基本序列和映射规则

<<Python基础编程>>

def checkIndex(key):  if not isinstance(key, (int, long)):    raise TypeError  if key<0:    raise IndexError    class ArithmeticSequence(object):  """docstring for ArithmeticSequence"""  def __init__(self, start=0, step=1):    super(ArithmeticSequence, self).__init__()    self.start = start    self.step = step    self.changed = {}  def __getitem__(self, key):    checkIndex(key)    try:      return self.changed[key]    except KeyError, e:      return self.start + self.step*key  def __setitem__(self, key, value):    checkIndex(key)    self.changed[key] = value