When Python, singleton mode, multi-instance mode, one-time initialization encounters together

1. In Python, the singleton pattern is easy to implement, and you can find many answers by flipping through the relevant tutorials on the Web.

such as this:

Class Hello (object):    def __new__ (CLS, *args, **kwargs):        if not ' _instance_one ' in VARs (CLS):            cls._instance _one=object.__new__ (CLS)            return cls._instance_one        return cls._instance_one    def __init__ (self):        Print Selfa=hello () B=hello () #************************** result *******************************<__main__.hello Object at 0x7f8afeaf1150><__main__.hello Object @ 0x7f8afeaf1150>process finished with exit code 0

As you can see, the memory address of the two instances is the same, which means that the two are the same instance.

Note: If we rewrite the __new__ function, we need to inherit the object class.

2. It is important to note that the self and cls._instance_one in the above example are actually the same thing, we can simply do a test:

Class Hello (object):    def __new__ (CLS, *args, **kwargs):        if not ' _instance_one ' in VARs (CLS):            cls._instance _one=object.__new__ (CLS)            print Cls._instance_one            return cls._instance_one        return Cls._instance_one    def __init__ (self):        print Selfa=hello () #************************************** result ******************* <__main__.hello object at 0x7fb31f65e150><__main__.hello object at 0x7fb31f65e150>process Finished with exit code 0

3. If we need to initialize the singleton mode only once, we only need to add a flag:

Class Hello (object):    def __new__ (CLS, *args, **kwargs):        if not ' _instance_one ' in VARs (CLS):            cls._instance _one=object.__new__ (CLS)            cls._instance_one._flag=1            return cls._instance_one        return Cls._instance_one    def __init__ (self):        if Self._flag:            print self            self._flag=0        print "End" A=hello () B=hello () #*** Result*********************************<__main__.hello Object at 0x7f14de3bd150>endend

4. Note that the _flag in the above example is written in a class, outside the class, we can also do an experiment:

Class Hello (object):    _flag=1    def __new__ (CLS, *args, **kwargs):        if not ' _instance_one ' in VARs (CLS):            cls._instance_one=object.__new__ (CLS)            return Cls._instance_one        if not ' _instance_two ' in VARs (CLS):            CLS . _instance_two=object.__new__ (CLS)            return cls._instance_two    def __init__ (self):        print Self._flag        self._flag=0        Print Self._flaga=hello () B=hello () #*************************************result ********* 1010Process finished with exit code 0

You can see that the two are equivalent.

When Python, singleton mode, multi-instance mode, one-time initialization encounters together