基本序列和映射规则

<>

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