What are decorators?

Colin J. Williams cjw at sympatico.ca
Mon Aug 9 21:57:59 CEST 2004



gohaku wrote:
> Hi everyone,
> The discussion on Python Decorators and "@" has piqued my interest on this
> "feature?" in programming languages such as Python and Java.
> can anybody what is the point in using Decorators?
> 
The term decorator is a bit misleading.  It does not decorate or adorn a 
function, class or method declaration, the declaration is transformed.

There are cases where one might wish to change the behaviour of a 
function to, for example ensure that the arguments being passed in
are of a certain class or that the object returned has given type.


> The examples I have seen written in Python of this "Not Yet Implemented" 
> feature
> are confusing to say the least and perplexes me as to its usefulness.
> 
> Thanks in advance.
> -gohaku
> 
In a posting yerterday, Dan bishop wrote:

 >
 > If I understand correctly, they'd be useful for anything where you'd
 > now use the syntax
 >
 > function = decorator(function)
 >
 > In addition to @staticmethod, you could have decorators for
 >
 > (1) Memoization.  Makes repeated function evaluation more efficient
 > without having to rewrite the function.
 >
 > class memoize(object):
 >    def __init__(self, func):
 >       self.__func = func
 >       self.__results = {}
 >    def __call__(self, *args):
 >       if args not in self.__results:
 >          self.__results[args] = self.__func(*args)
 >       return self.__results[args]
 >
 > def fibonacci(n):
 >    @memoize
 >    if n in (0, 1):
 >       return n
 >    return fibonacci(n - 1) + fibonacci(n - 2)
 >
 > (2) Debugging uses, like:
 >
 > class printreturns(object):
 >    "Behaves like f but prints its return values."
 >    def __init__(self, f):
 >       self.__f = f
 >    def __call__(self, *args):
 >       result = self.__f(*args)
 >       if debug:
 >          print 'f%r = %r' % (args, result)
 >       return
 > def somefunc(x, y):
 >    @printreturns
 >    ...

I found it helpful, I hope that you do.

Colin W.




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