Learning Python, 5th Edition（PDF 高清全文下载）

Preface

If you’re standing in a bookstore looking for the short story on this book, try this:

• Python is a powerful multiparadigm computer programming language, optimized for programmer productivity, code readability, and software quality.

• This book provides a comprehensive and in-depth introduction to the Python language itself. Its goal is to help you master Python fundamentals before moving on to apply them in your work. Like all its prior editions, this book is designed to serve as a single, all-inclusive learning resource for all Python newcomers, whether they will be using Python 2.X, Python 3.X, or both.

• This edition has been brought up to date with Python releases 3.3 and 2.7, and has been expanded substantially to reflect current practice in the Python world.

This preface describes this book’s goals, scope, and structure in more detail. It’s optional reading, but is designed to provide some orientation before you get started with the book at large.

This Book’s “Ecosystem”

Python is a popular open source programming language used for both standalone programs and scripting applications in a wide variety of domains. It is free, portable, powerful, and is both relatively easy and remarkably fun to use. Programmers from every corner of the software industry have found Python’s focus on developer productivity and software quality to be a strategic advantage in projects both large and small.

Whether you are new to programming or are a professional developer, this book is designed to bring you up to speed on the Python language in ways that more limited approaches cannot. After reading this book, you should know enough about Python to apply it in whatever application domains you choose to explore.

By design, this book is a tutorial that emphasizes the core Python language itself, rather than specific applications of it. As such, this book is intended to serve as the first in a two-volume set:

• Learning Python, this book, teaches Python itself, focusing on language fundamentals that span domains.

• Programming Python, among others, moves on to show what you can do with Python after you’ve learned it.

This division of labor is deliberate. While application goals can vary per reader, the need for useful language fundamentals coverage does not. Applications-focused books such as Programming Python pick up where this book leaves off, using realistically scaled examples to explore Python’s role in common domains such as the Web, GUIs, systems, databases, and text. In addition, the book Python Pocket Reference provides reference materials not included here, and it is designed to supplement this book.

Because of this book’s focus on foundations, though, it is able to present Python language fundamentals with more depth than many programmers see when first learning the language. Its bottom-up approach and self-contained didactic examples are de signed to teach readers the entire language one step at a time.

The core language skills you’ll gain in the process will apply to every Python software system you’ll encounter—be it today’s popular tools such as Django, NumPy, and App Engine, or others that may be a part of both Python’s future and your programming career.

Because it’s based upon a three-day Python training class with quizzes and exercises throughout, this book also serves as a self-paced introduction to the language. Although its format lacks the live interaction of a class, it compensates in the extra depth and flexibility that only a book can provide. Though there are many ways to use this book, linear readers will find it roughly equivalent to a semester-long Python class.

About This Fifth Edition

The prior fourth edition of this book published in 2009 covered Python versions 2.6 and 3.0.1 It addressed the many and sometimes incompatible changes introduced in the Python 3.X line in general. It also introduced a new OOP tutorial, and new chapters on advanced topics such as Unicode text, decorators, and metaclasses, derived from both the live classes I teach and evolution in Python “best practice.”

This fifth edition completed in 2013 is a revision of the prior, updated to cover both Python 3.3 and 2.7, the current latest releases in the 3.X and 2.X lines. It incorporates

1. And 2007’s short-lived third edition covered Python 2.5, and its simpler—and shorter—single-line Python world. Over the years, this book has grown in size and complexity in direct proportion to Python’s own growth. Per Appendix C, Python 3.0 alone introduced 27 additions and 57 changes in the language that found their way into this book, and Python 3.3 continues this trend. Today’s Python programmer faces two incompatible lines, three major paradigms, a plethora of advanced tools, and a blizzard of feature redundancy—most of which do not divide neatly between the 2.X and 3.X lines. That’s not as daunting as it may sound (many tools are variations on a theme), but all are fair game in an inclusive, comprehensive Python text.

all language changes introduced in each line since the prior edition was published, and has been polished throughout to update and sharpen its presentation. Specifically:

• Python 2.X coverage here has been updated to include features such as dictionary and set comprehensions that were formerly for 3.X only, but have been back-ported for use in 2.7.

