python常用的魔法方法(雙下劃線)

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類的對(duì)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 = 占用的是操作系統(tǒng)資源

    def __del__(self):
        print('run')
        # 發(fā)起系統(tǒng)調(diào)用，告訴操作系統(tǒng)回收相關(guān)的系統(tǒng)資源
        self.x.close()

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

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

    def __enter__(self):
        print('with語句執(zhí)行時(shí)會(huì)首先執(zhí)行的方法，返回值會(huì)賦值給as聲明的變量')
        return self.name

    def __exit__(self, exc_type, exc_val, exc_tb):
        print('with中的代碼塊執(zhí)行完畢時(shí)執(zhí)行exit')
        print(exc_type, '如果出現(xiàn)異常表示異常類型')
        print(exc_val, '表示異常的值')
        print(exc_tb, '表示異常的溯源信息')
        return 123  # 非零 非空 非None為真

    
with Open() as test:
    print(test)
    raise TypeError('看一下錯(cuò)誤信息')
print('我會(huì)不會(huì)被執(zhí)行呢')  # 當(dāng)__exit__方法返回值為真時(shí)，會(huì)被執(zhí)行，否則不會(huì)被執(zhí)行

class MyDict(dict):

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

    def __getitem__(self, item):
        print('執(zhí)行g(shù)etitem', item)  # 執(zhí)行g(shù)etitem x
        print(self.__dict__[item])  # 1

    def __delitem__(self, key):
        print('執(zhí)行delitem', key)  # 執(zhí)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/修改屬性setattr')
        self.__dict__[key] = value
        # self.key = value  # 會(huì)出現(xiàn)無線遞歸，因?yàn)閷?duì)象.屬性會(huì)調(diào)用__setattr__方法

    def __delattr__(self, item):
        print('刪除屬性delattr')
        self.__dict__.pop(item)

    def __getattr__(self, item):
        print('屬性不存在時(shí)調(diào)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                 # 有實(shí)例則直接返回
        # else:
        #     cls._instance = super().__new__(cls) # 沒有實(shí)例則new一個(gè)并保存
        #     return cls._instance                 # 這個(gè)返回是給是給init，再實(shí)例化一次，也沒有關(guān)系

        if not cls._instance:                         # 這是簡化的寫法，上面注釋的寫法更容易提現(xiàn)判斷思路
            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                    # 將記錄類的實(shí)例對(duì)象的數(shù)據(jù)屬性放在元類中自動(dòng)定義了

    def __call__(cls, *args, **kwargs):                   # 此call會(huì)在類被調(diào)用（即實(shí)例化時(shí)觸發(fā)）
        if cls._instance:      # 判斷類有沒有實(shí)例化對(duì)象
            return cls._instance
        else:        # 沒有實(shí)例化對(duì)象時(shí)，控制類造空對(duì)象并初始化
            obj = cls.__new__(cls, *args, **kwargs)
            obj.__init__(*args, **kwargs)
            cls._instance = obj             # 保存對(duì)象，下一次再實(shí)例化可以直接返回而不用再造對(duì)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