Magic Methods in Python

 

What Are Dunder Methods?

In Python, special methods are a set of predefined methods you can use to enrich your classes.

 They are easy to recognize because they start and end with double underscores, for example __init__ or __str__.

Dunder methods let you emulate the behavior of built-in types.

 For example, to get the length of a string you can call len('string'). But an empty class definition doesn’t support this behavior out of the box:

These “dunders” or “special methods” in Python are also sometimes called “magic methods.”

class NoLenSupport:
    pass

>>> obj = NoLenSupport()
>>> len(obj)
TypeError: "object of type 'NoLenSupport' has no len()"

To fix this, you can add a __len__ dunder method to your class:

class LenSupport:
    def __len__(self):
        return 42

>>> obj = LenSupport()
>>> len(obj)
42

Object Initialization: __init__

"__init__" is a reserved method in python classes. 
It is called as a constructor in object oriented terminology. 
This method is called when an object is created from a class and it allows the class to initialize the attributes of the class.
class Hospital:
    """A simple Hospital class"""

    def __init__(self, doctor, salary):
        """
        This is the constructor that lets us create
        objects from this class
        """
        self.doctor = doctor
        self.salary = salary
Python __eq__
Introduction to the Python __eq__ method
By default, all Python objects inherit the __eq__ method from the base class object, which provides a basic equality comparison. This default behavior checks if two objects have the same identity, meaning they refer to the same memory location.
 
We will override the default method to implement a different use case

Suppose that you have the following Person class with three instance attributes: first_name, last_name, and age:
class Person: def __init__(self, first_name, last_name, age):    self.first_name = first_name     self.last_name = last_name      self.age = age
Code language: Python (python)
And you create two instances of the Person class:
ravi = Person('Ravi', 'shankar', 25)hema = Person('hema', 'malini', 25)
Code language: Python (python)
In this example, the john and jane objects are not the same object. And you can check it using the is operator:
print(ravi is hema)  # False
Code language: Python (python)
Also, when you compare john with jane using the equal operator (==), you’ll get the result of False:
print(ravi == hema) # False
Code language: Python (python)
Since john and jane have the same age, you want them to be equal. In other words, you want the following expression to return True:
ravi == hema
To do it, you can implement the __eq__ dunder method in the Person class.
Python automatically calls the __eq__ method of a class when you use the == operator to compare the instances of the class. By default, Python uses the is operator if you don’t provide a specific implementation for the __eq__ method.
The following shows how to implement the __eq__ method in the Person class that returns True if two person objects have the same age:
class Person: def __init__(self, first_name, last_name, age):    self.first_name = first_name      self.last_name = last_name     self.age = age     def __eq__(self, other):
                return self.age == other.age
Code language: Python (python)
Now, if you compare two instances of the Person class with the same age, it returns True:
ravi = Person('Ravi', 'shankar', 25) hema = Person('hema', 'malini', 25)print(ravi == hema) # True
Code language: Python (python)
And if two instances of the Person class don’t have the same age, the == operator returns False:
john = Person('John', 'Doe', 25)
mary = Person('Mary', 'Doe', 27)
print(john == mary)  # False
Code language: Python (python)ws
__del__ 
The default behavior of the __del__ method inherited from object is to do nothing special when an object is deleted. It's essentially a no-op (no operation). Python's garbage collector automatically reclaims memory used by objects when they are no longer referenced, and the __del__ method is not typically needed for basic memory management.
 This is used to delete an instance and we cannot perform any operations related to that instance after that.
class Student:
 def__init__(self,name,age):
  self.name = name
  self.age = age
 def__del__(self):
  print('Instance is deleted')stud1=Student('tom',12)
del stud1
print(stud1.name)Instance is deleted
NameError: name 'stud1' is not defined  --> raising an error. __lt__ in python:class student:
    def __init__(self,name,age):
        self.name=name
        self.age=age

    def __lt__(self,other):
        return self.age<other.age

ajay=student("Ajay",32)
vinith=student("Arun",12)

print(ajay<vinith)#Output Falsethe __lt__ function in above compares the age of two students.
__gt__ in python:class student:
    def __init__(self,name,age):
        self.name=name
        self.age=age

    def __gt__(self,other):
        return self.age>other.age

ajay=student("Ajay",32)
vinith=student("Arun",12)

print(ajay>vinith)#Output Truethe __gt__ function in above compares the age of two students.