`__new__` and `__init__` on class and object
In Python 3, when defining a subclass, why do you need to use
cls as the first argument of
__new__, but not to use
self as the first argument of
class MyClass(object): def __new__(cls, *args, **kwargs): return super(MyClass, cls).__new__(cls, *args, **kwargs) # with `cls` def __init__(self, *args, **kwargs): return super(MyClass, self).__init__(*args, **kwargs) # without `self`
When I compared these functions I got more confused:
>>> cls = object >>> self = cls() >>> cls.__new__ is self.__new__ True >>> cls.__init__ is self.__init__ False >>> self.__init__() >>> cls.__init__() Traceback (most recent call last): ...
So, what are the differences between
__init__ behind these results? Which methods are bound and which are free? Why are you able to call
self.__init__() but not
cls.__init__ a method defined in
cls itself or in its metaclass?
The biggest part of the picture you're probably missing is that
__new__ is a staticmethod, special-cased to be one even if you don't use the
When calling a method through
super() performs the same kind of argument binding that would be performed normally for that kind of method (using the descriptor protocol). For a staticmethod like
__new__, that means no arguments are automatically bound, so
cls has to be passed explicitly. For an instance method like
__init__, that means
self is bound automatically, which is why you don't have to pass
Professional Python, Augment the class' original `__init__` method to also store a # `_created` attribute on the _created # Done; return the class object. return cls The first thing that is a new method that will be assigned to __init__, and this method first calls the How to create a Class and it's Object in Python? How to create data attributes of a class in run-time in python? Decorator for both class methods and functions in Python; How to use the __init__() method to assign values to data attributes in Python? Single, Multiple and Multi-level Inheritance in Python
cls stands for class itself, while
self stands for object itself. These are just conventions.
__new__ method is called before the object is created, in fact,
__new__should create the object and return it. Therefore, it needs a class to create object. After that, the
__init__ is called to initialize the object, so it needs the object as the first argument.
class MyClass: def __new__(cls, *args, **kwargs): # cls == MyClass return super().__new__(cls, *args, **kwargs) # cls here is because __new__ is staticmethods so you have to pass the cls explicitly # You can't use cls() here because it will call this methods again and again # causing recusion error def __init__(self, *args, **kwargs): # Here self is the instance(or object) of MyClass # So you can initialize it by self.xxx self.xxx = 'xxx'
__new__ is static method, so the class and instance share the same
__init__ is the method of instance. If you want to call it via class, you need to pass the instance as the first argument explicitly.
Anything in Python is object, including the class itself. So for class, it has its own
__init__ which are used by
metaclass to create class and initialize class.
These are meta programming of Python, I suggest to read the ninth chapter of Python Cookbook.
__new__ and __init__, The object base class brings methods/properties that are common to all new-style classes. Throughout the rest of the article we will examine the __new__ and __ The __new__ and __init__ methods behave differently between themselves and between the old-style versus new-style python class definitions. Understanding the difference between __new__ and __init__ The major difference between these two methods is that __new__ handles object creation and __init__ handles object initialization.
According to the docs:
__new__()is a static method (special-cased so you need not declare it as such)
__init__ on the other hand, is a proper instance method. It can be called multiple times on the same instance, by the way.
That should be enough to explain your terminal session:
>>> cls = object >>> self = cls()
You just called
object.__call__, which essentially does
self = cls.__new__() if isinstance(self, cls): cls.__init__(self) return self
Notice that the return value of
__new__ is not required to be an instance of the class it belongs to, but
__init__ is called only if it is. In your case, it is.
>>> cls.__new__ is self.__new__ True
__new__ is a static method, so attempting to bind it to the instance does nothing: it stays a class method. This is the same reason that you have to pass
cls explicitly when calling
super().__new__: it's the same free function, unbound to either class or instance.
>>> cls.__init__ is self.__init__ False
Not only are these not the same thing, but their types are different.
cls.__init__ is a regular function.
self.__init__ is a bound method which lacks the first parameter of
This has already been called, but for
object, it's a no-op you can call as many times as you like. Notice that the first parameter is not being passed in, being as it is a bound method.
This is calling the raw
__init__ function, which requires that the
self parameter be passed in. Since you don't do that, it raises. Try this instead:
Issue 36827: Overriding __new__ method with itself changes , By the way, I understand why `object`'s `__init__` can complain about a __new__(cls) class E(C, T): @staticmethod def __new__(*args, Differences between __new__()and __init__()methods in Python. 1 new() __new__()method in Python class is responsible for creating a new instance object of the class. __new__()is similar to the constructor method in other OOP (Object Oriented Programming) languages such as C++, Javaand so on.
Classes, Derived class objects can access and modify the attributes of its base classes: it creates a new instance of the decorated class and calls its `__init__` method. In Python, the method __init__()is called as constructor but it does not creates an object instead it instatiates the object. To create an object, Python uses a special method __new__(). When a class has both __init__() and __new__() methods, the __new__() method overrides __init__() method.
Explain classes, __init__ and self like I'm five. : learnpython, So I sort of see Python as a giant class that has methods we can call with the '. class Airplane: def __init__(self): print "A new instance got made!" So what's In Python, the __new__ method is similar to the __init__ method, but if both exist, __new__ method executes first. In the base class object, __new__ is defined as a static method and needs to pass a parameter cls. cls represent the classe that need to be instantiated, and this parameter is provided automatically by python parser at instantiation time.
`bytes` should implement `__new__` · Issue #2630 · python , `bytes` should implement `__new__` #2630. Open class bytes(ByteString):. @overload def __init__(self, o: object = ) -> None: . By default, the __new__() method invokes the __init__ method. B. The __new__() method is defined in the object class. C. The __init__() method is defined in the object class. D. The __str__() method is defined in the object class. E. The __eq__(other) method is defined in the object class.