A summary of __init__, __new__, __call__ in Python _python

1.__new__ (CLS, *args, **kwargs) is invoked when an object is created, returning an instance of the current object; Note: The first argument here is the CLS, the class itself.
2.__init__ (self, *args, **kwargs) is invoked after the object is created, with some initialization of the instance of the current object, no return value, that is, after the call to __new__, based on the returned instance; Note that the first argument here is self, the object itself. Notice the difference with new "
3.__call__ (self, *args, **kwargs) If the class implements this method, it is equivalent to using this type of object as a function, equivalent to overloading the bracket operator

See the specific example:

Copy Code code as follows:

Class O (object):

def __init__ (self, *args, **kwargs):
Print "Init"
Super (O, self). __init__ (*args, **kwargs)

Def __new__ (CLS, *args, **kwargs):
Print "New", CLS
Return Super (O, CLS). __new__ (CLS, *args, **kwargs)

def __call__ (self, *args, **kwargs):
Print "Call"

oo = O ()
Print "________"
OO ()

The print out is:

Copy Code code as follows:

New
Init
________
Call

For example: Python Singleton (single case mode) implementation, then we are not just overloading some __new__ methods on it

Copy Code code as follows:

Class Singleton1 (object):
"" "Overload New Method" ""
Def __new__ (CLS, *args, **kwargs):
If not ' _instance ' in VARs (CLS):
Cls._instance = Super (Singleton1, CLS). __new__ (CLS, *args, **kwargs)
Return cls._instance

Can you overload the __init__ method? Obviously not, because __init__ before the __new__ method, this time has generated an object, no way to implement a single case pattern