【python】__new__和__init__區別

原文： http://blog.csdn.net/cnmilan/article/details/8849680

__new__：建立對象時調用，返回當前對象的一個實例 __init__：建立完對象後調用，對當前對象的實例的一些初始化，無返回值 測試1： >>> class A(object): def __init__(self): print("in init") def __new__(self): print("in new") >>> A() in new 測試2： class A(object): def __new__(Class): Object = super(A, Class).__new__(Class) print "in New" return Object def __init__(self): print "in init" class B(A): def __init__(self): print "in B's init" B() >>> in New in B's init 對於New來講： Called to create a new instance of class cls. __new__() is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of __new__() should be the new object instance (usually an instance ofcls). Typical implementations create a new instance of the class by invoking the superclass’s __new__() method using super(currentclass, cls).__new__(cls[, ...]) with appropriate arguments and then modifying the newly-created instance as necessary before returning it. 這也是上面的例子2 If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[, ...]), where self is the new instance and the remaining arguments are the same as were passed to __new__(). If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked. __new__() is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation. object.__init__(self[, ...]) Called when the instance is created. The arguments are those passed to the class constructor expression. If a base class has an __init__() method, the derived class’s __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: BaseClass.__init__(self, [args...]). As a special constraint on constructors, no value may be returned; doing so will cause a TypeError to be raised at runtime.