{"id":212,"date":"2024-01-25T16:08:10","date_gmt":"2024-01-25T16:08:10","guid":{"rendered":"https:\/\/learnpython.elegantwallp.com\/?p=212"},"modified":"2024-01-25T16:08:12","modified_gmt":"2024-01-25T16:08:12","slug":"python-__new__","status":"publish","type":"post","link":"https:\/\/learnpython.elegantwallp.com\/2024\/01\/25\/python-__new__\/","title":{"rendered":"Python __new__"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you\u2019ll learn about the Python __new__ method and understand how Python uses it to create a new object.<\/p>\n\n\n\n

Introduction to the Python __new__ method<\/h2>\n\n\n\n

When you create an instance of a\u00a0class, Python first calls the\u00a0__new__()<\/code>\u00a0method to create the object and then calls the\u00a0__init__()<\/code>\u00a0method to initialize the object\u2019s attributes.<\/p>\n\n\n\n

The\u00a0__new__()<\/code>\u00a0is a\u00a0static method\u00a0of the\u00a0object<\/code>\u00a0class. It has the following signature:<\/p>\n\n\n\n

object.__new__(class, *args, **kwargs)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
The first argument of the __new__<\/code> method is the class<\/code> of the new object that you want to create.<\/p>\n\n\n\n
The *args<\/code> and **kwargs<\/code> parameters must match the parameters of the __init__()<\/code> of the class. However, the __new__()<\/code> method does use them.<\/p>\n\n\n\n
The __new__()<\/code> method should return a new object of the class. But it doesn\u2019t have to.<\/p>\n\n\n\n
When you define a new class, that class implicitly inherits from the object<\/code> class. It means that you can override the __new__<\/code> static method and do something before and after creating a new instance of the class.<\/p>\n\n\n\n
To create the object of a class, you call the super().__new__()<\/code> method.<\/p>\n\n\n\n
Technically, you can call the object.__new__()<\/code> method to create an object manually. However, you need to call the __init__()<\/code> yourself manually after. Python will not call the __init__()<\/code> method automatically if you explicitly create a new object using the object.__new__()<\/code> method.<\/p>\n\n\n\n
Python __new__ method example<\/h2>\n\n\n\n
The following defines the\u00a0Person<\/code>\u00a0class with the\u00a0name<\/code>\u00a0attribute and create a new instance of the\u00a0Person<\/code>\u00a0class:<\/p>\n\n\n\n
class Person: def __init__(self, name): self.name = name person = Person('John')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
In Python, a class is\u00a0callable. When you call the class to create a new object:<\/p>\n\n\n\n
person = Person('John')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Python will call the\u00a0__new__()<\/code>\u00a0and\u00a0__init__()<\/code>\u00a0methods. It\u2019s equivalent to the following method calls:<\/p>\n\n\n\n
person = object.__new__(Person, 'John') person.__init__('John')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
The following shows the contents of the\u00a0__dict__<\/code>\u00a0of the\u00a0person<\/code>\u00a0object after calling the\u00a0__new__()<\/code>\u00a0and\u00a0__init__()<\/code>\u00a0methods:<\/p>\n\n\n\n
person = object.__new__(Person, 'John') print(person.__dict__) person.__init__('John') print(person.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
{} {'name': 'John'}<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
As you can see clearly from the output, after calling the __new__()<\/code> method, the person.__dict__<\/code> is empty. And after calling the __init__()<\/code> method, the person.__dict__<\/code> contains the name<\/code> attribute with the value \u2018John'<\/code>.<\/p>\n\n\n\n
The following illustrates the sequence which Python calls the\u00a0__new__<\/code>\u00a0and\u00a0__init__<\/code>\u00a0method when you create a new object by calling the class:<\/p>\n\n\n\n
class Person: def __new__(cls, name): print(f'Creating a new {cls.__name__} object...') obj = object.__new__(cls) return obj def __init__(self, name): print(f'Initializing the person object...') self.name = name person = Person('John')<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Creating a new Person object... Initializing the person object...<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
In this example, Python calls the __new__ method and the __init__ method after that.<\/p>\n\n\n\n
When using the __new__ method<\/h2>\n\n\n\n
The following example defines the\u00a0SquareNumber<\/code>\u00a0class that inherits from the built-in int type:<\/p>\n\n\n\n
class SquareNumber(int): def __new__(cls, value): return super().__new__(cls, value ** 2) x = SquareNumber(3) print(x) # 9<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
In this example, the\u00a0__new__()<\/code>\u00a0method of the\u00a0SquareNumber<\/code>\u00a0class accepts an integer and returns the square number.\u00a0x<\/code>\u00a0is an instance of the\u00a0SquareNumber<\/code>\u00a0class and also an instance of the\u00a0int<\/code>\u00a0built-in type:<\/p>\n\n\n\n
print(isinstance(x, int)) # True<\/em><\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Note that you cannot do this by using the\u00a0__init__()<\/code>\u00a0method because the\u00a0__init__()<\/code>\u00a0method of the built-in\u00a0int<\/code>\u00a0takes no argument. The following code will result in an error:<\/p>\n\n\n\n
class SquareNumber(int): def __init__(self, value): super().__init__(value ** 2) x = SquareNumber(3)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Error:<\/p>\n\n\n\n
TypeError: object.__init__() takes exactly one argument (the instance to initialize)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
In practice, you use the __new__()<\/code> method when you want to tweak the object at the instantiated time.<\/p>\n\n\n\n
For example, the following defines the\u00a0Person<\/code>\u00a0class and uses the\u00a0__new__<\/code>method to inject the\u00a0full_name<\/code>\u00a0attribute to the Person\u2019s object:<\/p>\n\n\n\n
class Person: def __new__(cls, first_name, last_name): # create a new object<\/em> obj = super().__new__(cls) # initialize attributes<\/em> obj.first_name = first_name obj.last_name = last_name # inject new attribute<\/em> obj.full_name = f'{first_name} {last_name}' return obj person = Person('John', 'Doe') print(person.full_name) print(person.__dict__)<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
John Doe {'first_name': 'John', 'last_name': 'Doe', 'full_name': 'John Doe'}<\/code>Code language: Python (python)<\/small><\/code><\/pre>\n\n\n\n
Typically, when you override the __new__()<\/code> method, you don\u2019t need to define the __init__()<\/code> method because everything you can do in the __init__()<\/code> method, you can do it in the __new__()<\/code> method.<\/p>\n","protected":false},"excerpt":{"rendered":"
Summary: in this tutorial, you\u2019ll learn about the Python __new__ method and understand how Python uses it to create a new object. Introduction to the Python __new__ method When you create an instance of a\u00a0class, Python first calls the\u00a0__new__()\u00a0method to create the object and then calls the\u00a0__init__()\u00a0method to initialize the object\u2019s attributes. The\u00a0__new__()\u00a0is a\u00a0static method\u00a0of […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[32],"tags":[],"class_list":["post-212","post","type-post","status-publish","format-standard","hentry","category-8-metaprogramming-exceptions"],"_links":{"self":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/212","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/comments?post=212"}],"version-history":[{"count":1,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/212\/revisions"}],"predecessor-version":[{"id":213,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/posts\/212\/revisions\/213"}],"wp:attachment":[{"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/media?parent=212"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/categories?post=212"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/learnpython.elegantwallp.com\/wp-json\/wp\/v2\/tags?post=212"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}