Question

I am trying to simply my Web application handlers, by using Python decorators.

Essentially I want to use decorators to abstract code that checks for authenticated sessions and the other that checks to see if the cache provider (Memcache in this instance) has a suitable response.

Consider this method definition with the decorators:

@auth.login_required
@cache.clear
def post(self, facility_type_id = None):

auth.login_required checks to see if the user is logged in, otherwise returns an appropriate error message, or executes the original function.

cache.clear would check to to see if the cache has a particular key and drop that, before it executes the calling method.

Both auth.login_required and cache.clear would want to eventually execute the calling method (post).

From what I’ve read both, doing what I am doing now would execute the calling method (post) twice.

My question, how do I chain decorators that end up executing the calling method, but ensure that it’s only called once.

Appreciate any pointers and thanks for your time.

Answer 1

Each successive decorator receives the previously wrapped function, so the function itself only gets called once at the end of the chain. Here’s a simple example:

def dec1(f):
    def wrapped():
        print 'dec1'
        return f()
    return wrapped

def dec2(f):
    def wrapped():
        print 'dec2'
        return f()
    return wrapped

@dec2
@dec1
def spam(): print 'spam'

>>> spam()
dec2
dec1
spam