Think Python: An Introduction to Software Design: How To Think Like A Computer Scientist PDF
Preview Think Python: An Introduction to Software Design: How To Think Like A Computer Scientist
Think Python How to Think Like a Computer Scientist Version1.1.19 Think Python How to Think Like a Computer Scientist Version1.1.19 Allen Downey Green Tea Press Needham,Massachusetts Copyright©2008AllenDowney. Printinghistory: April2002: FirsteditionofHowtoThinkLikeaComputerScientist. August2007: Majorrevision,changedtitletoHowtoThinkLikea(Python)Programmer. June2008: Majorrevision,changedtitletoThinkPython:HowtoThinkLikeaComputerScientist. GreenTeaPress 9WashburnAve NeedhamMA02492 Permissionisgrantedtocopy,distribute,and/ormodifythisdocumentunderthetermsoftheGNUFreeDoc- umentation License, Version 1.1 or any later version published by the Free Software Foundation; with no InvariantSections,noFront-CoverTexts,andwithnoBack-CoverTexts. The GNU Free Documentation License is available from www.gnu.org or by writing to the Free Software Foundation,Inc.,59TemplePlace,Suite330,Boston,MA02111-1307,USA. TheoriginalformofthisbookisLATEXsourcecode. CompilingthisLATEXsourcehastheeffectofgenerating adevice-independentrepresentationofatextbook,whichcanbeconvertedtootherformatsandprinted. TheLATEXsourceforthisbookisavailablefromhttp://www.thinkpython.com Preface The strange history of this book InJanuary1999IwaspreparingtoteachanintroductoryprogrammingclassinJava. Ihadtaught it three times and I was getting frustrated. The failure rate in the class was too high and, even for studentswhosucceeded,theoveralllevelofachievementwastoolow. OneoftheproblemsIsawwasthebooks.Theyweretoobig,withtoomuchunnecessarydetailabout Java,andnotenoughhigh-levelguidanceabouthowtoprogram. Andtheyallsufferedfromthetrap dooreffect: theywouldstartouteasy,proceedgradually,andthensomewherearoundChapter5the bottomwouldfallout. Thestudentswouldgettoomuchnewmaterial,toofast,andIwouldspend therestofthesemesterpickingupthepieces. Twoweeksbeforethefirstdayofclasses,Idecidedtowritemyownbook. Mygoalswere: • Keepitshort. Itisbetterforstudentstoread10pagesthannotread50pages. • Becarefulwithvocabulary. Itriedtominimizethejargonanddefineeachtermatfirstuse. • Build gradually. To avoid trap doors, I took the most difficult topics and split them into a seriesofsmallsteps. • Focusonprogramming,nottheprogramminglanguage.Iincludedtheminimumusefulsubset ofJavaandleftouttherest. Ineededatitle,soonawhimIchoseHowtoThinkLikeaComputerScientist. My first version was rough, but it worked. Students did the reading, and they understood enough thatIcouldspendclasstimeonthehardtopics, theinterestingtopicsand(mostimportant)letting thestudentspractice. IreleasedthebookundertheGNUFreeDocumentationLicense,whichallowsuserstocopy,modify, anddistributethebook. Whathappenednextisthecoolpart.JeffElkner,ahighschoolteacherinVirginia,adoptedmybook andtranslateditintoPython. Hesentmeacopyofhistranslation,andIhadtheunusualexperience oflearningPythonbyreadingmyownbook. Jeff and I revised the book, incorporated a case study by Chris Meyers, and in 2001 we released How to Think Like a Computer Scientist: Learning with Python, also under the GNU Free Doc- umentation License. As Green Tea Press, I published the book and started selling hard copies throughAmazon.comandcollegebookstores. OtherbooksfromGreenTeaPressareavailableat greenteapress.com. vi Chapter0. Preface In2003IstartedteachingatOlinCollegeandIgottoteachPythonforthefirsttime. Thecontrast