• Python 3.X coverage has been augmented for new yield and raise syntax; the__pycache__ bytecode model; 3.3 namespace packages; PyDoc’s all-browser mode; Unicode literal and storage changes; and the new Windows launcher shipped with 3.3.

• Assorted new or expanded coverage for JSON, timeit, PyPy, os.popen, generators, recursion, weak references, __mro__, __iter__, super, __slots__, metaclasses, descriptors, random, Sphinx, and more has been added, along with a general increase in 2.X compatibility in both examples and narrative.

This edition also adds a new conclusion as Chapter 41 (on Python’s evolution), two new appendixes (on recent Python changes and the new Windows launcher), and one new chapter (on benchmarking: an expanded version of the former code timing example). See Appendix C for a concise summary of Python changes between the prior edition and this one, as well as links to their coverage in the book. This appendix also summarizes initial differences between 2.X and 3.X in general that were first addressed in the prior edition, though some, such as new-style classes, span versions and simply become mandated in 3.X (more on what the X’s mean in a moment).

because it gives fuller coverage to more advanced language features—which many of us have tried very hard to ignore as optional for the last decade, but which have now grown more common in Python code. As we’ll see, these tools make Python more powerful, but also raise the bar for newcomers, and may shift Python’s scope and definition. Because you might encounter any of these, this book covers them head-on, instead of

pretending they do not exist.

Despite the updates, this edition retains most of the structure and content of the prioredition, and is still designed to be a comprehensive learning resource for both the 2.X and 3.X Python lines. While it is primarily focused on users of Python 3.3 and 2.7—the latest in the 3.X line and the likely last in the 2.X line—its historical perspective also makes it relevant to older Pythons that still see regular use today.

Though it’s impossible to predict the future, this book stresses fundamentals that have been valid for nearly two decades, and will likely apply to future Pythons too. As usual, I’ll be posting Python updates that impact this book at the book’s website described ahead. The “What’s New” documents in Python’s manuals set can also serve to fill in the gaps as Python surely evolves after this book is published.

The Python 2.X and 3.X Lines

Because it bears heavily on this book’s content, I need to say a few more words about the Python 2.X/3.X story up front. When the fourth edition of this book was written in 2009, Python had just become available in two flavors:

• Version 3.0 was the first in the line of an emerging and incompatible mutation of the language known generically as 3.X.

• Version 2.6 retained backward compatibility with the vast body of existing Python code, and was the latest in the line known collectively as 2.X.

While 3.X was largely the same language, it ran almost no code written for prior releases. It:

• Imposed a Unicode model with broad consequences for strings, files, and libraries

• Elevated iterators and generators to a more pervasive role, as part of fuller functional paradigm

• Mandated new-style classes, which merge with types, but grow more powerful and complex

• Changed many fundamental tools and libraries, and replaced or removed others entirely

The mutation of print from statement to function alone, aesthetically sound as it may be, broke nearly every Python program ever written. And strategic potential aside, 3.X’s mandatory Unicode and class models and ubiquitous generators made for a different programming experience.

Python 2.X was still very widely used and was to be supported in parallel with Python 3.X for years to come. The majority of Python code in use was 2.X, and migration to 3.X seemed to be shaping up to be a slow process.

The 2.X/3.X Story Today

As this fifth edition is being written in 2013, Python has moved on to versions 3.3 and 2.7, but this 2.X/3.X story is still largely unchanged. In fact, Python is now a dual-version world, with many users running both 2.X and 3.X according to their software goals and dependencies. And for many newcomers, the choice between 2.X and 3.X remains one of existing software versus the language’s cutting edge. Although many major Python

packages have been ported to 3.X, many others are still 2.X-only today.

To some observers, Python 3.X is now seen as a sandbox for exploring new ideas, while 2.X is viewed as the tried-and-true Python, which doesn’t have all of 3.X’s features but is still more pervasive. Others still see Python 3.X as the future, a view that seems supported by current core developer plans: Python 2.7 will continue to be supported but is to be the last 2.X, while 3.3 is the latest in the 3.X line’s continuing evolution.

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