Learning Python, 4th Edition

Chapter 14. Iterations and Comprehensions, Part 1

In the prior chapter we met Python’s two looping statements, while and for. Although they can handle most repetitive tasks programs need to perform, the need to iterate over sequences is so common and pervasive that Python provides additional tools to make it simpler and more efficient. This chapter begins our exploration of these tools. Specifically, it presents the related concepts of Python’s iteration protocol—a method-call model used by the for loop—and fills in some details on list comprehensions—a close cousin to the for loop that applies an expression to items in an iterable.

Because both of these tools are related to both the for loop and functions, we’ll take a two-pass approach to covering them in this book: this chapter introduces the basics in the context of looping tools, serving as something of continuation of the prior chapter, and a later chapter (Chapter 20) revisits them in the context of function-based tools. In this chapter, we’ll also sample additional iteration tools in Python and touch on the new iterators available in Python 3.0.

One note up front: some of the concepts presented in these chapters may seem advanced at first glance. With practice, though, you’ll find that these tools are useful and powerful. Although never strictly required, because they’ve become commonplace in Python code, a basic understanding can also help if you must read programs written by others.

Iterators: A First Look

In the preceding ...