以xy坐标为例，定义一个类：

 1 class Coordinate(object):
 2     def __init__(self, x, y):
 3         self.x = x
 4         self.y = y
 5     def distance(self, other):
 6         x_diff_sq = (self.x - other.x)**2
 7         y_diff_sq = (self.y - other.y)**2
 8         return (x_diff_sq + y_diff_sq)**0.5
 9     def __str__(self):
10         return "<" + str(self.x) + "," + str(self.y) + ">"

 __init__ ：构造函数，定义类中的属性；参数self代表对象本身，例如 c = Coordinate(3, 4) ，c作为self参数被输入构造函数中

 __str__ ：print self

Other special operators: https://docs.python.org/3/reference/datamodel.html#basic-customization

E.g.,  __add__ +,  __sub__ -,  __eq__ ==,  __lt__ <,  __len__ len(), ...