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Iterators: A First Look

In the preceding chapter, I mentioned that the for loop can work on any sequence type in Python, including lists, tuples, and strings, like this:

>>> for x in [1, 2, 3, 4]: print(x ** 2, end=' ')
...
1 4 9 16

>>> for x in (1, 2, 3, 4): print(x ** 3, end=' ')
...
1 8 27 64

>>> for x in 'spam': print(x * 2, end=' ')
...
ss pp aa mm

Actually, the for loop turns out to be even more generic than this—it works on any iterable object. In fact, this is true of all iteration tools that scan objects from left to right in Python, including for loops, the list comprehensions we’ll study in this chapter, in membership tests, the map built-in function, and more.

The concept of “iterable objects” is relatively recent in Python, but it has come to permeate the language’s design. It’s essentially a generalization of the notion of sequences—an object is considered iterable if it is either a physically stored sequence or an object that produces one result at a time in the context of an iteration tool like a for loop. In a sense, iterable objects include both physical sequences and virtual sequences computed on demand.[33]