Python Magic Methods, Properties, and Iterators (Computer Science and Engineering Notes)

 

Python Magic Methods, Properties, and Iterators

Python provides powerful features that make objects more expressive and flexible. Among these are magic methods, properties, and iterators, which allow developers to customize behavior, control attribute access, and enable iteration in a clean, Pythonic way.

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✨ Magic Methods

Magic methods (also called dunder methods because they start and end with double underscores) allow you to define how objects behave with built-in operations.

Initialization and Representation

class Person:
    def __init__(self, name):
        self.name = name
    
    def __str__(self):
        return f"Person named {self.name}"

p = Person("Alice")
print(p)  # Person named Alice

• __init__: Called when an object is created.
• __str__: Defines the string representation of the object.

Arithmetic Operators

class Vector:
    def __init__(self, x, y):
        self.x, self.y = x, y
    
    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)
    
    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(2, 3)
v2 = Vector(4, 1)
print(v1 + v2)  # Vector(6, 4)

Here, __add__ customizes the + operator.

Comparison Operators

class Student:
    def __init__(self, grade):
        self.grade = grade
    
    def __eq__(self, other):
        return self.grade == other.grade

s1 = Student(90)
s2 = Student(90)
print(s1 == s2)  # True

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🏛️ Properties

Properties allow controlled access to attributes, enabling encapsulation without changing syntax.

Using @property

class Circle:
    def __init__(self, radius):
        self._radius = radius
    
    @property
    def radius(self):
        return self._radius
    
    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value
    
    @property
    def area(self):
        import math
        return math.pi * self._radius ** 2

c = Circle(5)
print(c.area)   # 78.5398...
c.radius = 10
print(c.area)   # 314.159...

• @property: Defines a getter.
• @radius.setter: Defines a setter with validation.
• Properties make attributes behave like variables but with controlled logic.

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🔄 Iterators

Iterators allow objects to be traversed using for loops and other iteration contexts.

Iterator Protocol

• An object is an iterator if it implements:
  • __iter__(): Returns the iterator object itself.
  • __next__(): Returns the next value or raises StopIteration.

Example: Custom Iterator

class Countdown:
    def __init__(self, start):
        self.current = start
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        self.current -= 1
        return self.current

for num in Countdown(5):
    print(num)
# Output: 4 3 2 1 0

This defines a countdown iterator that decrements until zero.

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📊 Comparison Table

Feature	Purpose	Example
Magic Methods	Customize object behavior with operators and built-ins	__add__, __eq__, __str__
Properties	Control attribute access with getters/setters	@property, @radius.setter
Iterators	Enable iteration over objects	__iter__, __next__

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📖 Conclusion

Python’s magic methods, properties, and iterators provide powerful ways to make objects more expressive, flexible, and Pythonic. Magic methods let you redefine operators, properties enable controlled attribute access, and iterators allow seamless traversal of custom objects. Together, they form the backbone of writing elegant, object-oriented Python code.