What is wrong this wrapper (decorator)?

[fl]

Hi,

I follow a tutorial to learn decorator:

http://simeonfranklin.com/blog/2012/jul/1/python-decorators-in-12-steps/

I use Enthought Canopy to run the following code.
It is really strange that the wrapper does not take effect. 
In fact, I go back to the basic way (not with @):

wrapper(sub(two, one))
Out[38]: <function checker>


When I use the non-wrapper mode, it has Coord print out.
If I use wrapper, it has nothing to print out.
Due to no debug mode help, I don't see anything wrong yet.


(add(two, one))
                                          # nothing print out
(sub(two, three))
Out[43]: Coord:-- {'y': 300, 'x': 400}    # correct is here


Anyone can help?
Thanks,



............
class Coordinate(object):
    def __init__(self, y, x):
        self.y = y
        self.x = x
    def __repr__(self):
        return "Coord:-- " + str(self.__dict__)

def add(a, b):
    return Coordinate(a.x + b.x, a.y + b.y)

def sub(a, b):
    return Coordinate(a.x - b.x, a.y - b.y)

def wrapper(func):
    def checker(a, b): # 1
        if a.x < 0 or a.y < 0:
            a = Coordinate(a.x if a.x > 0 else 0, a.y if a.y > 0 else 0)
            if b.x < 0 or b.y < 0:
                b = Coordinate(b.x if b.x > 0 else 0, b.y if b.y > 0 else 0)
            ret = func(a, b)
            if ret.x < 0 or ret.y < 0:
                ret = Coordinate(ret.x if ret.x > 0 else 0, ret.y if ret.y > 0 else 0)
            return ret
    return checker
    
one = Coordinate(100, 200)
two = Coordinate(300, 200)
three = Coordinate(-100, -100)

add = wrapper(add)
#sub = wrapper(sub)