第7页 | Learning Python | 阅读 ‧ 电子书库

Coverage for Both 3.0 and 2.6

To address this dichotomy and to meet the needs of all potential readers, this edition of this book has been updated to cover both Python 3.0 and Python 2.6 (and later releases in the 3.X and 2.X lines). It’s intended for programmers using Python 2, programmers using Python 3, and programmers stuck somewhere between the two.

That is, you can use this book to learn either Python line. Although the focus here is on 3.0 primarily, 2.6 differences and tools are also noted along the way for programmers using older code. While the two versions are largely the same, they diverge in some important ways, and I’ll point these out along the way.

For instance, I’ll use 3.0 print calls in most examples, but will describe the 2.6 print statement, too, so you can make sense of earlier code. I’ll also freely introduce new features, such as the nonlocal statement in 3.0 and the string format method in 2.6 and 3.0, and will point out when such extensions are not present in older Pythons.

If you are learning Python for the first time and don’t need to use any legacy code, I encourage you to begin with Python 3.0; it cleans up some longstanding warts in the language, while retaining all the original core ideas and adding some nice new tools. Many popular Python libraries and tools will likely be available for Python 3.0 by the time you read these words, especially given the file I/O performance improvements expected in the upcoming 3.1 release. If you are using a system based on Python 2.X, however, you’ll find that this book addresses your concerns, too, and will help you migrate to 3.0 in the future.

By proxy, this edition addresses other Python version 2 and 3 releases as well, though some older version 2.X code may not be able to run all the examples here. Although class decorators are available in both Python 2.6 and 3.0, for example, you cannot use them in an older Python 2.X that did not yet have this feature. See Tables 1 and 2 later in this Preface for summaries of 2.6 and 3.0 changes.

Note

Shortly before going to press, this book was also augmented with notes about prominent extensions in the upcoming Python 3.1 release—comma separators and automatic field numbering in string format method calls, multiple context manager syntax in with statements, new methods for numbers, and so on. Because Python 3.1 was targeted primarily at optimization, this book applies directly to this new release as well. In fact, because Python 3.1 supersedes 3.0, and because the latest Python is usually the best Python to fetch and use anyhow, in this book the term “Python 3.0” generally refers to the language variations introduced by Python 3.0 but that are present in the entire 3.X line.