Introducing Python Modern Computing in Simple Packages (Bill Lubanovic) (z-library.sk, 1lib.sk, z-lib.sk)

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Introducing Python SECOND EDITION Modern Computing in Simple Packages Bill Lubanovic

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Introducing Python by Bill Lubanovic Copyright © 2020 Bill Lubanovic. 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: Tyler Ortman Development Editor: Corbin Collins Production Editor: Christopher Faucher Copyeditor: Jasmine Kwityn Proofreader: Octal Publishing, LLC Indexer: WordCo Indexing Services, Inc. Interior Designer: David Futato Cover Designer: Karen Montgomery Illustrator: Rebecca Demarest November 2019: Second Edition Revision History for the Second Edition

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2019-11-06: First Release See http://oreilly.com/catalog/errata.csp?isbn=9781492051367 for release details. The O’Reilly logo is a registered trademark of O’Reilly Media, Inc. Introducing 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. 978-1-492-05136-7 [LSI]

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Dedication With love to Mary, Tom & Roxie, and Karin & Erik

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Preface As the title promises, this book will introduce you to one of the world’s most popular programming languages: Python. It’s aimed at beginning programmers as well as more experienced programmers who want to add Python to the languages they already know. In most cases, it’s easier to learn a computer language than a human language. There’s less ambiguity and fewer exceptions to keep in your head. Python is one of the most consistent and clear computer languages. It balances ease of learning, ease of use, and expressive power. Computer languages are made of data (like nouns in spoken languages) and instructions or code (like verbs). You need both. In alternating chapters, you’ll be introduced to Python’s basic code and data structures, learn how to combine them, and build up to more advanced ones. The programs that you read and write will get longer and more complex. Using a woodworking analogy, we’ll start with a hammer, nails, and scraps of wood. Over the first half of this book, we’ll introduce more specialized components, up to the equivalents of lathes and other power tools. You’ll not only learn the language, but also what to do with it. We’ll begin with the Python language and its “batteries included” standard library, but I’ll also show you how to find, download, install, and use some good third- party packages. My emphasis is on whatever I’ve actually found useful in more than 10 years of production Python development, rather than fringe topics or complex hacks. Although this is an introduction, some advanced topics are included because I want to expose them to you. Areas like databases and the web are still covered, but technology changes fast. A Python programmer might now be expected to know something about cloud computing, machine learning, or event streaming. You’ll find something here on all of these.

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Python has some special features that work better than adapting styles from other languages that you may know. For example, using for and iterators is a more direct way of making a loop than manually incrementing some counter variable. When you’re learning something new, it’s hard to tell which terms are specific instead of colloquial, and which concepts are actually important. In other words, “Is this on the test?” I’ll highlight terms and ideas that have specific meaning or importance in Python, but not too many at once. Real Python code is included early and often. NOTE I’ll include a note such as this when something might be confusing, or if there’s a more appropriate Pythonic way to do it. Python isn’t perfect. I’ll show you things that seem odd or that should be avoided—and offer alternatives you can use, instead. Now and then, my opinions on some subjects (such as object inheritance, or MVC and REST designs for the web) may vary a bit from the common wisdom. See what you think. Audience This book is for anybody interested in learning one of the world’s most popular computing languages, regardless of whether you have previously learned any programming. Changes in the Second Edition What’s changed since the first edition? About a hundred more pages, including cat pictures.

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Twice the chapters, each shorter now. An early chapter devoted to data types, variables, and names. New standard Python features like f-strings. New or improved third-party packages. New code examples throughout. An appendix on basic hardware and software, for new programmers. An appendix on asyncio, for not-so-new programmers. “New stack” coverage: containers, clouds, data science, and machine learning. Hints on getting a job programming in Python. What hasn’t changed? Examples using bad poetry and ducks. These are evergreen. Outline Part I (Chapters 1–11) explains Python’s basics. You should read these chapters in order. I work up from the simplest data and code structures, combining them on the way into more detailed and realistic programs. Part II (Chapters 12–22) shows how Python is used in specific application areas such as the web, databases, networks, and so on; read these chapters in any order you like. Here’s a brief preview of the chapters and appendixes, including some of the terms that you’ll run into there: Chapter 1, A Taste of Py Computer programs are not that different from directions that you see every day. Some little Python programs give you a glimpse of the language’s looks, capabilities, and uses in the real world. You’ll see how

