Python for Everyone: Exploring Data Using Python 3.pdf

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Why should you learn to write programs?

Creativity and motivation

Computer hardware architecture

Understanding programming

Words and sentences

Conversing with Python

Terminology: interpreter and compiler

Writing a program

What is a program?

The building blocks of programs

What could possibly go wrong?

Debugging

The learning journey

Glossary

Exercises

Variables, expressions, and statements

Values and types

Variables

Variable names and keywords

Statements

Operators and operands

Expressions

Order of operations

Modulus operator

String operations

Asking the user for input

Comments

Choosing mnemonic variable names

Debugging

Glossary

Exercises

Conditional execution

Boolean expressions

Logical operators

Conditional execution

Alternative execution

Chained conditionals

Nested conditionals

Catching exceptions using try and except

Short-circuit evaluation of logical expressions

Debugging

Glossary

Exercises

Functions

Function calls

Built-in functions

Type conversion functions

Math functions

Random numbers

Adding new functions

Definitions and uses

Flow of execution

Parameters and arguments

Fruitful functions and void functions

Why functions?

Debugging

Glossary

Exercises

Iteration

Updating variables

The while statement

Infinite loops

Infinite loops and break

Finishing iterations with continue

Definite loops using for

Loop patterns

Counting and summing loops

Maximum and minimum loops

Debugging

Glossary

Exercises

Strings

A string is a sequence

Getting the length of a string using len

Traversal through a string with a loop

String slices

Strings are immutable

Looping and counting

The in operator

String comparison

string methods

Parsing strings

Format operator

Debugging

Glossary

Exercises

Files

Persistence

Opening files

Text files and lines

Reading files

Searching through a file

Letting the user choose the file name

Using try, except, and open

Writing files

Debugging

Glossary

Exercises

Lists

A list is a sequence

Lists are mutable

Traversing a list

List operations

List slices

List methods

Deleting elements

Lists and functions

Lists and strings

Parsing lines

Objects and values

Aliasing

List arguments

Debugging

Glossary

Exercises

Dictionaries

Dictionary as a set of counters

Dictionaries and files

Looping and dictionaries

Advanced text parsing

Debugging

Glossary

Exercises

Tuples

Tuples are immutable

Comparing tuples

Tuple assignment

Dictionaries and tuples

Multiple assignment with dictionaries

The most common words

Using tuples as keys in dictionaries

Sequences: strings, lists, and tuples - Oh My!

Debugging

Glossary

Exercises

Regular expressions

Character matching in regular expressions

Extracting data using regular expressions

Combining searching and extracting

Escape character

Summary

Bonus section for Unix / Linux users

Debugging

Glossary

Exercises

Networked programs

HyperText Transfer Protocol - HTTP

The World's Simplest Web Browser

Retrieving an image over HTTP

Retrieving web pages with urllib

Reading binary files using urllib

Parsing HTML and scraping the web

Parsing HTML using regular expressions

Parsing HTML using BeautifulSoup

Bonus section for Unix / Linux users

Glossary

Exercises

Using Web Services

eXtensible Markup Language - XML

Parsing XML

Looping through nodes

JavaScript Object Notation - JSON

Parsing JSON

Application Programming Interfaces

Google geocoding web service

Security and API usage

Glossary

Exercises

Object-Oriented Programming

Managing Larger Programs

Getting Started

Using Objects

Starting with Programs

Subdividing a Problem - Encapsulation

Our First Python Object

Classes as Types

Object Lifecycle

Many Instances

Inheritance

Summary

Glossary

Using Databases and SQL

What is a database?

Database concepts

Database Browser for SQLite

Creating a database table

Structured Query Language summary

Spidering Twitter using a database

Basic data modeling

Programming with multiple tables

Constraints in database tables

Retrieve and/or insert a record

Storing the friend relationship

Three kinds of keys

Using JOIN to retrieve data

Summary

Debugging

Glossary

Visualizing data

Building a Google map from geocoded data

Visualizing networks and interconnections

Visualizing mail data

Contributions

Contributor List for Python for Everybody

Contributor List for Python for Informatics

Preface for Think Python

The strange history of Think Python

Acknowledgements for Think Python

Contributor List for Think Python

Python for Everybody Exploring Data Using Python 3 Charles R. Severance

Credits Editorial Support: Elliott Hauser, Sue Blumenberg Cover Design: Aimee Andrion Printing History • 2016-Jul-05 First Complete Python 3.0 version • 2015-Dec-20 Initial Python 3.0 rough conversion Copyright Details Copyright ~2009- Charles Severance. This work is licensed under a Creative Commons Attribution-NonCommercial- ShareAlike 3.0 Unported License. This license is available at http://creativecommons.org/licenses/by-nc-sa/3.0/ You can see what the author considers commercial and non-commercial uses of this material as well as license exemptions in the Appendix titled “Copyright Detail”.

