[Chicago] FW: Playing with generics

[RCRamsdell@gldd.com]

I have a function that works on a pair of x, y coordinates.  I wanted to
be able to pass either a 2-tuple, or x and y.  Remembering Ian's
presentation, this sounded like a great case for generics!  

So I produced listing 1.  Wow, lots of lines there, and some
duplication.  Plus I have to add a dependency (to the dispatch module).

I had a similar case in another module from the same project, so I tried
it in-line (Listing 2).  Much shorter, particularly when you consider
that I am checking for an additional case, that of no argument passed
in.

Is this just a case where the requirement is too simple to warrant the
overhead?  Yes, but: if I later wanted to add support for (x,y,z)
coords, where betaad might have a quite different calculation, then the
dispatch would probably pay off.


# Listing 1:
@dispatch.generic()
    def canvasCoords(self, *args):
        """Stub function for canvasCoords"""
    @canvasCoords.when("len(args)==2")
    def canvasCoords(self, x, y):
        """
        Calculate the coordinates on the canvas of the given dredge
coordinates
        x, y are coordinates in the dredging scheme, center of rotation
is (0,0)
        """
        return x/self.scalefactor+self.xoffset,
y/self.scalefactor+self.yoffset
    @canvasCoords.when("len(args)==1")
    def canvasCoords(self, xy):
        """
        Calculate the coordinates on the canvas of the given dredge
coordinates
        (x, y) are coordinates in the dredging scheme, center of
rotation is (0,0)
        """
        return xy[0]/self.scalefactor+self.xoffset,
xy[1]/self.scalefactor+self.yoffset

# Listing 2: 
def betaad(self, *args):
        """betaa - The angle of the line through the anchor in degrees
            the args are either a 2-tuple with the anchor coords, or
anchor
            x and y
        """
        if len(args)==0:
            xa, ya = self.Xa, self.Ya
        elif len(args) == 1 and len(args[0])==2:
            xa, ya = args[0]
        elif len(args) == 2:
            xa, ya = args
        else:
            raise TypeError, "HyAnchor.betaad - Wrong number of
arguments, expects a pair or 2-tuple"
        betaa = atan(xa/ya)
        return betaa*180/pi

Robert C. Ramsdell III
Production Engineering Manager
Great Lakes Dredge & Dock Company
2122 York Road
Oakbrook, IL 60523
rcramsdell at gldd.com
Phone: (630) 574-3463
Mobile: (630) 805-1776 
Fax: (630) 574-2909