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to run a Python program within its interactive interpreter (or shell), or from a text file saved on your computer. Chapter 2, Data: Types, Values, Variables, and Names Computer languages mix data and instructions. Different types of data are stored and treated differently by the computer. They may allow their values to be changed (mutable) or not (immutable). In a Python program, data can be literal (numbers like 78, text strings like "waffle") or represented by named variables. Python treats variables like names, which is different from many other languages and has some important consequences. Chapter 3, Numbers This chapter shows Python’s simplest data types: booleans, integers, and floating-point numbers. You’ll also learn the basic math operations. The examples use Python’s interactive interpreter like a calculator. Chapter 4, Choose with if We’ll bounce between Python’s nouns (data types) and verbs (program structures) for a few chapters. Python code normally runs a line at a time, from the start to the end of a program. The if code structure lets you run different lines of code, depending on some data comparison. Chapter 5, Text Strings Back to nouns, and the world of text strings. Learn how to create, combine, change, retrieve, and print strings. Chapter 6, Loop with while and for Verbs again, and two ways to make a loop: for and while. You’ll be introduced to a core Python concept: iterators. Chapter 7, Tuples and Lists It’s time for the first of Python’s higher-level built-in data structures: lists and tuples. These are sequences of values, like LEGO for

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building much more complex data structures. Step through them with iterators, and build lists quickly with comprehensions. Chapter 8, Dictionaries and Sets Dictionaries (aka dicts) and sets let you save data by their values rather than their position. This turns out to be very handy and will be among your favorite Python features. Chapter 9, Functions Weave the data and code structures of the previous chapters to compare, choose, or repeat. Package code in functions and handle errors with exceptions. Chapter 10, Oh Oh: Objects and Classes The word object is a bit fuzzy, but important in many computer languages, including Python. If you’ve done object-oriented programming in other languages, Python is a bit more relaxed. This chapter explains how to use objects and classes, and when it’s better to use alternatives. Chapter 11, Modules, Packages, and Goodies This chapter demonstrates how to scale out to larger code structures: modules, packages, and programs. You’ll see where to put code and data, how to get data in and out, handle options, tour the Python Standard Library, and take a glance at what lies beyond. Chapter 12, Wrangle and Mangle Data Learn to manage (or mangle) data like a pro. This chapter is all about text and binary data, joy with Unicode characters, and regex text searching. It also introduces the data types bytes and bytearray, counterparts of strings that contain raw binary values instead of text characters. Chapter 13, Calendars and Clocks

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Dates and times can be messy to handle. This chapter shows common problems and useful solutions. Chapter 14, Files and Directories Basic data storage uses files and directories. This chapter shows you how to create and use them. Chapter 15, Data in Time: Processes and Concurrency This is the first hard-core system chapter. Its theme is data in time— how to use programs, processes, and threads to do more things at a time (concurrency). Python’s recent async additions are mentioned, with details in Appendix C. Chapter 16, Data in a Box: Persistent Storage Data can be stored and retrieved with basic flat files and directories within filesystems. They gain some structure with common text formats such as CSV, JSON, and XML. As data get larger and more complex, they need the services of databases—traditional relational ones, and some newer NoSQL data stores. Chapter 17, Data in Space: Networks Send your code and data through space in networks with services, protocols, and APIs. Examples range from low-level TCP sockets, to messaging libraries and queuing systems, to cloud deployment. Chapter 18, The Web, Untangled The web gets its own chapter—clients, servers, APIs, and frameworks. You’ll crawl and scrape websites, and then build real websites with request parameters and templates. Chapter 19, Be a Pythonista This chapter contains tips for Python developers—installation with pip and virtualenv, using IDEs, testing, debugging, logging, source control, and documentation. It also helps you to find and install useful

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third-party packages, package your own code for reuse, and learn where to get more information. Chapter 20, Py Art People are doing cool things with Python in the arts: graphics, music, animation, and games. Chapter 21, Py at Work Python has specific applications for business: data visualization (plots, graphs, and maps), security, and regulation. Chapter 22, Py Sci In the past few years, Python has emerged as a top language for science: math and statistics, physical science, bioscience, and medicine. Data science and machine learning are notable strengths. This chapter touches on NumPy, SciPy, and Pandas. Appendix A, Hardware and Software for Beginning Programmers If you’re fairly new to programming, this describes how hardware and software actually work. It introduces some terms that you’ll keep running into. Appendix B, Install Python 3 If you don’t already have Python 3 on your computer, this appendix shows you how to install it, whether you’re running Windows, macOS, Linux, or some other variant of Unix. Appendix C, Something Completely Different: Async Python has been adding asynchronous features in different releases, and they’re not easy to understand. I mention them as they come up in various chapters, but save a detailed discussion for this appendix. Appendix D, Answers to Exercises

