class Attribute:
    __name = None
    __value = None

    def __init__(self, name, value):
        """
        Initializes an attribute.
        """
        self.__name = name
        self.__value = value


class PointSet:
    """
    Useful point set for storing coordinates and attributes. It is
    backed by a set to provide nice convenience functions.
    """
    __points = set()
    __attributes = {}

    def add_point(self, x, y, attrs=[]):
        """
        Adds a point in screen coordinates and an optional attribute to
        the list.

        @param x The x-coordinate.
        @param y The y-coordinate.
        @param attr An optional attribute.
        """

        if attrs != [] and not all(isinstance(x, Attribute) for x in attrs):
            raise ValueError("Attributes in add_point must be an " +
                             "attribute array.")

        point = (x, y)
        self.__points.add(point)
        self.__attributes[point] = attrs


    def remove_point(self, x, y):
        """
        Removes a point and it's attributes from the point set.
        """
        point = (x, y)
        self.__points.discard(point)
        self.__attributes.pop(point)


    def attributes(self, x, y):
        """
        Returns the attribute array for a given point.

        @param x The x-coordinate of the point.
        @param y The y-coordinate of the point.
        """
        return self.__attributes[(x, y)]

    @property
    def points(self):
        """
        Getter for points. Returns a generator for
        looping.
        """
        for point in self.__points:
            yield point