__new__ in Python

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.

__new__ method

Whenever a class is instantiated __new__ and __init__ methods are called. __new__ method will be called when an object is created and __init__ method will be called to initialize the object. In the base class object, the __new__ method is defined as a static method which requires to pass a parameter cls. cls represents the class that is needed to be instantiated, and the compiler automatically provides this parameter at the time of instantiation.

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:

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.

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.

Output:

Creating instance
Lazyroar
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:

Output:

Lazyroar