python的單例模式實現: 重寫類的__new__方法即可 參照官網的解釋: 3.4.1. Basic customization object.__new__(cls[, ...]) Called to create a new instance of class cls. __new__() is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of __new__() should be the new object instance (usually an instance of cls). Typical implementations create a new instance of the class by invoking the superclass’s __new__() method using super(currentclass, cls).__new__(cls[, ...]) with appropriate arguments and then modifying the newly-created instance as necessary before returning it. If __new__() returns an instance of cls, then the new instance’s __init__() method will be invoked like __init__(self[,...]), where self is the new instance and the remaining arguments are the same as were passed to __new__(). If __new__() does not return an instance of cls, then the new instance’s __init__() method will not be invoked. __new__() is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation. class db(object): __instance = None def __init__(self): self.host = '10.0.0.1' self.port = '3306' self.user = 'root' self.password = '123456' def __new__(cls,*args,**kwargs): if cls.__instance is None: cls.__instance = super(db,cls).__new__(cls,*args,**kwargs) return cls.__instance In [2]: a=db() In [3]: b=db() In [4]: c=db() In [5]: a Out[5]: <__main__.db at 0x1066fd390> In [6]: b Out[6]: <__main__.db at 0x1066fd390> In [7]: c Out[7]: <__main__.db at 0x1066fd390> In [8]: a.host Out[8]: '10.0.0.1' In [9]: b.port Out[9]: '3306' In [10]: c.user Out[10]: 'root' In [11]: c.password='uiso' In [12]: b.password Out[12]: 'uiso' In [13]: a.password Out[13]: 'uiso'
可以看到, 初始化&實例化所使用的都是同一個對象,用來長連接的保持會非常好. 但是這種寫法不是線程安全的。
在多線程下,我們需要導入使用threading的特性,使用鎖來保證線程安全
Lock = threading.Lock()
將__new__方法重寫爲:
def __new__(cls, *args, **kwargs): if not cls.__instance: try: Lock.acquire() if not cls.__instance: cls.__instance = super(db, cls).__new__(cls, *args, **kwargs) finally: Lock.release() return cls.__instance
double check來保證線程的安全。