Learning Python (Mark Lutz) (Z-Library)

Learning Python SIXTH EDITION Powerful Object-Oriented Programming Mark Lutz

Learning Python by Mark Lutz Copyright © 2025 Mark Lutz. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles (http://oreilly.com). For more information, contact our corporate/institutional sales department: 800-998-9938 or corporate@oreilly.com. Acquisitions Editor: Louise Corrigan Development Editor: Sara Hunter Production Editor: Kristen Brown Copyeditor: nSight, Inc. Proofreader: Piper Content Partners Indexer: nSight, Inc.

Interior Designer: David Futato Cover Designer: Karen Montgomery Illustrator: Kate Dullea March 2025: Sixth Edition Revision History for the Sixth Edition 2025-02-25: First Release See http://oreilly.com/catalog/errata.csp?isbn=9781098171308 for release details. The O’Reilly logo is a registered trademark of O’Reilly Media, Inc. Learning Python, the cover image, and related trade dress are trademarks of O’Reilly Media, Inc. The views expressed in this work are those of the author and do not represent the publisher’s views. While the publisher and the author have used good faith efforts to ensure that the information and instructions contained in this work are accurate, the publisher and the author disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of

or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any code samples or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights. 978-1-098-17130-8 [LSI]

[Dedication] To Vera. You are my life.

Preface If you’re browsing a bookstore and trying to make sense of this book, try this: Python is one of the most widely used programming languages in the world. It’s part of nearly every role that computers play in our lives, and its relative ease of use makes it an ideal way to get started with programming. This book is a tutorial that teaches Python language fundamentals in depth. Its content is aimed at Python newcomers of all stripes, applies to every role that Python plays, and is based on decades of feedback from real learners like you. This edition updates this book for a decade of changes in Python and its world. It drops coverage of the now-defunct Python 2.X, explores new tools added to Python through version 3.12, and applies to other Pythons past and future. The rest of this preface provides more background info on this book and its subject. It explains what’s changed since the prior edition, debuts the book’s examples package, and may help you get oriented before jumping into details.

Python By most metrics you’ll find on the web today, Python is now either the most-used programming language on the planet or very near the top of the list. The oft-cited TIOBE popularity index, for example, has ranked Python most popular for several years. As this edition is being written in 2024, it lists Python at #1 and well ahead of its nearest followers, C, C++, and Java. Popularity ranks are prone to change, of course, and rely on usage metrics that are open to debate that we’ll skip here. Based on every signal available, though, the Python revolution has clearly happened. It’s now a ubiquitous, go-to language in web development, scientific programming, systems administration, AI, and nearly everything else computers do today. Thanks to its relative simplicity, Python is also commonly used to introduce newcomers to computer science across the education spectrum. In fact, it’s now fairly safe to say that Python played a pivotal role in changing the world. By spearheading a shift from statically typed compiled languages to dynamically typed scripting languages, Python ushered in changes that were at least as profound as those of the earlier transition from machine language to compiled languages. The scripting-

language shift both enabled tasks formerly impractical, and opened the field to nonprofessional contributors. In the process, it propelled computers to a prominence that would have been unthinkable in decades past. The internet, for example, simply could not be what it is today without tools like Python. For better and worse, Python enables the new. Lofty goals aside, all of this has two tangible implications for this book. First, because this is now a post-revolution Python world, this edition does much less cheerleading than its predecessors. There’s no reason to waste your time promoting a tool that’s already arrived. This edition still summarizes Python’s value proposition in Chapter 1, but the domains and tools that you’re likely to explore after learning the basics here are readily available on the web and change too regularly to cover in a fundamentals book like this in any event. Second, Python’s popularity means that by reading this book, you’ll be adding a valuable skill to your toolset, which will help you in a wide variety of computer-software tasks. Learning Python will both make entire domains accessible to you and enable you to achieve programming goals that might otherwise be difficult or impossible. Python users now enjoy a wealth of prior art ready to be leveraged in a language that accelerates their work.

