[ Python 常見問題 ] Decorating Python class methods - how do I pass the instance to the decorator?

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Question
I have the following code. I'm decorating the foo() method in bar, using the dec_check class as a decorator:

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1. class dec_check(object):  
2.   
3.   def __init__(self, f):  
4.     self.func = f  
5.   
6.   def __call__(self):  
7.     print 'In dec_check.__init__()'  
8.     self.func()  
9.   
10. class bar(object):  
11.   
12.   @dec_check  
13.   def foo(self):  
14.     print 'In bar.foo()'  
15.   
16. b = bar()  
17. b.foo()  

When executing this I was hoping to see:

In dec_check.__init__()
In bar.foo()

But I'm getting "TypeError: foo() takes exactly 1 argument (0 given)" as .foo(), being an object method, takes self as an argument. I'm guessing problem is that the instance of bar doesn't actually exist when I'm executing the decorator code.

HowTo
You need to make the decorator into a descriptor -- either by ensuring its (meta)class has a __get__ method, or, way simpler, by using a decorator function instead of a decorator class (since functions are already descriptors). E.g.:

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1. def dec_check(f):  
2.   def deco(*args, **kargs):  
3.     print(f'In deco: *args={args}; **kargs={kargs}')  
4.     f(*args, **kargs)  
5.   return deco  
6.   
7. class bar(object):  
8.   def __init__(self, age):  
9.     self.age = age  
10.   
11.   @dec_check  
12.   def foo(self, name):  
13.     print(f'In bar.foo: Hello {name}, my age is {self.age}')  
14.   
15. b = bar(36)  
16. b.foo('John')  

Execution output:

In deco: *args=(<__main__.bar object at 0x7fa4e67b9880>, 'John'); **kargs={}
In bar.foo: Hello John, my age is 36