python 中的 @ 符号

1 . 修饰函数

看装饰器这一节

https://www.cnblogs.com/xumaomao/articles/10984271.html

2. @staticmethod

相当于C++里的静态函数

#!/usr/bin/ env python
# -*- coding:utf-8 -*-

class Student(object):
    #类属性
    name = 'fdzwdt'
    def __init__(self,name):
        #实例对象属性
        self.name = name
    
    #实例方法,只能由实例对象来调用
    def get_name_ins(self):
        return self.name
    
    #类方法，可以通过类来调用，也可以通过实例对象调用
    @classmethod
    def get_name_cls(cls):
        return cls.name
    
    #静态方法，可以通过类来调用，也可以通过实例对象调用
    #主要功能就是对__init__构造函数进行重载
    @staticmethod
    def get_name_sta(name):
        return Student(name).name
st = Student('weidt')
print('ins_name:'+st.get_name_ins())
print('cls_name:'+Student.get_name_cls())
print('sta_name:'+Student.get_name_sta('wdt'))

3. @classmethod

classmethod 修饰符对应的函数不需要实例化，不需要 self 参数，但第一个参数需要是表示自身类的 cls 参数，可以来调用类的属性，类的方法，实例化对象等。

#!/usr/bin/python
# -*- coding: UTF-8 -*-
 
class A(object):
    bar = 1
    def func1(self):  
        print ('foo') 
    @classmethod
    def func2(cls):
        print ('func2')
        print (cls.bar)
        cls().func1()   # 调用 foo 方法
 
A.func2()               # 不需要实例化

输出结果为：

func2
1
foo

4. 修饰类

1. 修改类函数。

场景： 如果要给一个类的所有方法加上计时，并打印出来

# -*- coding:utf-8 -*-
import time
def time_it(fn):
    "Example of a method decorator"
    def decorator(*args, **kwargs):
        t1=time.time()
        ret = fn(*args, **kwargs)
        print('		%d seconds taken for %s'%(time.time()-t1, fn.__name__))
        return ret

    return decorator

def class_decorator(*method_names):
    def class_rebuilder(cls):
        "The class decorator example"
        class NewClass(cls):
            "This is the overwritten class"
            def __getattribute__(self, attr_name):
                attr_val = super(NewClass, self).__getattribute__(attr_name)
                if callable(attr_val) and attr_name in method_names:
                    return time_it(attr_val)
                return attr_val

        return NewClass
    return class_rebuilder

@class_decorator('first_method', 'second_method')
class MySecondClass(object):
    """
    This class is decorated
    """
    def first_method(self, *args, **kwargs):
        print("	this is a the MySecondClass.first_method")
    time.sleep(2)

    def second_method(self, *args, **kwargs):
        print("	this is the MySecondClass.second_method")
    time.sleep(1)

if __name__ == "__main__":
    print("::: With a decorated class :::")
    z = MySecondClass()
    z.first_method()
    z.second_method()

输出：

::: With a decorated class :::
    this is a the MySecondClass.first_method
        0 seconds taken for first_method
    this is the MySecondClass.second_method
        0 seconds taken for second_method

2. 增加类成员

场景：比如统一给所有的模型增加id, created_time属性

# -*- coding:utf-8 -*-
import time
def cd(cls):
    def init(*args, **kwargs):
        cls_obj = cls(*args, **kwargs)
        setattr(cls_obj, 'id', time.time())
        return cls_obj
    return init
@cd
class A(object):
    def __init__(self, name, age, sex='f'):
        self.name=name
        self.age=age
        self.sex=sex
    def s(self):
        print self.id

if __name__=='__main__':
    print type(A)#<type 'function'>
    a=A('Alice', 22)
    print type(a)#<class '__main__.A'>
    print a#<__main__.A object at 0x7fe617baa690>
    print a.name, a.age, a.sex#Alice 22 f
    a.s()

输出

<type 'function'>
<class '__main__.A'>
<__main__.A object at 0x1005f4bd0>
Alice 22 f
1627034597.78