iii Preface Remixing an Open Book It is quite natural for academics who are continuously told to “publish or perish” to want to always create something from scratch that is their own fresh creation. This book is an experiment in not starting from scratch, but instead “remixing” the book titled Think Python: How to Think Like a Computer Scientist written by Allen B. Downey, Jeﬀ Elkner, and others. In December of 2009, I was preparing to teach SI502 - Networked Programming at the University of Michigan for the ﬁfth semester in a row and decided it was time to write a Python textbook that focused on exploring data instead of understanding algorithms and abstractions. My goal in SI502 is to teach people lifelong data handling skills using Python. Few of my students were planning to be professional computer programmers. Instead, they planned to be librarians, managers, lawyers, biologists, economists, etc., who happened to want to skillfully use technology in their chosen ﬁeld. I never seemed to ﬁnd the perfect data-oriented Python book for my course, so I set out to write just such a book. Luckily at a faculty meeting three weeks before I was about to start my new book from scratch over the holiday break, Dr. Atul Prakash showed me the Think Python book which he had used to teach his Python course that semester. It is a well-written Computer Science text with a focus on short, direct explanations and ease of learning. The overall book structure has been changed to get to doing data analysis problems as quickly as possible and have a series of running examples and exercises about data analysis from the very beginning. Chapters 2–10 are similar to the Think Python book, but there have been major changes. Number-oriented examples and exercises have been replaced with data- oriented exercises. Topics are presented in the order needed to build increasingly sophisticated data analysis solutions. Some topics like try and except are pulled forward and presented as part of the chapter on conditionals. Functions are given very light treatment until they are needed to handle program complexity rather than introduced as an early lesson in abstraction. Nearly all user-deﬁned functions have been removed from the example code and exercises outside of Chapter 4. The word “recursion”1 does not appear in the book at all. In chapters 1 and 11–16, all of the material is brand new, focusing on real-world uses and simple examples of Python for data analysis including regular expressions for searching and parsing, automating tasks on your computer, retrieving data across the network, scraping web pages for data, object-oriented programming, using web services, parsing XML and JSON data, creating and using databases using Structured Query Language, and visualizing data. The ultimate goal of all of these changes is to shift from a Computer Science to an Informatics focus and to only include topics into a ﬁrst technology class that can be useful even if one chooses not to become a professional programmer. 1Except, of course, for this line.

iv Students who ﬁnd this book interesting and want to further explore should look at Allen B. Downey’s Think Python book. Because there is a lot of overlap be- tween the two books, students will quickly pick up skills in the additional areas of technical programming and algorithmic thinking that are covered in Think Python. And given that the books have a similar writing style, they should be able to move quickly through Think Python with a minimum of eﬀort. As the copyright holder of Think Python, Allen has given me permission to change the book’s license on the material from his book that remains in this book from the GNU Free Documentation License to the more recent Creative Commons Attribu- tion — Share Alike license. This follows a general shift in open documentation licenses moving from the GFDL to the CC-BY-SA (e.g., Wikipedia). Using the CC-BY-SA license maintains the book’s strong copyleft tradition while making it even more straightforward for new authors to reuse this material as they see ﬁt. I feel that this book serves as an example of why open materials are so important to the future of education, and I want to thank Allen B. Downey and Cambridge University Press for their forward-looking decision to make the book available under an open copyright. I hope they are pleased with the results of my eﬀorts and I hope that you, the reader, are pleased with our collective eﬀorts. I would like to thank Allen B. Downey and Lauren Cowles for their help, patience, and guidance in dealing with and resolving the copyright issues around this book. Charles Severance www.dr-chuck.com Ann Arbor, MI, USA September 9, 2013 Charles Severance is a Clinical Associate Professor at the University of Michigan School of Information.

Contents 1 Why should you learn to write programs? 1.1 1.2 1.3 Creativity and motivation . . . . . . . . . . . . . . . . . . . . . . . Computer hardware architecture . . . . . . . . . . . . . . . . . . . Understanding programming . . . . . . . . . . . . . . . . . . . . . 1.4 Words and sentences . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 1.6 Conversing with Python . . . . . . . . . . . . . . . . . . . . . . . . Terminology: interpreter and compiler . . . . . . . . . . . . . . . . 1.7 Writing a program . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 What is a program? . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 3 4 5 6 8 10 10 1.9 The building blocks of programs . . . . . . . . . . . . . . . . . . . . 11 1.10 What could possibly go wrong? . . . . . . . . . . . . . . . . . . . . 1.11 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.12 The learning journey . . . . . . . . . . . . . . . . . . . . . . . . . . 1.13 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.14 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Variables, expressions, and statements 2.1 2.2 2.3 2.4 2.5 2.6 2.7 12 14 15 15 16 19 19 20 22 23 23 24 24 Values and types . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variable names and keywords . . . . . . . . . . . . . . . . . . . . . . 21 Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Operators and operands . . . . . . . . . . . . . . . . . . . . . . . . Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Order of operations . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Modulus operator . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 String operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