withJavawasstriking. Studentsstruggledless,learnedmore,workedonmoreinterestingprojects, andgenerallyhadalotmorefun. OverthelastfiveyearsIhavecontinuedtodevelopthebook,correctingerrors,improvingsomeof theexamplesandaddingmaterial,especiallyexercises.In2008Istartedworkonamajorrevision— atthesametime,IwascontactedbyaneditoratCambridgeUniversityPresswhowasinterestedin publishingthenextedition. Goodtiming! Theresultisthisbook,nowwiththelessgrandiosetitleThinkPython. Someofthechangesare: • Iaddedasectionaboutdebuggingattheendofeachchapter. Thesesectionspresentgeneral techniquesforfindingandavoidingbugs,andwarningsaboutPythonpitfalls. • Iremovedthematerialinthelastfewchaptersabouttheimplementationoflistsandtrees. I stilllovethosetopics,butIthoughttheywereincongruentwiththerestofthebook. • Iaddedmoreexercises,rangingfromshorttestsofunderstandingtoafewsubstantialprojects. • I added a series of case studies—longer examples with exercises, solutions, and discussion. SomeofthemarebasedonSwampy,asuiteofPythonprogramsIwroteforuseinmyclasses. Swampy,codeexamples,andsomesolutionsareavailablefromthinkpython.com. • Iexpandedthediscussionofprogramdevelopmentplansandbasicdesignpatterns. • The useof Python ismoreidiomatic. Thebook isstillabout programming, notPython, but nowIthinkthebookgetsmoreleveragefromthelanguage. Ihopeyouenjoyworkingwiththisbook,andthatithelpsyoulearntoprogramandthink,atleasta littlebit,likeacomputerscientist. AllenB.Downey NeedhamMA Allen Downey is an Associate Professor of Computer Science at the Franklin W. Olin College of Engineering. Acknowledgements Firstandmostimportantly,IthankJeffElkner,whotranslatedmyJavabookintoPython,whichgot thisprojectstartedandintroducedmetowhathasturnedouttobemyfavoritelanguage. I also thank Chris Meyers, who contributed several sections to How to Think Like a Computer Scientist. And I thank the Free Software Foundation for developing the GNU Free Documentation License, whichhelpedmakemycollaborationwithJeffandChrispossible. IalsothanktheeditorsatLuluwhoworkedonHowtoThinkLikeaComputerScientist. Ithankallthestudentswhoworkedwithearlierversionsofthisbookandallthecontributors(listed below)whosentincorrectionsandsuggestions. AndIthankmywife,Lisa,forherworkonthisbook,andGreenTeaPress,andeverythingelse,too. vii Contributor List Morethan100sharp-eyedandthoughtfulreadershavesentinsuggestionsandcorrectionsoverthe pastfewyears. Theircontributions,andenthusiasmforthisproject,havebeenahugehelp. If you have a suggestion or correction, please send email to [email protected]. If I makeachangebasedonyourfeedback,Iwilladdyoutothecontributorlist(unlessyouasktobe omitted). Ifyouincludeatleastpartofthesentencetheerrorappearsin,thatmakesiteasyformetosearch. Pageandsectionnumbersarefine,too,butnotquiteaseasytoworkwith. Thanks! • LloydHughAllensentinacorrectiontoSection8.4. • YvonBouliannesentinacorrectionofasemanticerrorinChapter5. • FredBremmersubmittedacorrectioninSection2.1. • JonahCohenwrotethePerlscriptstoconverttheLaTeXsourceforthisbookintobeautifulHTML. • MichaelConlonsentinagrammarcorrectioninChapter2andanimprovementinstyleinChapter1, andheinitiateddiscussiononthetechnicalaspectsofinterpreters. • BenoitGirardsentinacorrectiontoahumorousmistakeinSection5.6. • CourtneyGleasonandKatherineSmithwrotehorsebet.py,whichwasusedasacasestudyinanearlier versionofthebook.Theirprogramcannowbefoundonthewebsite. • LeeHarrsubmittedmorecorrectionsthanwehaveroomtolisthere,andindeedheshouldbelistedas