__new__ in Python - GeeksforGeeks

25 Nov, 2019
Python is an Object oriented programming language i.e everything in Python is an object. There are special kind of methods in Python known as magic methods or dunder methods (dunder here means “Double Underscores”). Dunder or magic methods in Python are the methods having two prefix and suffix underscores in the method name. These are commonly used for operator overloading.Few examples for magic methods are: __init__, __add__, __len__, __repr__ etc.
Note: To know more about Magic methods click here.
Syntax:
class class_name:
    def __new__(cls, *args, **kwargs):
        statements
        .
        .
        return super(class_name, cls).__new__(cls, *args, **kwargs)

Note: Instance can be created inside __new__ method either by using super function or by directly calling __new__ method over object, where if parent class is object. That is instance = super(MyClass, cls).__new__(cls, *args, **kwargs) or instance = object.__new__(cls, *args, **kwargs)
If both __init__ method and __new__ method exists in the class, then the __new__ method is executed first and decides whether to use __init__ method or not, because other class constructors can be called by __new__ method or it can simply return other objects as an instance of this class.
Example:
# Python program to # demonstrate __new__  # don't forget the object specified as baseclass A(object):    def __new__(cls):         print("Creating instance")         return super(A, cls).__new__(cls)      def __init__(self):        print("Init is called")  A()
Output:
Creating instance
Init is called

The above example shows that __new__ method is called automatically when calling the class name, whereas __init__ method is called every time an instance of the class is returned by __new__ method, passing the returned instance to __init__ as the self parameter, therefore even if you were to save the instance somewhere globally/statically and return it every time from __new__, then __init__ will be called every time you do just that.
This means that if the super is omitted for __new__ method the __init__ method will not be executed. Let’s see if that is the case.
# Python program to# demonstrate __new__  class A(object):    def __new__(cls):        print("Creating instance")      # It is not called    def __init__(self):        print("Init is called")  print(A())
Output:
Creating instance
None

In the above example, it can be seen that __init__ method is not called and the instantiation is evaluated to be None because the constructor is not returning anything. Let’s see what happens if both the __new__ and __init__ methods are returning something.
# Python program to# demonstrate __new__  class A(object):    # new method returning a string    def __new__(cls):        print("Creating instance")        return "GeeksforGeeks"  class B(object):    # init method returning a string    def __init__(self):        print("Initializing instance")        return "GeeksforGeeks"  print(A())print(B())
Output:
Creating instance
GeeksforGeeks
Initializing instance

Traceback (most recent call last):
  File "/home/62216eb30d3856048eff916fb8d4c32d.py", line 17, in 
    print(B())
TypeError: __init__() should return None, not 'str'

This TypeError is raised by the handler that calls __init__ method and it wouldn’t even make sense to return anything from __init__ method since it’s purpose is just to alter the fresh state of the newly created instance.
Let’s try an example in which __new__ method returns an instance of a different class.Example:
# Python program to# demonstrate __new__ method  # class whose object# is returnedclass GeeksforGeeks(object):    def __str__(self):        return "GeeksforGeeks"          # class returning object# of different classclass Geek(object):    def __new__(cls):        return GeeksforGeeks()              def __init__(self):        print("Inside init")          print(Geek())
Output:
GeeksforGeeks

Python-Functions
Python-OOP
Python
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Comments
Old Comments
Python Dictionary
How to Install PIP on Windows ?
Enumerate() in Python
Read a file line by line in Python
Iterate over a list in Python
Different ways to create Pandas Dataframe
Python program to convert a list to string
Reading and Writing to text files in Python
Python OOPs Concepts
Create a Pandas DataFrame from Lists