TableofContentsMasteringPythonDesignPatternsCreditsAbouttheAuthorAbouttheReviewerswww.PacktPub.comSupportfiles,eBooks,discountoffers,andmore

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PrefaceDesignpatternsCommonmisunderstandingsaboutdesignpatternsDesignpatternsandPythonWhatthisbookcoversWhatyouneedforthisbookWhothisbookisforConventionsReaderfeedbackCustomersupport

DownloadingtheexamplecodeErrataPiracyQuestions

1.TheFactoryPatternFactoryMethod

Areal-lifeexampleAsoftwareexampleUsecasesImplementation

AbstractFactoryAreal-lifeexampleAsoftwareexampleUsecasesImplementation

Summary2.TheBuilderPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

3.ThePrototypePatternAreal-lifeexampleAsoftwareexample

UsecasesImplementationSummary

4.TheAdapterPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

5.TheDecoratorPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

6.TheFacadePatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

7.TheFlyweightPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

8.TheModel-View-ControllerPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

9.TheProxyPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

10.TheChainofResponsibilityPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

11.TheCommandPatternAreal-lifeexample

AsoftwareexampleUsecasesImplementationSummary

12.TheInterpreterPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

13.TheObserverPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

14.TheStatePatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

15.TheStrategyPatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

16.TheTemplatePatternAreal-lifeexampleAsoftwareexampleUsecasesImplementationSummary

Index

MasteringPythonDesignPatterns

MasteringPythonDesignPatternsCopyright©2015PacktPublishing

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CreditsAuthor

SakisKasampalis

Reviewers

EvanDempsey

AmitabhSharma

YogendraSharma

PatrycjaSzabłowska

CommissioningEditor

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AbouttheAuthorSakisKasampalis(@SKasampalis)isasoftwareengineerlivingintheNetherlands.Heisnotdogmaticaboutparticularprogramminglanguagesandtools;hisprincipleisthattherighttoolshouldbeusedfortherightjob.OneofhisfavoritetoolsisPythonbecausehefindsitveryproductive.

SakiswasalsothetechnicalreviewerofMasteringObject-orientedPythonandLearningPythonDesignPatterns,publishedbyPacktPublishing.

Iwanttothankmysweetheart,Georgia,forsupportingthiseffort.ManythankstoOwenRobertswhoencouragedmetowritethisbook.IalsowanttothankSumeetSawantforbeingaverykindandcooperativecontentdevelopmenteditor.Lastbutnotleast,Iwanttothankthereviewersofthisbookfortheirvaluablefeedback.

AbouttheReviewersEvanDempseyisasoftwaredeveloperfromWaterford,Ireland.Whenheisn'thackinginPythonforfunandprofit,heenjoyscraftbeers,commonLisp,andkeepingupwithmodernresearchinmachinelearning.Heisacontributortoseveralopensourceprojects.

AmitabhSharmaisaprofessionalsoftwareengineer.Hehasworkedextensivelyonenterpriseapplicationsintelecommunicationsandbusinessanalytics.Hisworkisfocusedonservice-orientedarchitecture,datawarehouses,andlanguagessuchasJava,Python,andothers.

IwouldliketothankmygrandfatherandmyfatherforallowingmetolearnallthatIcan.Iwouldalsoliketothankmywife,Komal,forhersupportandencouragement.

YogendraSharmawasbornandbroughtupinasmallbutculturaltown,Pratapgarh,inthestateofRajasthan.Hisbasiceducationhasbeenimpartedinhishometownitself,andhecompletedhisBTechinComputerSciencefromJaipur.Heisbasicallyanengineerbyheartandatechnicalenthusiastbynature.

HehasvastexperienceinthefieldsofPython,Djangoframework,webappsecurity,networking,Web2.0,andC++.

AlongwithCCNA,manyotheresteemedcertificationshavebeenawardedtohim.HeisanactivememberofInternationalAssociationofEngineers,Ubuntu,India,andComputerSocietyofIndia.