oneoftheprincipaleditorsofthetext. • JamesKaylinisastudentusingthetext.Hehassubmittednumerouscorrections. • DavidKershawfixedthebrokencatTwicefunctioninSection3.10. • EddieLamhassentinnumerouscorrectionstoChapters1,2,and3. HealsofixedtheMakefilesothat itcreatesanindexthefirsttimeitisrunandhelpedussetupaversioningscheme. • Man-YongLeesentinacorrectiontotheexamplecodeinSection2.4. • DavidMayopointedoutthattheword“unconsciously”inChapter1neededtobechangedto“subcon- sciously”. • ChrisMcAloonsentinseveralcorrectionstoSections3.9and3.10. • MatthewJ.Moelterhasbeenalong-timecontributorwhosentinnumerouscorrectionsandsuggestions tothebook. • SimonDiconMontfordreportedamissingfunctiondefinitionandseveraltyposinChapter3. Healso founderrorsintheincrementfunctioninChapter13. • JohnOuztscorrectedthedefinitionof“returnvalue”inChapter3. • Kevin Parks sent in valuable comments and suggestions as to how to improve the distribution of the book. • DavidPoolsentinatypointheglossaryofChapter1,aswellaskindwordsofencouragement. • MichaelSchmittsentinacorrectiontothechapteronfilesandexceptions. • RobinShawpointedoutanerrorinSection13.1,wheretheprintTimefunctionwasusedinanexample withoutbeingdefined. • PaulSleighfoundanerrorinChapter7andabuginJonahCohen’sPerlscriptthatgeneratesHTML fromLaTeX. • CraigT.SnydalistestingthetextinacourseatDrewUniversity. Hehascontributedseveralvaluable suggestionsandcorrections. • IanThomasandhisstudentsareusingthetextinaprogrammingcourse. Theyarethefirstonestotest thechaptersinthelatterhalfofthebook,andtheyhavemadenumerouscorrectionsandsuggestions. viii Chapter0. Preface • KeithVerheydensentinacorrectioninChapter3. • PeterWinstanleyletusknowaboutalongstandingerrorinourLatininChapter3. • ChrisWrobelmadecorrectionstothecodeinthechapteronfileI/Oandexceptions. • MosheZadkahasmadeinvaluablecontributionstothisproject. Inadditiontowritingthefirstdraftof thechapteronDictionaries,heprovidedcontinualguidanceintheearlystagesofthebook. • ChristophZwerschkesentseveralcorrectionsandpedagogicsuggestions,andexplainedthedifference betweengleichandselbe. • JamesMayersentusawholeslewofspellingandtypographicalerrors,includingtwointhecontributor list. • HaydenMcAfeecaughtapotentiallyconfusinginconsistencybetweentwoexamples. • AngelArnalispartofaninternationalteamoftranslatorsworkingontheSpanishversionofthetext. HehasalsofoundseveralerrorsintheEnglishversion. • TauhidulHoqueandLexBerezhnycreatedtheillustrationsinChapter1andimprovedmanyoftheother illustrations. • Dr. MicheleAlzettacaughtanerrorinChapter8andsentsomeinterestingpedagogiccommentsand suggestionsaboutFibonacciandOldMaid. • AndyMitchellcaughtatypoinChapter1andabrokenexampleinChapter2. • KalinHarveysuggestedaclarificationinChapter7andcaughtsometypos. • ChristopherP.SmithcaughtseveraltyposandishelpinguspreparetoupdatethebookforPython2.2. • DavidHutchinscaughtatypointheForeword. • Gregor Lingl is teaching Python at a high school in Vienna, Austria. He is working on a German translationofthebook,andhecaughtacoupleofbaderrorsinChapter5. • JuliePeterscaughtatypointhePreface. • FlorinOprinasentinanimprovementinmakeTime,acorrectioninprintTime,andanicetypo. • D.J.WebresuggestedaclarificationinChapter3. • KenfoundafistfuloferrorsinChapters8,9and11. • IvoWevercaughtatypoinChapter5andsuggestedaclarificationinChapter3. • CurtisYankosuggestedaclarificationinChapter2. • BenLogansentinanumberoftyposandproblemswithtranslatingthebookintoHTML. • JasonArmstrongsawthemissingwordinChapter2. • LouisCordiernoticedaspotinChapter16wherethecodedidn’tmatchthetext. • BrianCainsuggestedseveralclarificationsinChapters2and3. • RobBlacksentinapasselofcorrections,includingsomechangesforPython2.2. • Jean-PhilippeReyatEcoleCentraleParissentanumberofpatches,includingsomeupdatesforPython 