print statement so you can make sense of earlier codeand will often use portable printing techniques that run on both lines 'll also freely introduce new featuressuch as the nonlocal statement in and the string format method available as of and and will point out when such extensions are not present in older pythons by proxythis edition addresses other python version and releases as wellthough some older version code may not be able to run all the examples here although class decorators are available as of both python and for exampleyou cannot use them in an older python that did not yet have this feature againsee the change tables in appendix for summaries of recent and changes which python should useversion choice may be mandated by your organizationbut if you're new to python and learning on your ownyou may be wondering which version to install the answer here depends on your goals here are few suggestions on the choice when to choose xnew featuresevolution if you are learning python for the first time and don' need to use any existing codei encourage you to begin with python it cleans up some longstanding warts in the language and trims some dated cruftwhile retaining all the original core ideas and adding some nice new tools for example ' seamless unicode model and broader use of generators and functional techniques are seen by many users as assets many popular python libraries and tools are already available for python xor will be by the time you read these wordsespecially given the continual improvements in the line all new language evolution occurs in onlywhich adds features and keeps python relevantbut also makes language definition constantly moving target-- tradeoff inherent on the leading edge when to choose xexisting codestability if you'll be using system based on python xthe line may not be an option for you today howeveryou'll find that this book addresses your concernstooand will help if you migrate to in the future you'll also find that you're in large company every group taught in was using onlyand still regularly see useful python software in -only form moreoverunlike is no longer being changed--which is either an asset or liabilitydepending on whom you ask there' nothing wrong with using and writing codebut you may wish to keep tabs on and its ongoing evolution as you do python' future remains to be writtenand is largely up to its usersincluding you when to choose bothversion-neutral code probably the best news here is that python' fundamentals are the same in both its lines-- and differ in ways that many users will find minorand this book is designed to help you learn both in factas long as you understand their differencesit' often straightforward to write version-neutral code that runs on both xxxviii preface