Morerecently,heparticipatedinbugbountyprogramsandwonmanybugbounties,includingtherespectedYahoo,Ebay,PayPalbugbounty.Hehasbeenappointedassecurityresearcherforseveralrespectedorganizations,suchasAdobe,Ebay,Avira,Moodle,Cisco,Atlassian,Basecamp,CodeClimate,Abacus,Rediff,Assembla,RecruiterBox,Tumbler,Wrike,Indeed,HybridSaaS,Sengrid,andSnapEngag.

Hehasreviewedmanybooksfromreputedpublishinghouses.YoucanfindhimonLinkedInathttp://in.linkedin.com/in/yogendra0sharma.

Iwouldliketothankallmyfriendswhoalwaysencouragedmetodosomethingnewandbelievinginme.

PatrycjaSzabłowskaisaPythondeveloperwithsomeJavabackground,withexperiencemainlyinbackenddevelopment.ShegraduatedfromNicolausCopernicusUniversityinToruń,Poland.

SheiscurrentlyworkinginWarsaw,Poland,atGrupaWirtualnaPolska.Sheisconstantlyexploringtechnicalnoveltiesandisopen-mindedandeagertolearnaboutthenextPython

http://in.linkedin.com/in/yogendra0sharma

libraryorframework.HerfavoriteprogrammingmottoisCodeisreadmuchmoreoftenthanitiswritten.

I'dliketothankmyhusband,Wacław,forencouragingmetoexplorenewfrontiers,andalsomyparentsforteachingmewhatmattersthemost.

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Preface

DesignpatternsInsoftwareengineering,adesignpatternisarecommendedsolutiontoasoftwaredesignproblem.Designpatternsgenerallydescribehowtostructureourcodetosolvecommondesignproblemsusingbestpractices.Itisimportanttonotethatadesignpatternisahigh-levelsolution;itdoesn'tfocusonimplementationdetailssuchasalgorithmsanddatastructures[GOF95,page13],[j.mp/srcmdp].Itisuptous,assoftwareengineers,todecidewhichalgorithmanddatastructureisoptimaltousefortheproblemwearetryingtosolve.

NoteIfyouarewonderingwhatisthemeaningofthetextwithin[],pleasejumptotheConventionssectionofthisprefaceforamomenttoseehowreferencesareformattedinthisbook.

Themostimportantpartofadesignpatternisprobablyitsname.Thebenefitofnamingallpatternsisthatwehave,onourhands,acommonvocabularytocommunicate[GOF95,page13].Thus,ifyousendsomecodeforreviewandyourpeerreviewergivesfeedbackmentioning"IthinkthatyoucanuseaStrategyhereinsteadof...",evenifyoudon'tknoworrememberwhatastrategyis,youcanimmediatelylookitup.

Asprogramminglanguagesevolve,somedesignpatternssuchasSingletonbecomeobsoleteorevenantipatterns[j.mp/jalfdp],othersarebuiltintheprogramminglanguage(iterator),andnewpatternsareborn(Borg/Monostate[j.mp/amdpp],[j.mp/wikidpc]).

http://j.mp/srcmdphttp://j.mp/jalfdphttp://j.mp/amdpphttp://j.mp/wikidpc

CommonmisunderstandingsaboutdesignpatternsThereareafewmisunderstandingsaboutdesignpatterns.Onemisunderstandingisthatdesignpatternsshouldbeusedrightfromthestartwhenwritingcode.Itisnotunusualtoseedevelopersstrugglingwithwhichpatterntheyshoulduseintheircode,eveniftheyhaven'tfirsttriedtosolvetheproblemintheirownway[j.mp/prsedp],[j.mp/stedp].

