Re: Interesting decorator use.

[Steven Bethard]

Tom Willis wrote:
Question on decorators in general. Can you parameterize those?
If I wanted to something and after the function call for example, I
would expect something like this would work.
def prepostdecorator(function,pre,post):
    def wrapper(*args,**kwargs):
        pre()
        result =  function(*args,**kwargs)
        post()
        return result
    return wrapper
def dopre():
    print "call pre"
def dopost():
    print "call post"
@prepostdecorator(pre,post)
def sayhello(Name):
    print "Hey %s, nice to meet you" % Name
#sayhello = prepostdecorator(sayhello,dopre,dopost)
if __name__=="__main__":
    sayhello("Dude")
but I get ...
TypeError: prepostdecorator() takes exactly 3 arguments (2 given)
You get this TypeError for the same reason that I get the following 
TypeError:

py> def prepostdecorator(function, pre, post):
...     pass
...
py> prepostdecorator(1, 2)
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
TypeError: prepostdecorator() takes exactly 3 arguments (2 given)
The expression after @ is a _normal Python expression_.  So since you 
couldn't call prepostdecorator with 2 arguments in any other situation, 
you still can't.

If you want to call prepostdecorator with 2 arguments, you need to write 
it this way.  A few options:

(1) Use nested functions:
py> def prepostdecorator(pre,post):
...     def decorator(function):
...         def wrapper(*args,**kwargs):
...             pre()
...             result =  function(*args,**kwargs)
...             post()
...             return result
...         return wrapper
...     return decorator
...
py> @prepostdecorator(dopre, dopost)
... def sayhello(name):
...     print "Hey %s, nice to meet you" % name
...
py> sayhello('Tom')
call pre
Hey Tom, nice to meet you
call post
(2) Use functional.partial (PEP 309[1])
py> def prepostdecorator(pre, post, function):
...     def wrapper(*args,**kwargs):
...         pre()
...         result =  function(*args,**kwargs)
...         post()
...         return result
...     return wrapper
...
py> @partial(prepostdecorator, dopre, dopost)
... def sayhello(name):
...     print "Hey %s, nice to meet you" % name
...
py> sayhello('Tom')
call pre
Hey Tom, nice to meet you
call post
(3) Use a class:
py> class prepostdecorator(object):
...     def __init__(self, pre, post):
...         self.pre, self.post = pre, post
...     def __call__(self, function):
...         def wrapper(*args,**kwargs):
...             self.pre()
...             result = self.function(*args,**kwargs)
...             self.post()
...             return result
...         return wrapper
py> @prepostdecorator(dopre, dopost)
... def sayhello(name):
...     print "Hey %s, nice to meet you" % name
...
py> sayhello('Tom')
call pre
Hey Tom, nice to meet you
call post
Note that in all of the above cases, the result of evaluating the 
expression after the @ is a callable object that takes _exactly one_ 
argument, the function to be decorated.

HTH,
STeVe
[1] http://www.python.org/peps/pep-0309.html
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