☯話不多提,單例的用處大家都知道.☯

上單例

方式一:

✿經典的重寫__new__方法實現單例✿

class Singleton(object):

    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, "_instance"):
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return cls._instance

方式二:

✿使用元類type

class Singleton(type):
    def __init__(cls, *args, **kwargs):
        cls.__instance = None
        super(Singleton, cls).__init__(*args, **kwargs)

    def __call__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = super(Singleton, cls).__call__(*args, **kwargs)
        return
 
 cls.__instance


class UseClass(metaclass=Singleton):  # metaclass= 為 python3的寫法
    # __metaclass__ = Singleton  # __metaclass__ 為 python2的寫法
    pass

方式三:

✿裝飾器方式實現單例✿

def singleton(cls, *args, **kwargs):
    instances = {}
    def _singleton():
        if cls not in instances:
            instances[cls] = cls(*args, **kwargs)
        return
 
 instances[cls]

    return _singleton


@singleton
class UseClass(object):
    pass

✿附: 以上三種的校驗.✿

if __name__ == '__main__':
    s1 = UseClass()
    s2 = UseClass()
    print(id(s1))  # 140365945542080
    print(id(s2))  # 140365945542080
    print(s1 == s2)  # True
    print(s1 is s2)  # True

方式四:

✿匯入建立好的物件✿

# a.py
class UseClass(object):  # 在a.py中建立好要成為單例的物件.
    pass
uc = UseClass()

# b.py
from a import uc  # 匯入已經a中已經建立好的物件
print(id(uc))

# c.py
from a import uc  # 匯入已經a中已經建立好的物件
print(id(uc))

(這個看看玩就行啦.)擴充套件實現???what??黑人問號???:

✿兩個物件使用同一個__dict__(目前沒發現有什麼特別的用處…)✿

class Singleton(object):
    _state = {}

    def __new__(cls, *args, **kwargs):
        obj = super(Singleton, cls).__new__(cls, *args, **kwargs)
        obj.__dict__ = obj._state
        return obj


class UseClass(Singleton):
    pass


uc1 = UseClass()
uc2 = UseClass()
print(id(uc1))  # 140520618571536
print(id(uc2))  # 140520618595384
print(id(uc1) == id(uc2))  # False

print(uc1.__dict__)  # {}
print(uc2.__dict__)  # {}
print(id(uc1.__dict__) == id(uc2.__dict__))  # True

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