Python Class Based Decorator with parameters that can decorate a method or a function

You don’t need to mess around with descriptors. It’s enough to create a wrapper function inside the __call__() method and return it. Standard Python functions can always act as either a method or a function, depending on context:

class MyDecorator(object):
    def __init__(self, argument):
        self.arg = argument

    def __call__(self, fn):
        @functools.wraps(fn)
        def decorated(*args, **kwargs):
            print "In my decorator before call, with arg %s" % self.arg
            result = fn(*args, **kwargs)
            print "In my decorator after call, with arg %s" % self.arg
            return result
        return decorated

A bit of explanation about what’s going on when this decorator is used like this:

@MyDecorator("some other func!")
def some_other_function():
    print "in some other function!"

The first line creates an instance of MyDecorator and passes "some other func!" as an argument to __init__(). Let’s call this instance my_decorator. Next, the undecorated function object — let’s call it bare_func — is created and passed to the decorator instance, so my_decorator(bare_func) is executed. This will invoke MyDecorator.__call__(), which will create and return a wrapper function. Finally this wrapper function is assigned to the name some_other_function.