Dunder Methods

What are special methods?

They are ‘predefined’ methods that allow you to customize the behavior of classes. Can also be referred to as “dunder” (double-underscore) methods.

You don’t write my_object.__len__(), you write len(my_object).

Difference with normal methods

• Implicit invocation: Special methods are called automatically by Python in specific situations, while regular methods need to be called explicitly.
• Syntax integration: Special methods allow your objects to work with Python’s built-in syntax and functions, whereas regular methods don’t.

class MyClass:
    def __init__(self, value):
        self.value = value
    
    def __str__(self):
        return f"MyClass instance with value: {self.value}"
    
    def __add__(self, other):
        return MyClass(self.value + other.value)
    
    def double_value(self):
        return self.value * 2
        
# Usage obj1 = MyClass(5) 
obj2 = MyClass(10) 

print(obj1) # Calls __str__ implicitly 
result = obj1 + obj2 # Calls __add__ implicitly 
doubled = obj1.double_value() # Regular method, called explicitl

Uses for Special Methods

1. Emulating Numeric Types

Several special methods allow user objects to respond to operators such as ‘+’ or *

2. String representation of objects

__repr__(self):Returns a detailed string representation of the object, used for debugging. Without a custom __repr__, Python’s console would display a Vector instance <Vector object at 0x10e100070>, but we can change this:

Example

class Vector:
	def __repr__(self):
		return f'Vector({self.x!r}, {self.y!r})'

v1 = Vector(2, 4)

print(v1) # Normally prints <__main__.Vector object at 0x7dc33ea23520>

print(v1) #But now it returns Vector(2, 4)

3. Boolean value of a Custom Type

if __bool__ is not implemented, Python tries to invoke x.__len__(), and if that returns zero, bool returns False. Otherwise bool returns True.

Common special methods:

1. __init__(self, ...): Constructor method, called when an object is created.

2. __str__(self): Returns a string representation of the object, used by str() and print().

3. __repr__(self): Returns a detailed string representation of the object, used for debugging.

4. __len__(self): Defines behavior for the len() function.

5. __getitem__(self, key): Allows indexing or key-based access to the object.

6. __setitem__(self, key, value): Allows setting values using indexing or key-based access.

7. __iter__(self): Makes an object iterable.

8. __eq__(self, other): Defines behavior for the equality operator (= =).

9. __lt__(self, other): Defines behavior for the less than operator (<).

10. __add__(self, other): Defines behavior for the addition operator (+).