python常用的魔法方法(双下划线)

class Test():
    def __init__(self, name):
        self.name = name
        
    def test(self):
        print(self.name)
   
t = Test('xu')
t1 = Test('python')

class Test():
    def __new__(cls，*args, **kwargs):
        return object.__new__(cls, *args, **kwargs) 
    
    def __init__(self, name):
        self.name = name

class Test():
    
    def __init__(self):
        self.name = 'python'
    
    def __call__(self, *args, **kwargs):  # self是Test类的对象
        print(self)  # <__main__.Test object at 0x000001C78CE78FD0>
        print(self.name)
        
t = Test()
t()  # python

class Test():
    def __init__(self, name):
        self.name = name
 
    def __str__(self):
        return self.name
   
t = Test('xu')
print(t1)  # xu

class Test:
    def __init__(self):
        self.x = open('a.txt',mode='w')
        # self.x = 占用的是操作系统资源

    def __del__(self):
        print('run')
        # 发起系统调用，告诉操作系统回收相关的系统资源
        self.x.close()

obj = T()
del obj # obj.__del__() 

class Open:
    def __init__(self):
        self.name = 'open'

    def __enter__(self):
        print('with语句执行时会首先执行的方法，返回值会赋值给as声明的变量')
        return self.name

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('with中的代码块执行完毕时执行exit')
        print(exc_type, '如果出现异常表示异常类型')
        print(exc_val, '表示异常的值')
        print(exc_tb, '表示异常的溯源信息')
        return 123  # 非零 非空 非None为真

    
with Open() as test:
    print(test)
    raise TypeError('看一下错误信息')
print('我会不会被执行呢')  # 当__exit__方法返回值为真时，会被执行，否则不会被执行

class MyDict(dict):

    def __setitem__(self, key, value):
        print('执行setitem', key, value)  # 执行setitem, x, 1
        self.__dict__[key] = value

    def __getitem__(self, item):
        print('执行getitem', item)  # 执行getitem x
        print(self.__dict__[item])  # 1

    def __delitem__(self, key):
        print('执行delitem', key)  # 执行delitem x
        self.__dict__.pop(key)


d = MyDict()
d['x'] = 1
print(d['x'])
del d['x']

class Test:
    def __init__(self):
        self.name = 'python'

    def __setattr__(self, key, value):
        print('添加/修改属性setattr')
        self.__dict__[key] = value
        # self.key = value  # 会出现无线递归，因为对象.属性会调用__setattr__方法

    def __delattr__(self, item):
        print('删除属性delattr')
        self.__dict__.pop(item)

    def __getattr__(self, item):
        print('属性不存在时调用getattr')
t = Test()
t.x = 'x'
print(t.y)
del t.x

# cls_singleton.py
class Foo(object):
    pass

instance = Foo()

# test.py
import cls_singleton

obj1 = cls_singleton.instance
obj2 = cls_singleton.instance
print(obj1 is obj2)  # True

class Test:

    _instance = None

    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __new__(cls, *args, **kwargs):
        # if cls._instance:
        #     return cls._instance                 # 有实例则直接返回
        # else:
        #     cls._instance = super().__new__(cls) # 没有实例则new一个并保存
        #     return cls._instance                 # 这个返回是给是给init，再实例化一次，也没有关系

        if not cls._instance:                         # 这是简化的写法，上面注释的写法更容易提现判断思路
            cls._instance = super().__new__(cls)
        return cls._instance


t1 = Test('python', 18)
t2 = Test('python1', 18)
print(t1 is t2)  # True

class Mymeta(type):

    def __init__(cls, name, bases, dic):
        super().__init__(name, bases, dic)
        cls._instance = None                    # 将记录类的实例对象的数据属性放在元类中自动定义了

    def __call__(cls, *args, **kwargs):                   # 此call会在类被调用（即实例化时触发）
        if cls._instance:      # 判断类有没有实例化对象
            return cls._instance
        else:        # 没有实例化对象时，控制类造空对象并初始化
            obj = cls.__new__(cls, *args, **kwargs)
            obj.__init__(*args, **kwargs)
            cls._instance = obj             # 保存对象，下一次再实例化可以直接返回而不用再造对象
            return obj


class Test(metaclass=Mymeta):
    def __init__(self, name, age):
        self.name = name
        self.age = age


t1 = Test('python', 18)
t2 = Test('python1', 18)
print(t1 is t2)  # True