215. Immutable Class Design

🔹 1. Basic Immutable Class Using property

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class ImmutablePoint:
    def __init__(self, x, y):
        self._x = x  # Private variable
        self._y = y

    @property
    def x(self):
        return self._x

    @property
    def y(self):
        return self._y

p = ImmutablePoint(3, 4)
print(p.x)  # ✅ 3
p.x = 10  # ❌ AttributeError: can't set attribute

🔍 Key Takeaway:

• Using @property, we prevent modifications after object creation.

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🔹 2. Immutable Class with __slots__

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class ImmutablePerson:
    __slots__ = ("_name", "_age")  # Restricts instance attributes

    def __init__(self, name, age):
        self._name = name
        self._age = age

    @property
    def name(self):
        return self._name

    @property
    def age(self):
        return self._age

p = ImmutablePerson("Alice", 30)
print(p.name)  # ✅ Alice
p.age = 35  # ❌ AttributeError: can't set attribute

🔍 Key Takeaway:

• __slots__ prevents dynamic attribute creation, making the class memory-efficient and immutable.

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🔹 3. Using namedtuple for Immutable Objects

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from collections import namedtuple

ImmutableCar = namedtuple("ImmutableCar", ["brand", "model", "year"])

car = ImmutableCar("Tesla", "Model S", 2023)
print(car.brand)  # ✅ Tesla
car.brand = "Ford"  # ❌ AttributeError: can't set attribute

🔍 Key Takeaway:

• namedtuple is a built-in immutable class with fixed fields.

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🔹 4. Using dataclasses with frozen=True

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from dataclasses import dataclass

@dataclass(frozen=True)
class ImmutableUser:
    username: str
    email: str

user = ImmutableUser("john_doe", "[e-mail]")
print(user.username)  # ✅ john_doe
user.username = "new_user"  # ❌ FrozenInstanceError: cannot assign to field

🔍 Key Takeaway:

• Setting frozen=True in @dataclass makes the class immutable.

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🔹 5. Enforcing Immutability with __setattr__

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class ImmutableConfig:
    def __init__(self, setting):
        super().__setattr__("_setting", setting)  # Bypass __setattr__

    @property
    def setting(self):
        return self._setting

    def __setattr__(self, key, value):
        raise AttributeError("Cannot modify an immutable object")

config = ImmutableConfig("Dark Mode")
print(config.setting)  # ✅ Dark Mode
config.setting = "Light Mode"  # ❌ AttributeError: Cannot modify an immutable object

🔍 Key Takeaway:

• Overrides __setattr__ to block modifications.

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🔹 6. Preventing Deletion with __delattr__

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class ImmutableRecord:
    def __init__(self, value):
        self._value = value

    @property
    def value(self):
        return self._value

    def __delattr__(self, item):
        raise AttributeError("Cannot delete attributes")

record = ImmutableRecord(100)
del record.value  # ❌ AttributeError: Cannot delete attributes

🔍 Key Takeaway:

• __delattr__ prevents accidental deletions.

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🔹 7. Immutable Singleton Class

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class ImmutableSingleton:
    _instance = None

    def __new__(cls, value):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
            cls._instance._value = value
        return cls._instance

    @property
    def value(self):
        return self._value

singleton1 = ImmutableSingleton(42)
singleton2 = ImmutableSingleton(99)

print(singleton1.value)  # ✅ 42
print(singleton2.value)  # ✅ 42 (singleton1 and singleton2 are same)

🔍 Key Takeaway:

• Ensures only one immutable instance exists.

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🔹 8. Immutable Dictionary (MappingProxyType)

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from types import MappingProxyType

config_data = {"api_key": "12345", "mode": "production"}
immutable_config = MappingProxyType(config_data)

print(immutable_config["api_key"])  # ✅ 12345
immutable_config["api_key"] = "67890"  # ❌ TypeError: 'mappingproxy' object does not support item assignment

🔍 Key Takeaway:

• MappingProxyType creates a read-only dictionary.

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🔹 9. Immutable Object with Custom Hashing

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class ImmutableHashable:
    __slots__ = ("_data",)

    def __init__(self, data):
        self._data = data

    @property
    def data(self):
        return self._data

    def __hash__(self):
        return hash(self._data)

obj1 = ImmutableHashable(100)
obj2 = ImmutableHashable(100)

print(hash(obj1) == hash(obj2))  # ✅ True (Immutable objects can be hashed)

🔍 Key Takeaway:

• Immutable objects are hashable, allowing use in sets and dictionaries.

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🔹 10. Enforcing Attribute Restrictions with __slots__ and property

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class ImmutableEmployee:
    __slots__ = ("_name", "_salary")

    def __init__(self, name, salary):
        self._name = name
        self._salary = salary

    @property
    def name(self):
        return self._name

    @property
    def salary(self):
        return self._salary

emp = ImmutableEmployee("Alice", 50000)
print(emp.name)  # ✅ Alice
emp.salary = 60000  # ❌ AttributeError: can't set attribute

🔍 Key Takeaway:

• __slots__ and @property enforce immutability efficiently.

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🚀 Summary: Immutable Class Design Techniques

Method

Key Benefit

@property Decorators

Prevent direct modification

__slots__

Prevents dynamic attribute creation & saves memory

namedtuple

Built-in immutable tuple-like object

@dataclass(frozen=True)

Simplifies immutable class creation

Overriding __setattr__

Blocks modification of attributes

Overriding __delattr__

Prevents deletion of attributes

Singleton Pattern

Ensures only one immutable instance

MappingProxyType

Creates a read-only dictionary

Custom __hash__ Implementation

Allows immutable objects in sets/dicts