Dunder or magic methods in Python

Python magic (dunder) methods are special methods with double underscores __ that enable operator overloading and custom object behavior.

The below code displays the magic methods inherited by int class.

print(dir(int))

['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '_...

Explanation: dir(int) lists all attributes and methods of the int class, including magic (dunder) methods like __add__, __str__, etc.

You can list magic (dunder) methods for any Python object via the terminal by following these steps:

• Open the terminal or command prompt and type python3 to enter the Python console.
• Use the dir() function to list all methods, e.g.

>>> dir(int)

Output

Python Magic Methods

Below are the lists of Python magic methods and their uses.

1. Initialization and Construction

• __new__: To get called in an object's instantiation.
• __init__: To get called by the __new__ method.
• __del__: It is the destructor.

2. Numeric magic methods

• __trunc__(self): Implements behavior for math.trunc()
• __round__(self,n): Implements behavior for the built-in round()
• __abs__(self): Implements behavior for the built-in abs()
• __neg__(self): Implements behavior for negation
• __pos__(self): Implements behavior for unary positive 

3. Arithmetic operators

• __add__(self, other): Implements behavior for the + operator (addition).
• __sub__(self, other): Implements behavior for the - operator (subtraction).
• __mul__(self, other): Implements behavior for the * operator (multiplication).
• __truediv__(self, other): Implements behavior for the / operator (true division).
• __mod__(self, other): Implements behavior for the % operator (modulus).

4. String Magic Methods

• __str__(self): Defines behavior for when str() is called on an instance of your class.
• __repr__(self): To get called by built-int repr() method to return a machine readable representation of a type.
• __hash__(self): It has to return an integer, and its result is used for quick key comparison in dictionaries.
• __bool__(self): Returns True or False to determine the truth value of an object, used in conditions and bool() calls.
• __dir__(self): This method to return a list of attributes of a class.

5. Comparison magic methods

• __eq__(self, other): Defines behavior for the equality operator, ==.
• __ne__(self, other): Defines behavior for the inequality operator, !=.
• __lt__(self, other): Defines behavior for the less-than operator, <.
• __gt__(self, other): Defines behavior for the greater-than operator, >.
• __ge__(self, other): Defines behavior for the greater-than-or-equal-to operator, >=.

Examples of Magic Methods

1. __init__ Method

__init__ method is automatically called when a new instance of a class is created. It is used to initialize the object’s attributes, similar to constructors in languages like C++, Java, C#, or PHP.

class String:
    
    def __init__(self, string):
        self.string = string
        
if __name__ == '__main__':
    
    s1 = String('Hello')
    print(s1)

<__main__.String object at 0x791ce72aa900>

Explanation:

• __init__(self, string): Constructor that initializes the object with a string.
• s1= String('Hello'): Creates an instance of String with 'Hello'.
• print(s1): Tries to print the object.

2. __repr__ method

__repr__ method defines the official string representation of an object, primarily used for debugging. By default, it shows the type and memory address of the object.

class String:
    def __init__(self, string):
        self.string = string

    def __repr__(self):
        return 'Object: {}'.format(self.string)

if __name__ == '__main__':
    str1 = String('Hello')
    print(str1)

Object: Hello

Explanation:

• __repr__(self): Returns the official string representation of the object; used when printing or debugging.
• str1 = String('Hello'): Creates an instance of String with 'Hello'.

3. __add__ method

__add__ method method defines how objects of a class are added together using the '+' operator. It allows operator overloading.

Adding __add__ method to String class: 

class String:
    
    def __init__(self, string):
        self.string = string 
        
    def __repr__(self):
        return 'Object: {}'.format(self.string)
        
    def __add__(self, other):
        return self.string + other

if __name__ == '__main__':
    
    string1 = String('Hello')
    print(string1 +' Geeks')

Hello Geeks

Explanation:

• __init__(self, string): Initializes the object with a string.
• __repr__(self): Provides a readable string representation of the object for debugging.
• __add__(self, other): Overloads the + operator to allow adding a string to the object’s string attribute.
• string1 + ' Geeks' – Calls __add__, returning 'Hello Geeks'.

Related Articles:

• Customize your Python class with Magic or Dunder methods
• __new__ in Python