2.2andotherthoughtfulimprovements. • JasonMaderatGeorgeWashingtonUniversitymadeanumberofusefulsuggestionsandcorrections. • JanGundtofte-Bruunremindedusthat“aerror”isanerror. • AbelDavidandAlexisDinnoremindedusthatthepluralof“matrix”is“matrices”,not“matrixes”.This errorwasinthebookforyears,buttworeaderswiththesameinitialsreporteditonthesameday.Weird. • CharlesThayerencouragedustogetridofthesemi-colonswehadputattheendsofsomestatements andtocleanupouruseof“argument”and“parameter”. • RogerSperbergpointedoutatwistedpieceoflogicinChapter3. • SamBullpointedoutaconfusingparagraphinChapter2. • AndrewCheungpointedouttwoinstancesof“usebeforedef.” ix • C.CoreyCapelspottedthemissingwordintheThirdTheoremofDebuggingandatypoinChapter4. • AlessandrahelpedclearupsomeTurtleconfusion. • WimChampagnefoundabrain-oinadictionaryexample. • DouglasWrightpointedoutaproblemwithfloordivisioninarc. • JaredSpindorfoundsomejetsamattheendofasentence. • LinPeihengsentanumberofveryhelpfulsuggestions. • RayHagtvedtsentintwoerrorsandanot-quite-error. • TorstenHu¨bschpointedoutaninconsistencyinSwampy. • IngaPetuhhovcorrectedanexampleinChapter14. • ArneBabenhauserheidesentseveralhelpfulcorrections. • MarkE.Casidaisisgoodatspottingrepeatedwords. • ScottTylerfilledinathatwasmissing.Andthensentinaheapofcorrections. • GordonShephardsentinseveralcorrections,allinseparateemails. • AndrewTurnerspottedanerrorinChapter8. • AdamHobartfixedaproblemwithfloordivisioninarc. • Daryl Hammond and Sarah Zimmerman pointed out that I served up math.pi too early. And Zim spottedatypo. • GeorgeSassfoundabuginaDebuggingsection. • BrianBinghamsuggestedExercise11.9. • LeahEngelbert-FentonpointedoutthatIusedtupleasavariablename,contrarytomyownadvice. Andthenfoundabunchoftyposanda“usebeforedef.” • JoeFunkespottedatypo. • Chao-chaoChenfoundaninconsistencyintheFibonacciexample. • JeffPaineknowsthedifferencebetweenspaceandspam. • LubosPintessentinatypo. • GreggLindandAbigailHeithoffsuggestedExercise14.6. • MaxHailperinhassentinanumberofcorrectionsandsuggestions. Maxisoneoftheauthorsofthe extraordinaryConcreteAbstractions,whichyoumightwanttoreadwhenyouaredonewiththisbook. • ChotipatPornavalaifoundanerrorinanerrormessage. • StanislawAntolsentalistofveryhelpfulsuggestions. • EricPashmansentanumberofcorrectionsforChapters4–11. • MiguelAzevedofoundsometypos. • JianhuaLiusentinalonglistofcorrections. • NickKingfoundamissingword. • MartinZuthersentalonglistofsuggestions. • AdamZimmermanfoundaninconsistencyinmyinstanceofan“instance”andseveralothererrors. • RatnakarTiwarisuggestedafootnoteexplainingdegeneratetriangles. • Anurag Goel suggested another solution for is_abecedarian and sent some additional corrections. AndheknowshowtospellJaneAusten. • KelliKratzerspottedoneofthetypos. • MarkGriffithspointedoutaconfusingexampleinChapter3. • RoydanOngiefoundanerrorinmyNewton’smethod. • PatrykWolowiechelpedmewithaproblemintheHTMLversion. • MarkChonofskytoldmeaboutanewkeywordinPython3.0. • RussellColemanhelpedmewithmygeometry. • WeiHuangspottedseveraltypographicalerrors. • KarenBarberspottedthetheoldesttypointhebook. x Chapter0. Preface
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