phenomenal growth in popularity here' the damage so far 've now written python books ( of thisand of two others)which have together sold some , units by my data 've also been teaching python for over decade and halfhave taught some python training sessions in the europecanadaand mexicoand have met roughly , students along the way besides propelling me toward frequent flyer utopiathese classes helped me refine this text and my other python books teaching honed the booksand vice versawith the net result that my books closely parallel what happens in my classesand can serve as viable alternative to them as for python itselfin recent years it has grown to become one of the top to most widely used programming languages in the world (depending on which source you cite and when you cite itbecause we'll be exploring python' status in the first of this booki'll defer the rest of this story until then python thanks because teaching teaches teachers to teachthis book owes much to my live classes ' like to thank all the students who have participated in my courses during the last years along with changes in python itselfyour feedback played major role in shaping this textthere' nothing quite as instructive as watching , people repeat the same beginner mistakes live and in personthis book' recent editions owe their trainingbased changes primarily to recent classesthough every class held since has in some way helped refine this book ' like to thank clients who hosted classes in dublinmexico citybarcelonalondonedmontonand puerto ricosuch experiences have been one of my career' most lasting rewards because writing teaches writers to writethis book also owes much to its audience want to thank the countless readers who took time to offer suggestions over the last yearsboth online and in person your feedback has also been vital to this book' evolution and substantial factor in its successa benefit that seems inherent in the open source world reader comments have run the gamut from "you should be banned from writing booksto "god bless you for writing this book"if consensus is possible in such matters it probably lies somewhere between these twothough to borrow line from tolkienthe book is still too short ' also like to express my gratitude to everyone who played part in this book' production to all those who have helped make this book solid product over the years --including its editorsformattersmarketerstechnical reviewersand more and to 'reilly for giving me chance to work on book projectsit' been net fun (and only feels little like the movie groundhog dayadditional thanks is due to the entire python communitylike most open source systemspython is the product of many unsung efforts it' been my privilege to watch preface xlix