That said, Python isn’t the only game out there, and you’d also be well served by learning computer science from the ground up—the full stack in developer speak. Studying lower-level languages like C and Java, for example, can still give you a much more complete perspective than scripting languages alone and help you solve complex problems as they arise. Python itself, after all, is just a C program in its most-used flavor. Even so, Python is a great place to start, and enough for many a task. While it’s not without warts and suffers from the thrashing that’s endemic to software today, a multitude of developers still find Python a lot more fun to use than other tools. Java and C++, for example, seem languages designed for middle management: they hobble programmers with training wheels and bureaucratic hurdles that have little to do with your program’s goals. Python, in sharp contrast, remains more ally than obstacle. That viewpoint is naturally subjective, but you’ve come to the right place to do the math on this yourself. This Book This book is a tutorial on the Python language and a classic in its domain. It’s the product of three decades spent using,

promoting, and teaching Python, and dates back to the mid- 1990s, when Python was still at version 1.X, and the web was just something developers mused about over lunch. Although the focus here is firmly on the present, that legacy naturally adds some historical context that will help you understand Python more deeply. Despite what you may have heard, the past matters, especially in knowledge-based fields. Just as importantly, this book has always been based on live- and-in-person feedback from Python beginners struggling to learn Python for the first time. This feedback mostly owes to Python training classes taught over a period of two decades. While these classes have now gone the way of the dodo and Yahoo, this book takes care to retain its learner-inspired material because that’s much—if not most—of its value. As a result, if you’re like most of the thousands of learners whose experiences have been captured here, you’ll probably find that this book works like a self-paced version of the Python training sessions from which it arose. You may sometimes even find that it answers your questions before they are asked because a host of learners before you have had the same queries. This isn’t clairvoyance; it simply reflects the fact that learning resources do best when they listen to learners.

It’s also worth noting up front that this book sometimes critiques Python changes while presenting them. Critical thinking is crucial in engineering domains—especially in one caught up in an arms race that convolutes tools used by millions of people. On some levels, Python remains a constantly morphing sandbox of ideas that too often prioritizes changer hubris over user need, and this book is not shy about calling this out. That said, the main goal here is to educate, not criticize, and opinions are always, well, opinionated. Although views here reflect decades of using and teaching Python, you should always judge the net worth of Python changes for yourself in whatever world you’ve been cast. This Edition This edition completely drops coverage of Python 2.X, the earlier version of the language, and adds new coverage of recent changes in Python 3.X, the newer and incompatible version. When the prior edition was published in 2013, Python 2.X was still widely used and probably even dominant. Because of that, the prior edition had to cover both the established 2.X and the new and upcoming 3.X, which at times made for a twisted tale indeed.

Over a decade later, 2.X has been officially sunsetted, and the Python world has adopted 3.X so fully that 2.X constitutes an unwarranted distraction to today’s Python learners. Hence, after a decades-long tenure, 2.X-specific content has been cut here to make room for new 3.X topics and address book size in general. Formally, this edition has been updated to be current with Python 3.12 and its era, though it also previews 3.13 mods, and its focus on fundamentals makes it generally applicable to both older and newer Python versions. Before the emails start flooding in, this book wants to make clear that it regrets the loss of historical context (and secretly pines for the simpler days of 2.X too). But 3.X is a substantial topic all by itself, without bifurcating the story and increasing the page count for a Python version that is now little used. So go well into that good Python night, 2.X, and long live 3.X. Unless stated otherwise, “Python” in this edition simply means the 3.X line in general and 3.12 and later in particular. In terms of 3.X mods, this edition newly covers f'…' f-string literals, := named-assignment expressions, match statements, type hinting, async coroutines, star-unpacking proliferation, underscore digit separators, __main__.py package files, __getattr__ module hooks, except* exception groups, dictionary-key insertion order, positional-only function

arguments, hash-based bytecode files, and other additions, deprecations, and mutations that have cropped up over the last decade plus. Among these, type hinting and async coroutines are not covered in depth—by design. The former is an optional and academic tool wholly unused by Python itself and at odds with its core principles. The latter is an advanced applications tool and has morphed constantly since its inception. And both quickly head over complexity cliffs that push them out of scope for Python beginners. When needed, supplemental info on such narrow topics is always just a search away on the web. Here, the goal is learning to walk well before trying to run. Among other noteworthy changes this time around: The Unicode content in the advanced part’s Chapter 37 is new and improved because this topic is now an essential in Python 3.X and the world at large. Usage coverage, including the new Appendix A, gives more focus to macOS, Android, Linux, and iOS because not all of this book’s readers use Windows. Most code-file examples now have numbered captions because the extra formality distinguishes them better in the book, and it’s worth the space.

