):
        super().__init__()
        self.__data = "child stuff"     # -> _Child__data (different!)
    def reveal_child(self):
        return self.__data              # self._Child__data

c = Child()
print(c.reveal())        # "base secret"  <- Base's data intact!
print(c.reveal_child())  # "child stuff"
print(c.__dict__)        # {'_Base__data': ..., '_Child__data': ...}

• Child.__data ne Base.__data ko clobber kyun nahi kiya? Yeh step kyun: har __data ko compile time par mangle kiya gaya tha uss class ka use karke jisme yeh likha gaya tha, jisse do alag keys ban gayi. Mangling ka poora point yahi hai.

Worked Example 3 — Properties: Pythonic tarika hide karne ka

Name mangling internal names ko protect karta hai, lekin controlled access expose karne ka idiomatic tarika hai @property.

class Temperature:
    def __init__(self, celsius):
        self._celsius = celsius           # single underscore: "internal"
 
    @property
    def celsius(self):                    # getter
        return self._celsius
 
    @celsius.setter
    def celsius(self, value):
        if value < -273.15:               # invariant: above absolute zero
            raise ValueError("below absolute zero")
        self._celsius = value
 
    @property
    def fahrenheit(self):                 # computed, read-only
        return self._celsius * 9/5 + 32

• Property kyun, public attribute kyun nahi? Yeh step kyun: t.celsius = -300 ab setter chalata hai aur reject ho jaata hai, invariant protect karta hai — jabki callers phir bhi clean t.celsius syntax use karte hain (koi ugly get_/set_ nahi).
• fahrenheit read-only kyun hai? Yeh step kyun: koi setter define nahi, toh assign karne par AttributeError aata hai; yeh derived state hai aur independently store nahi hona chahiye.

Common Mistakes (Steel-manned)

Flashcards

Connections

• OOP Fundamentals
• Abstraction — hiding complexity behind interfaces
• Inheritance — base and derived classes
• Properties and Descriptors
• Class Invariants & Validation
• Access Modifiers (Java vs Python)

Concept Map