Implementing advanced python decorators

Python decorators may save a lot of time, through the ease of applying them with other functions/methods. In practice python decorators are something between the decorator pattern and macros. An example decorator that enables running a function only in a single process at a time is presented here.

Today I'll present how to implement a decorator that enables a few ways to handle the embedded function. Such a mechanism is used in the celery project.

A quick celery task usage example:

@celery.task
def add(x, y):
    return x + y

res = add.delay(5,6) #asynchronous task
res = add(5,6) #synchronous task 

This is quite convenient, you may have such decorators applied to functions and still dynamically choose how the decorator should act in a specific context. An implementation of a similar decorator is presented below (with some test code):

class SomeDecorator():

    def __call__(self, func, some_flag=False):

        def f(*args, **kwargs):
            if some_flag:
                print "Some flag is set"
            print "Some decorator routines"
            func(*args,**kwargs)

        setattr(f, "with_flag", lambda *args, **kwargs: \
                self.__call__(func, some_flag=True)(*args,**kwargs)) 

        return f


@SomeDecorator()
def test(a, b): 
    print a, b


if __name__ == '__main__':
    print "*****"
    test(5, b = "test")
    print "*****"
    test.with_flag(7, b="test2")

We have to be aware that after a decorator is used, each code referring to test will instead refer to f (function object type). To achieve our goal we have to provide f  capabilities of performing different type of decorator activities. In this example code I have added a function attribute (line 11) which executes the default __call__ method, and still passes arguments/keyword arguments to the embedded function.

The standard output of running the presented code:

*****
Some decorator routines
5 test
*****
Some flag is set
Some decorator routines
7 test2

Feel free to use / modify / upgrade this code to achieve desired results.

Cheers!
KR