Some redundancy has been trimmed, but not all, because repetition is useful and even important in learning resources. The size of this book was reduced by the prior bullet, rewrites and flow mods, and the net of 2.X cuts and 3.X inserts because it’s less to grok. The size of the print version of this book was further reduced by moving two advanced but optional chapters online (Chapters 38 and 39) because it’s less to lug. Fictitious names in examples are more gender neutral: “Bob” is now an ambiguous “Pat” unless paired with “Sue” as before because it better defuses bias. The Monty Python references have been dropped because they can be confusing and might be divisive, and borrowing personality from media seems cheap. Both first-person voice and personal anecdotes have been globally sacked because you’ve bought this book to learn Python, not an author’s life story. About the last two: Python’s namesake was funny stuff, to be sure, but compulsively aping the work of a nearly all-male comedy group can seem like the secret handshake of an exclusive boys’ club in hindsight. And while an occasional “I” or “my” might add color or credibility, overuse tends to come off as narcissism. Hence, the former 1k “spam” are now symbols

more inclusive, and this book’s three-decade tenure will have to speak for itself. Despite all the mods, this edition remains much more technical novel than reference manual, and meaty enough to be comparable to a full-semester class on Python and programming. It introduces topics and expands them in later chapters as recurring themes, accumulating comprehensive coverage along the way. There are Python quick-reference resources at python.org and a multitude of blogs and videos that promise to teach Python rapidly. This book is for those who know that learning something well requires a bit more. Media Choices As of this writing, this book is destined to be available in three forms: print (i.e., paper), ebook (e.g., PDF, ePub, and Kindle), and online (a.k.a. web). The latter means the publisher’s subscription service, currently branded as the O’Reilly learning platform (f.k.a. Safari). Naturally, each medium has valid uses that vary per reader. For instance, many prefer print for linear reads and electronic media for random searches and code copy/paste.

You’re welcome to use the forms that work best for you, of course, but should carefully weigh the inherent privacy trade- offs of online media. By now, it should be abundantly clear that online anything comes with a potential for covert use and sale of customer information and access, which print and ebook media largely avoid. While monetization schemes vary, online users just might have a legion of salespeople peering over their shoulders as they use products for which they’ve already paid in full. So please be careful out there. Unless you must use an employer’s online subscription, this book suggests vetting your media options wisely and generally recommends its print and ebook forms to protect your privacy whenever possible. Your life really shouldn’t be turned into a revenue stream unless you get reimbursed for it. One last media tip: this book may also be fed by its publisher into generative AI models, the current hot topic in the tech press. Although this may prove useful for looking up isolated facts, it’s not deep learning and isn’t necessarily any more reliable than the least of the gossip it regurgitates. Until the world figures this out, please use wisely.

Updates and Examples As most authors would attest, it’s shockingly easy to miss typos in material you’ve read hundreds of times, and incompatible change is a norm in computer book topics. Hence, this edition, like all its predecessors, expects to be updated regularly after its publication. This book’s supplements, example files, and clarifications and corrections (a.k.a. errata) will all be maintained on the web. Here are the main coordinates for these online resources; as usual, consult your local search engine if these change over time: Author website This site will be used to post general notes and updates related to this text or Python itself—a hedge against future changes and a sort of virtual appendix to this book. Book examples This site will host this book’s examples package, with both individual files to view and save online and a ZIP file of all the examples to download to your device. Publisher website

This site will maintain this edition’s errata list, and chronicle patches applied to the text in reprints. It will also link to other formats, including ebooks. The second of these is home to the examples’ source code. Please see the usage notes there, as well as the examples’ coverage in “Where to Run: Code Folders”. There is no plan to host the examples on GitHub today, because that site’s learning curve is a lot to ask of beginners, and its commercial agendas should be cause for concern. At any of the preceding websites, all error reports and suggestions for this book are welcome, and this feedback is invaluable for book quality. But please keep it fact-based and civil. Posts on the errata list have been mostly constructive, but the list has limited utility, and has been known to attract the usual trolls. Such is life in the age of global conversation. Conventions and Reuse This book’s mechanics will make more sense once you start reading it, but as a reference, this book uses the following typographical conventions: Italic

Used for email addresses, URLs, filenames, pathnames, and emphasizing new terms when they are first introduced Constant width Used for program code, the contents of files and the output from commands, and to designate modules, methods, statements, and system commands Constant width bold Used in code sections to show commands or text that would be typed by the user, and, occasionally, to highlight portions of code Constant width italic Used for replaceables (content you must fill in) and some comments in code sections NOTE This element indicates a tip, suggestion, or general note relating to the nearby text. The icon may make more sense if you imagine a crow sounding an alarm. Three more quick content notes here: first, you’ll find occasional sidebars (delimited by boxes) and footnotes throughout, which are often optional reading but provide