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def __setitem__(self, key, value): """ Change an item in the arithmetic sequence. """ checkIndex(key) self.changed[key] = value # Store the changed value

This implements an arithmetic sequence a sequence of numbers in which each is greater than the previous one by a constant amount. The first value is given by the constructor parameter start (defaulting to zero), while the step between the values is given by step (defaulting to one). You allow the user to change some of the elements by keeping the exceptions to the general rule in a dictionary called changed. If the element hasn t been changed, it is calculated as self.start + key*self.step. Here is an example of how you can use this class: >>> >>> 9 >>> >>> 2 >>> 11 s = ArithmeticSequence(1, 2) s[4] s[4] = 2 s[4] s[5]

Note that I want it to be illegal to delete items, which is why I haven t implemented __del__: >>> del s[4] Traceback (most recent call last): File "<stdin>", 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 (that is, negative in this case), an IndexError is raised: >>> s["four"] Traceback (most recent call last): File "<stdin>", 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 "<stdin>", line 1, in File "arithseq.py", line 31, in __getitem__ checkIndex(key) File "arithseq.py", line 11, in checkIndex if key<0: raise IndexError IndexError The index checking is taken care of by a utility function I ve written for the purpose, checkIndex. One thing that might surprise you about the checkIndex function is the use of isinstance (which you should rarely use because type or class checking goes against the grain of Python s polymorphism). I ve used it because the language reference explicitly states that the index should be an integer (this includes long integers). And complying with standards is one of the (very few) valid reasons for using type checking.

Note You can simulate slicing, too, if you like. When slicing an instance that supports __getitem__,

a slice object is supplied as the key. Slice objects are described in the Python Library Reference (http:// python.org/doc/lib) in Section 2.1, Built-in Functions, under the slice function. Python 2.5 also has the more specialized method called __index__, which allows you to use noninteger limits in your slices. This is mainly useful only if you wish to go beyond the basic sequence protocol, though.

While the four methods of the basic sequence/mapping protocol will get you far, the official language reference also recommends that several other magic and ordinary methods be implemented (see the section Emulating container types in the Python Reference Manual, http://www.python.org/doc/ref/sequence-types.html), including the __iter__ method, which I describe in the section Iterators, later in this chapter. Implementing all these methods (to make your objects fully polymorphically equivalent to lists or dictionaries) is a lot of work and hard to get right. If you want custom behavior in only one of the operations, it makes no sense that you should need to reimplement all of the others. It s just programmer laziness (also called common sense). So what should you do The magic word is inheritance. Why reimplement all of these things when you can inherit them The standard library comes with three ready-to-use implementations of the sequence and mapping protocols (UserList, UserString, and UserDict), and in current versions of Python, you can subclass the built-in types themselves. (Note that this is mainly useful if your class s behavior is close to the default. If you need to reimplement most of the methods, it might be just as easy to write a new class.)