Python Tutorial Release 3 7 0 Guido van Rossum and the Python development team September 02 2018 Python Software Foundation Email docs python org CONTENTS 1 Whetting Your Appetite 2 Using the Python Interpreter 2 1 2 2 The Interpreter and Its Environment Invoking the Interpreter 3 An Informal Introduction to Python 3 1 Using Python as a Calculator First Steps Towards Programming 3 2 4 More Control Flow Tools 4 1 if Statements 4 2 for Statements 4 3 The range Function break and continue Statements and else Clauses on Loops 4 4 4 5 pass Statements 4 6 De ning Functions 4 7 More on De ning Functions 4 8 Intermezzo Coding Style 5 Data Structures 5 1 More on Lists 5 2 The del statement 5 3 Tuples and Sequences 5 4 Sets 5 5 Dictionaries 5 6 Looping Techniques 5 7 More on Conditions 5 8 Comparing Sequences and Other Types 6 Modules 6 1 More on Modules 6 2 Standard Modules 6 3 The dir Function 6 4 Packages 7 Input and Output 7 1 7 2 Reading and Writing Files Fancier Output Formatting 8 Errors and Exceptions 3 5 5 6 9 9 16 19 19 19 20 21 22 22 24 29 31 31 35 36 37 38 39 40 40 43 44 46 47 48 53 53 57 61 i Syntax Errors 8 1 8 2 Exceptions 8 3 Handling Exceptions 8 4 Raising Exceptions 8 5 User de ned Exceptions 8 6 De ning Clean up Actions 8 7 Prede ned Clean up Actions 9 Classes 9 1 A Word About Names and Objects 9 2 Python Scopes and Namespaces 9 3 A First Look at Classes 9 4 Random Remarks 9 5 Inheritance 9 6 Private Variables 9 7 Odds and Ends 9 8 9 9 Generators 9 10 Generator Expressions Iterators 10 Brief Tour of the Standard Library 10 1 Operating System Interface 10 2 File Wildcards 10 3 Command Line Arguments 10 4 Error Output Redirection and Program Termination 10 5 String Pattern Matching 10 6 Mathematics 10 7 Internet Access 10 8 Dates and Times 10 9 Data Compression 10 10 Performance Measurement 10 11 Quality Control 10 12 Batteries Included 11 Brief Tour of the Standard Library Part II 11 1 Output Formatting 11 2 Templating 11 3 Working with Binary Data Record Layouts 11 4 Multi threading 11 5 Logging 11 6 Weak References 11 7 Tools for Working with Lists 11 8 Decimal Floating Point Arithmetic 12 Virtual Environments and Packages 12 1 Introduction 12 2 Creating Virtual Environments 12 3 Managing Packages with pip 61 61 62 64 65 66 66 69 69 69 72 75 77 78 79 79 80 81 83 83 83 84 84 84 84 85 85 86 86 87 87 89 89 90 91 91 92 93 93 94 97 97 97 98 101 14 Interactive Input Editing and History Substitution 103 14 1 Tab Completion and History Editing 103 14 2 Alternatives to the Interactive Interpreter 103 13 What Now ii 15 Floating Point Arithmetic Issues and Limitations 105 108 15 1 Representation Error 16 Appendix 111 16 1 Interactive Mode 111 B About these documents 127 B 1 Contributors to the Python Documentation 127 C History and License 129 C 1 History of the software 129 C 2 Terms and conditions for accessing or otherwise using Python 130 C 3 Licenses and Acknowledgements for Incorporated Software 133 A Glossary D Copyright Index 113 145 147 iii iv Python Tutorial Release 3 7 0 Python is an easy to learn powerful programming language It has e cient high level data structures and a simple but e ective approach to object oriented programming Python s elegant syntax and dynamic typing together with its interpreted nature make it an ideal language for scripting and rapid application development in many areas on most platforms The Python interpreter and the extensive standard library are freely available in source or binary form for all major platforms from the Python Web site https www python org and may be freely distributed The same site also contains distributions of and pointers to many free third party Python modules programs and tools and additional documentation The Python interpreter is easily extended with new functions and data types implemented in C or C or other languages callable from C Python is also suitable as an extension language for customizable applications This tutorial introduces the reader informally to the basic concepts and features of the Python language and system It helps to have a Python interpreter handy for hands on experience but all examples are self contained so the tutorial can be read o line as well For a description of standard objects and modules see library index reference index gives a more formal de nition of the language To write extensions in C or C read extending index and c api index There are also several books covering Python in depth This tutorial does not attempt to be comprehensive and cover every single feature or even every commonly used feature Instead it introduces many of Python s most noteworthy features and will give you a good idea of the language s avor and style After reading it you will be able to read and write Python modules and programs and you will be ready to learn more about the various Python library modules described in library index The Glossary is also worth going through CONTENTS 1 Python Tutorial Release 3 7 0 2 CONTENTS CHAPTER ONE WHETTING YOUR APPETITE If you do much work on computers eventually you nd that there s some task you d like to automate For example you may wish to perform a search and replace over a large number of text les or rename and rearrange a bunch of photo les in a complicated way Perhaps you d like to write a small custom database or a specialized GUI application or a simple game If you re a professional software developer you may have to work with several C C Java libraries but nd the usual write compile test re compile cycle is too slow Perhaps you re writing a test suite for such a library and nd writing the testing code a tedious task Or maybe you ve written a program that could use an extension language and you don t want to design and implement a whole new language for your application Python is just the language for you You could write a Unix shell script or Windows batch les for some of these tasks but shell scripts are best at moving around les and changing text data not well suited for GUI applications or games You could write a C C Java program but it can take a lot of development time to get even a rst draft program Python is simpler to use available on Windows Mac OS X and Unix operating systems and will help you get the job done more quickly Python is simple to use but it is a real programming language o ering much more structure and support for large programs than shell …