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This has the answers to the end-of-chapter exercises. Don’t peek here until you’ve tried the exercises yourself, or you might be turned into a newt. Appendix E, Cheat Sheets This appendix contains cheat sheets to use as a quick reference. Python Versions Computer languages change over time as developers add features and fix mistakes. The examples in this book were written and tested while running Python version 3.7. Version 3.7 was the most current as this book was being edited, and I’ll talk about its notable additions. Version 3.8 is scheduled for general release in late 2019, and I’ll include a few things to expect from it. If you want to know what was added to Python and when, try the What’s New in Python page. It’s a technical reference; a bit heavy when you’re just starting with Python, but may be useful in the future if you ever have to get programs to work on computers with different Python versions. Conventions Used in This Book The following typographical conventions are used in this book: Italic Indicates new terms, URLs, email addresses, filenames, and file extensions. Constant width Used for program listings, as well as within paragraphs to refer to program elements such as variables, functions, and data types. Constant width bold Shows commands or other text that should be typed literally by the user.

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Constant width italic Shows text that should be replaced with user-supplied values or by values determined by context. NOTE This icon signifies a tip, suggestion, or general note. WARNING This icon indicates a warning or caution. Using Code Examples The substantial code examples and exercises in this book are available online for you to download. This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Introducing Python by Bill Lubanovic (O’Reilly). Copyright 2020 Bill Lubanovic, 978- 1-492-05136-7.” If you feel your use of code examples falls outside fair use or the permission given here, feel free to contact us at permissions@oreilly.com.

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O’Reilly Online Learning NOTE For over 40 years, O’Reilly Media has provided technology and business training, knowledge, and insight to help companies succeed. Our unique network of experts and innovators share their knowledge and expertise through books, articles, conferences, and our online learning platform. O’Reilly’s online learning platform gives you on-demand access to live training courses, in-depth learning paths, interactive coding environments, and a vast collection of text and video from O’Reilly and 200+ other publishers. For more information, please visit http://oreilly.com. How to Contact Us Please address comments and questions concerning this book to the publisher: O’Reilly Media, Inc. 1005 Gravenstein Highway North Sebastopol, CA 95472 800-998-9938 (in the United States or Canada) 707-829-0515 (international or local) 707-829-0104 (fax) We have a web page for this book, where we list errata, examples, and any additional information. You can access this page at https://oreil.ly/introducing-python-2e.

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To comment or ask technical questions about this book, send an email to bookquestions@oreilly.com. For more information about our books, courses, conferences, and news, see our website at http://www.oreilly.com. Find us on Facebook: http://facebook.com/oreilly Follow us on Twitter: http://twitter.com/oreillymedia Watch us on YouTube: http://www.youtube.com/oreillymedia Acknowledgments My sincere thanks to the reviewers and readers who helped make this better: Corbin Collins, Charles Givre, Nathan Stocks, Dave George, and Mike James

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Part I. Python Basics

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Chapter 1. A Taste of Py Only ugly languages become popular. Python is the one exception. —Don Knuth Mysteries Let’s begin with two mini-mysteries and their solutions. What do you think the following two lines mean? (Row 1): (RS) K18,ssk,k1,turn work. (Row 2): (WS) Sl 1 pwise,p5,p2tog,p1,turn. It looks technical, like some kind of computer program. Actually, it’s a knitting pattern; specifically, a fragment describing how to turn the heel of a sock, like the one in Figure 1-1. Figure 1-1. Knitted socks

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This makes as much sense to me as a Sudoku puzzle does to one of my cats, but my wife understands it perfectly. If you’re a knitter, you do, too. Let’s try another mysterious text, found on an index card. You’ll figure out its purpose right away, although you might not know its final product: 1/2 c. butter or margarine 1/2 c. cream 2 1/2 c. flour 1 t. salt 1 T. sugar 4 c. riced potatoes (cold) Be sure all ingredients are cold before adding flour. Mix all ingredients. Knead thoroughly. Form into 20 balls. Store cold until the next step. For each ball: Spread flour on cloth. Roll ball into a circle with a grooved rolling pin. Fry on griddle until brown spots appear. Turn over and fry other side. Even if you don’t cook, you probably recognized that it’s a recipe : a list of food ingredients followed by directions for preparation. But what does it make? It’s lefse, a Norwegian delicacy that resembles a tortilla (Figure 1-2). Slather on some butter and jam or whatever you like, roll it up, and enjoy. 1

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Figure 1-2. Lefse The knitting pattern and the recipe share some features: A regular vocabulary of words, abbreviations, and symbols. Some might be familiar, others mystifying. Rules about what can be said, and where—syntax. A sequence of operations to be performed in order. Sometimes, a repetition of some operations (a loop), such as the method for frying each piece of lefse. Sometimes, a reference to another sequence of operations (in computer terms, a function). In the recipe, you might need to refer to another recipe for ricing potatoes. Assumed knowledge about the context. The recipe assumes you that know what water is and how to boil it. The knitting pattern