pygame学习教程（四）屏幕显示多个按钮

前一篇
说明我写这个系列是为了给初学者展示一些思路和技巧，很多代码不是最优的。如果有朋友对构架有不同看法，欢迎指正。
这里，继续上个例子展示一些python的技巧。
首先，我们修改Jbutton()类。需要涉及很多的向量计算，这里引入
DVerctor。需要将这个文件考到执行文件相同的目录，import。
DVerctor.py ，可以运行测试一下

################## http://www.pygame.org/wiki/2DVectorClass ##################
import operator
import math

class Vec2d(object):
    """2d vector class, supports vector and scalar operators,
       and also provides a bunch of high level functions
       """
    __slots__ = ['x', 'y']

    def __init__(self, x_or_pair, y = None):
        if y == None:
            self.x = x_or_pair[0]
            self.y = x_or_pair[1]
        else:
            self.x = x_or_pair
            self.y = y

    def __len__(self):
        return 2

    def __getitem__(self, key):
        if key == 0:
            return self.x
        elif key == 1:
            return self.y
        else:
            raise IndexError("Invalid subscript "+str(key)+" to Vec2d")

    def __setitem__(self, key, value):
        if key == 0:
            self.x = value
        elif key == 1:
            self.y = value
        else:
            raise IndexError("Invalid subscript "+str(key)+" to Vec2d")

    # String representaion (for debugging)
    def __repr__(self):
        return 'Vec2d(%s, %s)' % (self.x, self.y)

    # Comparison
    def __eq__(self, other):
        if hasattr(other, "__getitem__") and len(other) == 2:
            return self.x == other[0] and self.y == other[1]
        else:
            return False

    def __ne__(self, other):
        if hasattr(other, "__getitem__") and len(other) == 2:
            return self.x != other[0] or self.y != other[1]
        else:
            return True

    def __nonzero__(self):
        return bool(self.x or self.y)

    # Generic operator handlers
    def _o2(self, other, f):
        "Any two-operator operation where the left operand is a Vec2d"
        if isinstance(other, Vec2d):
            return Vec2d(f(self.x, other.x),
                         f(self.y, other.y))
        elif (hasattr(other, "__getitem__")):
            return Vec2d(f(self.x, other[0]),
                         f(self.y, other[1]))
        else:
            return Vec2d(f(self.x, other),
                         f(self.y, other))

    def _r_o2(self, other, f):
        "Any two-operator operation where the right operand is a Vec2d"
        if (hasattr(other, "__getitem__")):
            return Vec2d(f(other[0], self.x),
                         f(other[1], self.y))
        else:
            return Vec2d(f(other, self.x),
                         f(other, self.y))

    def _io(self, other, f):
        "inplace operator"
        if (hasattr(other, "__getitem__")):
            self.x = f(self.x, other[0])
            self.y = f(self.y, other[1])
        else:
            self.x = f(self.x, other)
            self.y = f(self.y, other)
        return self

    # Addition
    def __add__(self, other):
        if isinstance(other, Vec2d):
            return Vec2d(self.x + other.x, self.y + other.y)
        elif hasattr(other, "__getitem__"):
            return Vec2d(self.x + other[0], self.y + other[1])
        else:
            return Vec2d(self.x + other, self.y + other)
    __radd__ = __add__

    def __iadd__(self, other):
        if isinstance(other, Vec2d):
            self.x += other.x
            self.y += other.y
        elif hasattr(other, "__getitem__"):
            self.x += other[0]
            self.y += other[1]
        else:
            self.x += other
            self.y += other
        return self

    # Subtraction
    def __sub__(self, other):
        if isinstance(other, Vec2d):
            return Vec2d(self.x - other.x, self.y - other.y)
        elif (hasattr(other, "__getitem__")):
            return Vec2d(self.x - other[0], self.y - other[1])
        else:
            return Vec2d(self.x - other, self.y - other)
    def __rsub__(self, other):
        if isinstance(other, Vec2d):
            return Vec2d(