Notonlyisthiswrong,butitisalsoagainstthenatureofdesignpatterns.Designpatternsarediscovered(incontrasttoinvented)asbettersolutionsoverexistingsolutions.Ifyouhavenoexistingsolution,itdoesn'tmakesensetolookforabetterone.Justgoaheadanduseyourskillstosolveyourproblemasbestasyouthink.Ifyourcodereviewershavenoobjectionsandthroughtimeyouseethatyoursolutionissmartandflexibleenough,itmeansthatyoudon'tneedtowasteyourtimeonstrugglingaboutwhichpatterntouse.Youmighthaveevendiscoveredabetterdesignpatternthantheexistingone.Whoknows?Thepointisdonotlimityourcreativityinfavorofforcingyourselftouseexistingdesignpatterns.

Asecondmisunderstandingisthatdesignpatternsshouldbeusedeverywhere.Thisresultsincreatingcomplexsolutionswithunnecessaryinterfacesandhierarchies,whereasimplerandstraightforwardsolutionwouldbesufficient.Donotreatdesignpatternsasapanaceabecausetheyarenot.Theymustbeusedonlyifthereisproofthatyourexistingcode"smells",andishardtoextendandmaintain.Trythinkingintermsofyouaren'tgonnaneedit(YAGNI[j.mp/c2yagni])andKeepitsimplestupid(KISS[j.mp/wikikis]).Usingdesignpatternseverywhereisasevilasprematureoptimization[j.mp/c2pro].

http://j.mp/prsedphttp://j.mp/stedphttp://j.mp/c2yagnihttp://j.mp/wikikishttp://j.mp/c2pro

DesignpatternsandPythonThisbookfocusesondesignpatternsinPython.Pythonisdifferentthanmostcommonprogramminglanguagesusedinpopulardesignpatternsbooks(usuallyJava[FFBS04]orC++[GOF95]).Itsupportsduck-typing,functionsarefirst-classcitizens,andsomepatterns(forinstance,iteratoranddecorator)arebuilt-infeatures.Theintentofthisbookistodemonstratethemostfundamentaldesignpatterns,notallpatternsthathavebeendocumentedsofar[j.mp/wikidpc].ThecodeexamplesfocusonusingidiomaticPythonwhenapplicable[j.mp/idiompyt].IfyouarenotfamiliarwiththeZenofPython,itisagoodideatoopenthePythonREPLrightnowandexecuteimportthis.TheZenofPythonisbothamusingandmeaningful.

http://j.mp/wikidpchttp://j.mp/idiompyt

WhatthisbookcoversPart1:Creationalpatternspresentsdesignpatternsthatdealwithobjectcreation.

Chapter1,TheFactoryPattern,willteachyouhowtousetheFactorydesignpattern(FactoryMethodandAbstractFactory)toinitializeobjects,andcoverthebenefitsofusingtheFactorydesignpatterninsteadofdirectobjectinstantiation.

Chapter2,TheBuilderPattern,willteachyouhowtosimplifythecreationofobjectsthataretypicallycomposedbymorethanonerelatedobjects.

Chapter3,ThePrototypePattern,willteachyouhowtocreateanewobjectthatisafullcopy(hence,thenameclone)ofanexistingobject.

Part2:Structuralpatternspresentsdesignpatternsthatdealwithrelationshipsbetweentheentities(classes,objects,andsoon)ofasystem.

Chapter4,TheAdapterPattern,willteachyouhowtomakeyourexistingcodecompatiblewithaforeigninterface(forexample,anexternallibrary)withminimalchanges.

Chapter5,TheDecoratorPattern,willteachyouhowtoenhancethefunctionalityofanobjectwithoutusinginheritance.

Chapter6,TheFacadePattern,willteachyouhowtocreateasingleentrypointtohidethecomplexityofasystem.

Chapter7,TheFlyweightPattern,willteachyouhowtoreuseobjectsfromanobjectpooltoimprovethememoryusageandpossiblytheperformanceofyourapplications.

Chapter8,TheModel-View-ControllerPattern,willteachyouhowtoimprovethemaintainabilityofyourapplicationsbyavoidingmixingthebusinesslogicwiththeuserinterface.

Chapter9,TheProxyPattern,willteachyouhowtoimprovethesecurityofyourapplicationbyaddinganextralayerofprotection.

Part3:Behavioralpatternspresentsdesignpatternsthatdealwiththecommunicationofthesystem'sentities.

