Beginning Python - From Novice to Professional

182 CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS
>>> del s[4]
Traceback (most recent call last):
File "", line 1, in ?
AttributeError: ArithmeticSequence instance has no attribute '__delitem__'
Also, the class has no __len__ method because it is of infinite length.
If an illegal type of index is used, a TypeError is raised, and if the index is the correct type
but out of range (negative in the last of the following two examples), an IndexError is raised:
>>> s["four"]
Traceback (most recent call last):
File "", line 1, in ?
File "arithseq.py", line 31, in __getitem__
checkIndex(key)
File "arithseq.py", line 10, in checkIndex
if not isinstance(key, int): raise TypeError
TypeError
>>> s[-42]
Traceback (most recent call last):
File "", line 1, in ?
File "arithseq.py", line 31, in __getitem__
checkIndex(key)
File "arithseq.py", line 11, in checkIndex
if key