new實現(xiàn)單例
new至少要有一個參數(shù)cls,代表要實例化的類央渣,new方法負責創(chuàng)建一個實例對象厌杜,在對象被創(chuàng)建的時候調(diào)用該方法它是一個類方法,new方法負責創(chuàng)建一個實例對象,在對象被創(chuàng)建的時候調(diào)用該方法它是一個類方法畔勤。new方法在返回一個實例之后笋庄,會自動的調(diào)用init方法脉顿,對實例進行初始化。如果new方法不返回值义矛,或者返回的不是實例妨托,那么它就不會自動的去調(diào)用init方法缸榛。此參數(shù)在實例化時由Python解釋器自動提供
class Singleton(object):
def __new__(cls, *args, **kwargs):
print cls,'xxxx'
if not hasattr(cls,"_instance"):
cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
return cls._instance
元類實現(xiàn)單例
class Singleton(type):
def __init__(self, *args, **kwargs):
print "__init__"
self.__instance = None
super(Singleton,self).__init__(*args, **kwargs)
def __call__(self, *args, **kwargs):
print "__call__"
if self.__instance is None:
self.__instance = super(Singleton,self).__call__(*args, **kwargs)
return self.__instance
class Foo(object):
__metaclass__ = Singleton #在代碼執(zhí)行到這里的時候,元類中的__new__方法和__init__方法其實已經(jīng)被執(zhí)行了兰伤,而不是在Foo實例化的時候執(zhí)行内颗。且僅會執(zhí)行一次。只有再需要對原類做變動時才需要寫new方法
為了更好理解上面單例敦腔,注意下面兩種寫法
class UpperAttrMetaClass(type):
def __new__(upperattr_metaclass, future_class_name, future_class_parents, future_class_attr):
print 'xxxxxxx'
uppercase_attr = {}
for name, value in future_class_attr.items():
if not name.startswith('__'):
uppercase_attr[name.upper()] = value
else:
uppercase_attr[name] = value
return type.__new__(upperattr_metaclass,future_class_name, future_class_parents, uppercase_attr)#返回一個對象均澳,但同時這個對象是一個類
def __init__(upperattr_metaclass,*args, **kwargs):
print 'init'
upperattr_metaclass.__instance = None
super(UpperAttrMetaClass,upperattr_metaclass).__init__(*args, **kwargs)
def myprint(self, param):
print param,'xxxxx'
def __call__(upperattr_metaclass,*args, **kwargs):
print 'call'
# return upperattr_metaclass
if upperattr_metaclass.__instance is None:
upperattr_metaclass.__instance = super(UpperAttrMetaClass,upperattr_metaclass ).__call__(*args, **kwargs)
return upperattr_metaclass.__instance
下面兩種寫法 ,
class Test(object):
__metaclass__ = UpperAttrMetaClass
def __call__(self):
print 'test call'
def __new__(cls, *args, **kwargs):
print 'test new'
return super(Test, cls).__new__(cls,*args, **kwargs)
def __init__(self):
print 'test init'
def myprint(self, param):
print param
aaaa = 22222
class Test(object):
def __call__(self):
print 'test call'
def myprint(self, param):
print param
aaaa = 22222
Test = UpperAttrMetaClass('Test', (object,), {'myprint':Test().myprint, '__call__':Test().__call__, 'aaaa':Test().aaaa})
這里我們重寫了call方法,一開始不理解
super(UpperAttrMetaClass,upperattr_metaclass ).__call__(*args, **kwargs)
為什么能得到實例符衔,如果我們不重寫call方法找前,Test()將得到實例對象,那么重寫的話判族,重寫的格式就是這樣躺盛,在Test類中重寫new,init方法這時被調(diào)用。
t = Test()
t.MYPRINT('xxx')
t2 = Test()
print t
print t2.AAAA
print Test()