Chapter10,TheChainofResponsibilityPattern,willteachyouhowtosendarequesttomultiplereceivers.

Chapter11,TheCommandPattern,willteachyouhowtomakeyourapplicationcapableofrevertingalreadyappliedoperations.

Chapter12,TheInterpreterPattern,willteachyouhowtocreateasimplelanguageontopofPython,whichcanbeusedbydomainexpertswithoutforcingthemtolearnhowtoprograminPython.

Chapter13,TheObserverPattern,willteachyouhowtosendnotificationstotheregisteredstakeholdersofanobjectwheneveritsstatechanges.

Chapter14,TheStatePattern,willteachyouhowtocreateastatemachinetomodelaproblemandthebenefitsofthistechnique.

Chapter15,TheStrategyPattern,willteachyouhowtopick(duringruntime)analgorithmbetweenmanyavailablealgorithms,basedonsomeinputcriteria(forexample,theelementsize).

Chapter16,TheTemplatePattern,willteachyouhowtomakeaclearseparationbetweenthecommonanddifferentpartsofanalgorithmtoavoidunnecessarycodeduplication.

WhatyouneedforthisbookThecodeiswrittenexclusivelyinPython3.Python3is,inmanyaspects,notcompatiblewithPython2.x[j.mp/p2orp3].ThefocusisonPython3.4.0butusingPython3.3.0shouldalsobefine,sincetherearenosyntaxdifferencesbetweenPython3.3.0andPython3.4.0[j.mp/py3dot4].Ingeneral,ifyouinstallthelatestPython3versionfromwww.python.org,youshouldbefinewithrunningtheexamples.Mostmodules/librariesthatareusedintheexamplesareapartofthePython3distribution.Ifanexamplerequiresanyextramodulestobeinstalled,instructionsonhowtoinstallthemaregivenbeforepresentingtherelatedcode.

http://j.mp/p2orp3http://j.mp/py3dot4http://www.python.org

WhothisbookisforTheaudienceofthisbookisPythonprogrammerswithanintermediatebackgroundandaninterestindesignpatternsimplementedinidiomaticPython.ProgrammersofotherlanguageswhoareinterestedinPythoncanalsobenefit,butit'sbetteriftheyfirstreadsomematerialsthatexplainhowthingsaredoneinPython[j.mp/idiompyt],[j.mp/dspython].

http://j.mp/idiompythttp://j.mp/dspython

ConventionsInthisbook,youwillfindanumberoftextstylesthatdistinguishbetweendifferentkindsofinformation.Herearesomeexamplesofthesestylesandanexplanationoftheirmeaning.

Codewordsintext,databasetablenames,foldernames,filenames,fileextensions,pathnames,dummyURLs,userinput,andTwitterhandlesareshownasfollows:"WewillusetwolibrariesthatarepartofthePythondistributionforworkingwithXMLandJSON:xml.etree.ElementTreeandjson."

Ablockofcodeissetasfollows:

@propertydefparsed_data(self):returnself.data

Whenwewishtodrawyourattentiontoaparticularpartofacodeblock,therelevantlinesoritemsaresetinbold:

@propertydefparsed_data(self):returnself.data

Anycommand-lineinputoroutputiswrittenasfollows:

>>>python3factory_method.py

Newtermsandimportantwordsareshowninbold.Wordsthatyouseeonthescreen,forexample,inmenusordialogboxes,appearinthetextlikethis:"ClickingtheNextbuttonmovesyoutothenextscreen."

NoteWarningsorimportantnotesappearinaboxlikethis.

TipTipsandtricksappearlikethis.

Bookreferencesfollowtheformat[Author,page].Forexample,thereference[GOF95,page10]referstothe10thpageoftheGOF(DesignPatterns:ElementsofReusableObject-OrientedSoftware)book.Attheendofthebook,thereisasectiondevotedtoallbookreferences.

