Python 2.2 quick reference

New features in Python 2.2 generalize the process of visiting the elements of a sequence.

These features all affect the way for statements work. In the general case, a for statement looks like this:

for v in S:
    B

where the block B is executed once for each element of the sequence S, with some destination v set to each element of S in turn.

An iterator is a new concept that generalizes the sequence S so that you can use objects other than regular sequence types like lists and tuples. An iterator is an object that knows how to visit a sequence of values.

The way for statements actually work now is that it calls the built-function iter(S) to convert the sequence into an iterator that knows how to visit the elements of S. Calling iter(S) where S is a list or tuple returns an iterator that visits each element of the list or tuple in turn, with index values 0, 1, 2, ....

Any object with the special .__getitem__() method can be used as the sequence in a for statement. Values 0, 1, ... are used for the index, and the for statement terminates when .__getitem__() raises IndexError.

Objects may also have an .__iter__() method, which supersedes the .__getitem__() method in for statements. This method implements the iter() function for the class, and returns an iterator for the instance.

An iterator must be an object with a .next() method that takes no arguments and returns the next element in sequence. This method should raise the special exception StopIteration to signify that there are no more elements.