[ Python 常見問題 ] what is difference between __init__ and __call__?

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Question 
I want to know what is difference between __init__ and __call__ methods? For example : 

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1. class test:  
2.   def __init__(self):  
3.     self.a = 10  
4.   
5.   def __call__(self):   
6.     b = 20  

Answer: 
The first one __init__ is used to initialize newly created object, and receives arguments used to do that: 

Called after the instance has been created (by __new__()), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an __init__() method, the derived class’s __init__() method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: BaseClass.__init__(self, [args...]).

Sample code: 

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1. class foo:  
2.     def __init__(self, a, b, c):  
3.         print "foo's __init__ ongoing!"  
4.         self.a=a  
5.         self.b=b  
6.         self.c=c  
7.     def __str__(self):  
8.         return "(a={0};b={1};c={2})".format(self.a, self.b, self.c)  
9.   
10. x = foo(1, 2, 3) # __init__  
11. print "x={0}".format(x)  

Execution result: 

foo's __init__ ongoing!
x=(a=1;b=2;c=3)

The second implements function __call__: 

Called when the instance is “called” as a function; if this method is defined, x(arg1, arg2, ...) is a shorthand for x.__call__(arg1, arg2, ...).

Sample Code: 

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1. class foo:  
2.     def __init__(self, a, b, c):  
3.         print "foo's __init__ ongoing!"  
4.         self.a=a  
5.         self.b=b  
6.         self.c=c  
7.           
8.     def __str__(self):  
9.         return "(a={0};b={1};c={2})".format(self.a, self.b, self.c)  
10.       
11.     def __call__(self, a, b, c):  
12.         print"Object(a={0};b={1};c={2})".format(a, b, c)  
13.   
14. x = foo(1, 2, 3) # __init__  
15. print "x={0}".format(x)  
16. x(3,4,5) # __call__  
17. x.__call__(4, 5, 6)  

Execution result: 

foo's __init__ ongoing!
x=(a=1;b=2;c=3)
Object(a=3;b=4;c=5)
Object(a=4;b=5;c=6)

Supplement 
* Python __new__ 、__init__、 __call__