function not being overridden in child class

Question

I have these two classes:

class Shape(object):
    def __init__(self, start_point, *args):
        self.vertices = []
        self.__make_vertices(start_point, *args)

    def __make_vertices(self, start_point, *args):
        print "Not Implemented: __make_vertices"

    def __getitem__(self, *args):
        return self.vertices.__getitem__(*args)

class Cube(Shape):
    def __init__(self, start_point, side_length):
        Shape.__init__(self, start_point, side_length)

    def __make_vertices(self, start_point, side_length):
        append = self.vertices.append
        start_point = Vector(*(start_point))
        i, j, k = side_length*I, side_length*J, side_length*K
        append(start_point)
        append(self.vertices[-1] - k)
        append(self.vertices[-1] - j)
        append(self.vertices[-1] + k)
        append(self.vertices[-1] - i)
        append(self.vertices[-1] - k)
        append(self.vertices[-1] + j)
        append(self.vertices[-1] + k)
        print self.vertices

When I make a new Cube, I expected that the __make_vertices function I defined in the Cube class would be called, but instead I keep getting the message that the Shape classes __make_vertices function prints out. What am I misunderstanding?

Answer 1

You are missing name mangling:

Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped. This mangling is done without regard to the syntactic position of the identifier, as long as it occurs within the definition of a class.

In other words, you should only use attribute/method names beginning with two underscores when you specifically don't want subclasses to override them, but rather to provide their own private version of them. This is rare, so usually you won't need to use double-underscore names at all.