[Python 文章收集] Python Decorator 四種寫法範例 Code

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Preface
Python Decorator 可以以 function 與 class 實作出來，且有無參數的寫法也會因此不一樣，以下記錄了從 Stack Overflow 學習來的 decorator 寫法 {function, class} X {with, without} arguments = 4 種.

Decorator Function

沒有參數的 Function 版本
使用 closure 去處理 function:

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1. def decorateApple(f):  
2.     print "=== Enter decorator with %s" % (f)  
3.     def d_f(*args, **kargs):  
4.         print("=== apple before call")  
5.         result = f(*args, **kargs)  
6.         print("=== apple after call")  
7.         return result  
8.     print "=== Exit decorator"  
9.     return d_f  
10.   
11. @decorateApple  
12. def print_hello(name):  
13.     print("Hello %s." % (name))  
14.   
15. print_hello('John')  

Execution output:

=== Enter decorator with
=== Exit decorator
=== apple before call
Hello John.
=== apple after call

有參數的 Decorator Function
需要多一層的 outter function 來傳入參數:

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1. def decorateApple(*dargs, **dkargs):  
2.     print "=== Enter decorator"  
3.     for a in dargs:  
4.         print "==== Arg=%s" % a  
5.     def outer_d_f(f):  
6.         print "===== Enter outer_d_f"  
7.         def d_f(*args, **kargs):  
8.             print("======= apple before call")  
9.             result = f(*args, **kargs)  
10.             print("======= apple after call")  
11.             return result  
12.         print "===== Exit outer_d_f"  
13.         return d_f  
14.     print "=== Exit decorator"  
15.     return outer_d_f  
16.   
17. @decorateApple('banana')  
18. def print_hello(name):  
19.     print("Hello %s." % (name))  
20.   
21. print_hello('John')  

Execution output:

=== Enter decorator
==== Arg=banana
=== Exit decorator
===== Enter outer_d_f
===== Exit outer_d_f
======= apple before call
Hello John.
======= apple after call

Decorator Class

沒有參數版本的 Decorator Class
Python 會將被修飾的 function 傳入 __init__，這時就將它存入到 decorator 的 data member self.f 。另外再直接覆寫 __call__ ，然後在呼叫 self.f 的前後作額外的修飾動作。這裡的 __call__ 的 signature ，就是跟 被修飾的 function 一樣，所以就加上動態的 *args, 跟 **kargs 就可以完整 delegate了。

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1. #!/usr/bin/env python  
2. class decorateAppleClass(object):  
3.     def __init__(self, f):  
4.         self.f = f  
5.   
6.     def __call__(self, *args, **kargs):  
7.         print("===== apple before call")  
8.         result = self.f(*args, **kargs)  
9.         print("===== apple after call")  
10.         return result  
11.   
12.   
13. @decorateAppleClass  
14. def print_hello4(name):  
15.     print("Hello %s." % (name))  
16.   
17. print_hello4('John')  

Execution output:

===== apple before call
Hello John.
===== apple after call

帶有參數版本的 Decorator Class
當有參數版本的 decorator class 在修飾 function 時，function 就不會像沒有參數版本的 decorator class 一樣，會將 function 傳入 __init__，而是會傳入 decorator 的參數。function 本體則是會在 __call__ 中被傳入，這是一個比較不一樣的地方。

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1. #!/usr/bin/env python  
2. class decorateAppleClass(object):  
3.     def __init__(self, *dargs, **dkw):  
4.         print("===== Enter decorator __init__")  
5.         for a in dargs:  
6.             print("-----> Arg=%s" % (a))  
7.   
8.     def __call__(self, f):  
9.         def d_f(*args, **kargs):  
10.             print("===== apple before call")  
11.             result = f(*args, **kargs)  
12.             print("===== apple after call")  
13.             return result  
14.         return d_f  
15.   
16.   
17. @decorateAppleClass('banana')  
18. def print_hello4(name):  
19.     print("Hello %s." % (name))  
20.   
21. print_hello4('John')  

Execution output:

===== Enter decorator __init__
-----> Arg=banana
===== apple before call
Hello John.
===== apple after call