How do I implement two decorators in Python both of which would eventually want to call the calling function

Peter Otten __peter__ at
Sat Aug 6 03:59:00 EDT 2011

Devraj wrote:

> Hi all,
> 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.

You typically don't need to do anything special for the above to work:

>>> def v(f):
...     print "decorator v, wrapping", f.__name__, "into a"
...     def a(*args, **kw):
...             print "calling", f.__name__, "from a"
...             return f(*args, **kw)
...     return a
>>> def w(f):
...     print "decorator w, wrapping", f.__name__, "into b"
...     def b(*args, **kw):
...             print "calling", f.__name__, "from b"
...             return f(*args, **kw)
...     return b
>>> @v
... @w
... def f(s):
...     print s
decorator w, wrapping f into b
decorator v, wrapping b into a
>>> f("hello")
calling b from a
calling f from b

The output shows that w wraps f into b, but v then doesn't get to see the 
original f, it wraps b into a. Put another way

def f(): ...

is the same as

def f(): ...

f = v(w(f))

and calling f() now is equivalent to calling a() which may or may not invoke 
b() which may or may not invoke the original f().

Translated into your example:

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

The cache should only be cleared after a successful login, and the original 
post() will only be invoked after a successful login and with a cleared 

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