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特种方式之new

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特种方式之new

python 特殊形式之new,pythonnew

object.``__new__(cls[, ...])

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 of cls).

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.

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.

调用产生三个新的类的实例,cls__new__()是一个静态方法(不需要声明),类本身(cls)作为第一个参数,其他的的参数是传递给对象构造函数的表达式(对类的调用),__new()__的返回值应该是一个新的对象实例(一般是cls的实例)。典型的实现方法就是在返回新生成的实例之前,调用父类的__new()__方法(super(currentclass, cls).__new__(cls[, ...]))来改变这个实例对象,比如说可以把实例里面字符的空格去掉等等(这句是我自己加的)。

如果__new()__回到了三个cls的实例对象,然后就能够调用这一个新的实例的__init()__方法(__init__[,...]),self指新创立的实例其他的参数和传递给__new()的一样。

如果__new()__从未有过成功再次来到一个cls的实例,就不会调用那一个实例的init()方法。

__new()__重视用来进行不可变类型(疑似int,str,恐怕元组)的子类自定义实例的创造。也能够重写自定义元类来扩充自定义类的始建。

举个例子来讲:在实例化对象此前,先将字符串做二个管理,就可以用__new__,上面包车型大巴例证正是做叁个去空格管理。

图片 1

class Word(str):

    def __new__(cls,word):

        if ' ' in word:
            print("there is qutos")
            word = ''.join(word.split())
        return str.__new__(cls,word)

a = Word('hello sherry')
print(a)

View Code

 

特殊情势之new,pythonnew object. __new__ ( cls [, ... ] ) Called to create a new instance of class cls . __new__() is a static method (special-cased so you need not de...

official site:
https://docs.python.org/3/reference/datamodel.html

object._ new _(cls[, ...])

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 of cls).

Typical implementations create a new instance of the class by invoking the superclass’s __new__() method using super().__new__(cls[, ...]) with appropriate arguments and then modifying the newly-created instance as necessary before returning it.

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 after the instance has been created (by __new__()), but before it is returned to the caller. 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: super().__init__([args...]).

Because __new__() and __init__() work together in constructing objects (__new__() to create it, and __init__() to customize it), no non-None value may be returned by __init__(); doing so will cause a TypeError to be raised at runtime.

object._ del _(self)

Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a __del__() method, the derived class’s __del__() method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.

It is possible (though not recommended!) for the __del__() method to postpone destruction of the instance by creating a new reference to it. This is called object resurrection. It is implementation-dependent whether __del__() is called a second time when a resurrected object is about to be destroyed; the current CPythonimplementation only calls it once.

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits.

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