class my_decorator(object):

    def __init__(self, f):
        print("inside my_decorator.__init__()")
        f() # Prove that function definition has completed

    def __call__(self):
        print("inside my_decorator.__call__()")

@my_decorator
def aFunction():
    print("inside aFunction()")

inside my_decorator.__init__()
inside aFunction()

print("Finished decorating aFunction()")

Finished decorating aFunction()

aFunction()

inside my_decorator.__call__()