Python Magic methods are the methods starting and ending with double underscores ‘__’. They are defined by built-in classes in Python and are commonly used for operator overloading. 

They are also called Dunder methods, Dunder here means “Double Under (Underscores)”.

Python Magic Methods

Built-in classes define many magic methods, dir() function can show you magic methods inherited by a class.

Example:

This code displays the magic methods inherited by int class.

# code 
print(dir(int))

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

Python Magic Methods

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

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.

Numeric magic methods

• __trunc__(self): Implements behavior for math.trunc()
• __ceil__(self): Implements behavior for math.ceil()
• __floor__(self): Implements behavior for math.floor()
• __round__(self,n): Implements behavior for the built-in round()
• __invert__(self): Implements behavior for inversion using the ~ operator.
• __abs__(self): Implements behavior for the built-in abs()
• __neg__(self): Implements behavior for negation
• __pos__(self): Implements behavior for unary positive 

Arithmetic operators

• __add__(self, other): Implements behavior for math.trunc()
• __sub__(self, other): Implements behavior for math.ceil()
• __mul__(self, other): Implements behavior for math.floor()
• __floordiv__(self, other): Implements behavior for the built-in round()
• __div__(self, other): Implements behavior for inversion using the ~ operator.
• __truediv__(self, other): Implements behavior for the built-in abs()
• __mod__(self, other): Implements behavior for negation
• __divmod__(self, other): Implements behavior for unary positive 
• __pow__: Implements behavior for exponents using the ** operator.
• __lshift__(self, other): Implements left bitwise shift using the << operator.
• __rshift__(self, other): Implements right bitwise shift using the >> operator.
• __and__(self, other): Implements bitwise and using the & operator.
• __or__(self, other): Implements bitwise or using the | operator.
• __xor__(self, other): Implements bitwise xor using the ^ operator.

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.
• __unicode__(self): This method to return an unicode string of a type.
• __format__(self, formatstr): return a new style of string.
• __hash__(self): It has to return an integer, and its result is used for quick key comparison in dictionaries.
• __nonzero__(self): Defines behavior for when bool() is called on an instance of your class. 
• __dir__(self): This method to return a list of attributes of a class.
• __sizeof__(self): It return the size of the object.

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, >.
• __le__(self, other): Defines behavior for the less-than-or-equal-to operator, <=.
• __ge__(self, other): Defines behavior for the greater-than-or-equal-to operator, >=.

Dunder or Magic Methods in Python

Let’s see some of the Python magic methods with examples:

1. __init__ method

The __init__ method for initialization is invoked without any call, when an instance of a class is created, like constructors in certain other programming languages such as C++, Java, C#, PHP, etc.

These methods are the reason we can add two strings with the ‘+’ operator without any explicit typecasting. 

# declare our own string class 
class String: 
	
	# magic method to initiate object 
	def __init__(self, string): 
		self.string = string 
		
# Driver Code 
if __name__ == '__main__': 
	
	# object creation 
	string1 = String('Hello') 

	# print object location 
	print(string1) 

<__main__.String object at 0x7f538c059050>

2. __repr__ method

__repr__ method in Python defines how an object is presented as a string.

The below snippet of code prints only the memory address of the string object. Let’s add a __repr__ method to represent our object. 

# declare our own string class 
class String: 
	
	# magic method to initiate object 
	def __init__(self, string): 
		self.string = string 
		
	# print our string object 
	def __repr__(self): 
		return 'Object: {}'.format(self.string) 

# Driver Code 
if __name__ == '__main__': 
	
	# object creation 
	string1 = String('Hello') 

	# print object location 
	print(string1) 

Object: Hello