Solution 19.1.

Exercise 19.5

class PointFactory:

    def __init__(self):
        self._points = {}

    def create(self, x, y):
        if self._points.has_key((x,y)):
            return self._points[(x,y)]
        point = Point(x,y)
        self._points[(x,y)] = point
        return point

    def delete(self, x, y):
        del self._points[(x,y)]