>>> def checkIndex(key):
...     if not isinstance(key,(int,long)):raise TypeError
...     if key<0:raise IndexError
...
>>> class ArithneticSequence:
...     def __init__(self,start=0,step=1):
...             self.start=start
...             self.step=step
...             self.changed={}
...     def __getitem__(self,key):
...             checkIndex(key)
...             try:return self.changed[key]
...             except KeyError:
...                     return self.start+key*self.step
...     def __setitem__(self,key,value):
...             checkIndex(key)
...             self.changed[key]=value
...
>>> s=ArithneticSequence(1,2)
>>> s[4]
9
>>> s[4]=2
>>> s[4]=2
>>> s[4]
2
>>> s[5]
11
>>> del s[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: ArithneticSequence instance has no attribute '__delitem__'
>>> s["four"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in __getitem__
  File "<stdin>", line 2, in checkIndex
TypeError
>>> s[-42]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in __getitem__
  File "<stdin>", line 3, in checkIndex
IndexError