python - Difference between decorator classes and decorator functions

Question

I guess that's how they are called, but I will give examples just in case.

Decorator class:

class decorator(object):
    def __init__(self, func):
        self.func = func

    def __call__(self, *args, **kwargs):
        print 'something'
        self.func(*args, **kwargs)

Decorator function:

def decorator(func):
    def wrapper(*args, **kwargs):
        print 'something'
        return func(*args, **kwargs)
    return wrapper

Is using one or the other just a matter of taste? Is there any practical difference?

Answer 1

If you can write a function to implement your decorator you should prefer it. But not all decorators can easily be written as a function - for example when you want to store some internal state.

class counted(object):
    """ counts how often a function is called """
    def __init__(self, func):
        self.func = func
        self.counter = 0

    def __call__(self, *args, **kwargs):
        self.counter += 1
        return self.func(*args, **kwargs)


@counted
def something():
    pass

something()
print something.counter

I've seen people (including myself) go through ridiculous efforts to write decorators only with functions. I still have no idea why, the overhead of a class is usually totally negligible.