>>> 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