[ Python 常見問題 ] Python object field being passed to decorator as arguments?

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Question 
How do I pass a object field to a decorator on a class method as an argument? What I want to do is something like: 
- test3836.py 

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1. #!/usr/bin/env python  
2. import functools  
3.   
4. def check_authorization(*args, **kw):  
5.     print "Doing authorization"  
6.     for a in args:  
7.         print "\t%s" % (a)  
8.     def wrapper(f):  
9.         def dfun(*args, **kw):  
10.             result = f(*args, **kw)  
11.             return result  
12.         return dfun  
13.     return wrapper  
14.   
15. class Client(object):  
16.     def __init__(self, url):  
17.         self.url = url  
18.   
19.     @check_authorization('some_attr', "self.url")  
20.     def get(self):  
21.         print "Get"  
22.         self.do_work()  
23.   
24.     def do_work(self):  
25.         print "Doing work"  
26.   
27.   
28. client = Client("http://test.org")  
29. client.get()  

Execution output: 

Doing authorization
some_attr
self.url
Get
Doing work

How-To 
Instead of passing in the instance attribute at class definition time, check it at runtime: 

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1. #!/usr/bin/env python  
2. import functools  
3.   
4. def check_authorization(*args, **kw):  
5.     for a in args:  
6.         print "Decorator argu=%s" % (a)  
7.     def wrapper(f):  
8.         def dfun(*args, **kw):  
9.             print "Doing authorization on URL=%s" % (args[0].url)  # args[0] -> self  
10.             result = f(*args, **kw)  
11.             return result  
12.         return dfun  
13.     return wrapper  
14.   
15. class Client(object):  
16.     def __init__(self, url):  
17.         self.url = url  
18.   
19.     @check_authorization('some_attr')  
20.     def get(self):  
21.         print "Get"  
22.         self.do_work()  
23.   
24.     def do_work(self):  
25.         print "Doing work"  
26.   
27.   
28. client = Client("http://test.org")  
29. client.get()  

Execution output: 

Decorator argu=some_attr
Doing authorization on URL=http://test.org
Get
Doing work

The decorator intercepts the method arguments; the first argument is the instance, so it reads the attribute off of that. You can pass in the attribute name as a string to the decorator and use getattr if you don't want to hardcode the attribute name: 

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1. #!/usr/bin/env python  
2. import functools  
3.   
4. def check_authorization(*dargs, **dkw):  
5.     for a in dargs:  
6.         print "Decorator argu=%s" % (a)  
7.     def wrapper(f):  
8.         def dfun(*args, **kw):  
9.             print "Doing authorization on URL=%s" % (getattr(args[0], dargs[0]))  
10.             result = f(*args, **kw)  
11.             return result  
12.         return dfun  
13.     return wrapper  
14.   
15. class Client(object):  
16.     def __init__(self, url):  
17.         self.url = url  
18.   
19.     @check_authorization('url')  
20.     def get(self):  
21.         print "Get"  
22.         self.do_work()  
23.   
24.     def do_work(self):  
25.         print "Doing work"  
26.   
27.   
28. client = Client("http://test.org")  
29. client.get()  

Execution output: 

Decorator argu=url
Doing authorization on URL=http://test.org
Get
Doing work

Supplement 
* [Python 學習筆記] 進階議題 : 修飾器 (函式修飾器) 
* [ Python 文章收集 ] Decorators I: Introduction to Python Decorators