What are decorated functions?

[Gabriel Genellina]

At Tuesday 22/8/2006 17:19, Wolfgang Draxinger wrote:

>I'm just reading the python language reference and came around
>the term "decorated functions", but I have no idea, for what
>they could be used.

A decorator takes a function/method/callable, "decorates" (modifies) 
it in a certain way, and returns it.
@classmethod and @staticmethod are examples: they take a (normal) 
method, and transform it into another thing.
You can write your own decorators. An example similar to one 
discussed on this list a few days ago: suppose you have a pure 
function taking a long time to compute; you could save the results in 
a dictionary using the arguments as key, and retrieve them later when 
a call is made exactly with the same arguments. This is a known 
pattern ("memoize").

def memoized(function):
     function.cache={}
     def f(*args):
         try: return function.cache[args]
         except KeyError:
             result = function.cache[args] = function(*args)
             return result
     return f

@memoized
def fibo(n):
     time.sleep(2) # a sloooooow function
     if n<2: return 1
     return fibo(n-1)+fibo(n-2)

The original function is fibo() - it works OK but assume it's slow. 
So you "decorate" it with @memoized to improve performance.
(This is just an example, of course - I'm not saying memoizing is the 
right thing to do here, nor this is the best way to do it... just to 
demonstrate what a decorator could be used for)



Gabriel Genellina
Softlab SRL 


	
	
		
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