vi CONTENTS 2.10 Asking the user for input . . . . . . . . . . . . . . . . . . . . . . . 2.11 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Choosing mnemonic variable names . . . . . . . . . . . . . . . . . 2.13 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.14 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.15 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Conditional execution 25 26 27 28 29 30 31 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Boolean expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Logical operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conditional execution . . . . . . . . . . . . . . . . . . . . . . . . . Alternative execution . . . . . . . . . . . . . . . . . . . . . . . . . Chained conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . Nested conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . Catching exceptions using try and except . . . . . . . . . . . . . . Short-circuit evaluation of logical expressions . . . . . . . . . . . . Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 32 33 34 35 36 38 39 39 40 43 43 43 44 45 46 47 48 49 49 3.10 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.11 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Functions 4.1 4.2 4.3 Function calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Built-in functions . . . . . . . . . . . . . . . . . . . . . . . . . . . Type conversion functions . . . . . . . . . . . . . . . . . . . . . . . 4.4 Math functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 4.6 4.7 4.8 4.9 Random numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . Adding new functions . . . . . . . . . . . . . . . . . . . . . . . . . Deﬁnitions and uses . . . . . . . . . . . . . . . . . . . . . . . . . . Flow of execution . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters and arguments . . . . . . . . . . . . . . . . . . . . . . 4.10 Fruitful functions and void functions . . . . . . . . . . . . . . . . . . 51 4.11 Why functions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.12 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.13 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.14 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 52 53 54

CONTENTS 5 Iteration Updating variables . . . . . . . . . . . . . . . . . . . . . . . . . . . The while statement . . . . . . . . . . . . . . . . . . . . . . . . . Inﬁnite loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . “Inﬁnite loops” and break . . . . . . . . . . . . . . . . . . . . . . Finishing iterations with continue . . . . . . . . . . . . . . . . . . Deﬁnite loops using for . . . . . . . . . . . . . . . . . . . . . . . . Loop patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 5.7.1 Counting and summing loops . . . . . . . . . . . . . . . . . . 61 5.7.2 Maximum and minimum loops . . . . . . . . . . . . . . . . Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Strings vii 57 57 57 58 58 59 60 62 64 64 64 67 67 68 68 69 70 70 74 74 75 76 77 79 79 80 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 A string is a sequence . . . . . . . . . . . . . . . . . . . . . . . . . Getting the length of a string using len . . . . . . . . . . . . . . . Traversal through a string with a loop . . . . . . . . . . . . . . . . String slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strings are immutable . . . . . . . . . . . . . . . . . . . . . . . . . Looping and counting . . . . . . . . . . . . . . . . . . . . . . . . . The in operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 String comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 string methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 6.10 Parsing strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.11 Format operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.12 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.13 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.14 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Files 7.1 7.2 7.3 7.4 Persistence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Opening ﬁles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Text ﬁles and lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Reading ﬁles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

CONTENTS Searching through a ﬁle . . . . . . . . . . . . . . . . . . . . . . . . Letting the user choose the ﬁle name . . . . . . . . . . . . . . . . . Using try, except, and open . . . . . . . . . . . . . . . . . . . . 7.8 Writing ﬁles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.9 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.10 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii 7.5 7.6 7.7 8 Lists 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 83 85 86 87 88 89 89 91 92 92 93 94 94 95 96 97 98 99 A list is a sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Lists are mutable . . . . . . . . . . . . . . . . . . . . . . . . . . . . Traversing a list . . . . . . . . . . . . . . . . . . . . . . . . . . . . List operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deleting elements . . . . . . . . . . . . . . . . . . . . . . . . . . . Lists and functions . . . . . . . . . . . . . . . . . . . . . . . . . . . Lists and strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 Parsing lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.11 Objects and values . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.12 Aliasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 8.13 List arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 8.14 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 8.15 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8.16 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 9 Dictionaries 107 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Dictionary as a set of counters . . . . . . . . . . . . . . . . . . . . 109 Dictionaries and ﬁles . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Looping and dictionaries . . . . . . . . . . . . . . . . . . . . . . . . 111 Advanced text parsing . . . . . . . . . . . . . . . . . . . . . . . . . 113 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

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