Webreferencesfollowtheformat[j.mp/shortened].TheseareshortenedURLaddressesthatyoucantypeorcopy/pasteintoyourwebbrowserandberedirectedtothereal(usuallylongerandsometimesuglier)webreference.Forexample,typingj.mp/idiompytinyouwebbrowser'saddressbarshouldredirectyoutohttp://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html.

http://j.mp/idiompythttp://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html

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Chapter1.TheFactoryPatternCreationaldesignpatternsdealwithanobjectcreation[j.mp/wikicrea].Theaimofacreationaldesignpatternistoprovidebetteralternativesforsituationswhereadirectobjectcreation(whichinPythonhappensbythe__init__()function[j.mp/divefunc],[Lott14,page26])isnotconvenient.

IntheFactorydesignpattern,aclientasksforanobjectwithoutknowingwheretheobjectiscomingfrom(thatis,whichclassisusedtogenerateit).Theideabehindafactoryistosimplifyanobjectcreation.Itiseasiertotrackwhichobjectsarecreatedifthisisdonethroughacentralfunction,incontrasttolettingaclientcreateobjectsusingadirectclassinstantiation[Eckel08,page187].Afactoryreducesthecomplexityofmaintaininganapplicationbydecouplingthecodethatcreatesanobjectfromthecodethatusesit[Zlobin13,page30].

Factoriestypicallycomeintwoforms:theFactoryMethod,whichisamethod(orinPythonicterms,afunction)thatreturnsadifferentobjectperinputparameter[j.mp/factorympat];theAbstractFactory,whichisagroupofFactoryMethodsusedtocreateafamilyofrelatedproducts[GOF95,page100],[j.mp/absfpat].

FactoryMethodIntheFactoryMethod,weexecuteasinglefunction,passingaparameterthatprovidesinformationaboutwhatwewant.Wearenotrequiredtoknowanydetailsabouthowtheobjectisimplementedandwhereitiscomingfrom.

Areal-lifeexampleAnexampleoftheFactoryMethodpatternusedinrealityisinplastictoyconstruction.Themoldingpowderusedtoconstructplastictoysisthesame,butdifferentfigurescanbeproducedusingdifferentplasticmolds.ThisislikehavingaFactoryMethodinwhichtheinputisthenameofthefigurethatwewant(duckandcar)andtheoutputistheplasticfigurethatwerequested.Thetoyconstructioncaseisshowninthefollowingfigure,whichisprovidedbywww.sourcemaking.com[j.mp/factorympat].

http://j.mp/wikicreahttp://j.mp/divefunchttp://j.mp/factorympathttp://j.mp/absfpathttp://www.sourcemaking.comhttp://j.mp/factorympat

AsoftwareexampleTheDjangoframeworkusestheFactoryMethodpatternforcreatingthefieldsofaform.TheformsmoduleofDjangosupportsthecreationofdifferentkindsoffields(CharField,EmailField)andcustomizations(max_length,required)[j.mp/djangofacm].

UsecasesIfyourealizethatyoucannottracktheobjectscreatedbyyourapplicationbecausethecodethatcreatesthemisinmanydifferentplacesinsteadofasinglefunction/method,youshouldconsiderusingtheFactoryMethodpattern[Eckel08,page187].TheFactoryMethodcentralizesanobjectcreationandtrackingyourobjectsbecomesmucheasier.NotethatitisabsolutelyfinetocreatemorethanoneFactoryMethod,andthisishowitistypicallydoneinpractice.EachFactoryMethodlogicallygroupsthecreationofobjectsthathavesimilarities.Forexample,oneFactoryMethodmightberesponsibleforconnectingyoutodifferentdatabases(MySQL,SQLite),anotherFactoryMethodmightberesponsibleforcreatingthegeometricalobjectthatyourequest(circle,triangle),andsoon.

TheFactoryMethodisalsousefulwhenyouwanttodecoupleanobjectcreationfromanobjectusage.Wearenotcoupled/boundtoaspecificclasswhencreatinganobject,wejustprovidepartialinformationaboutwhatwewantbycallingafunction.Thismeansthatintroducingchangestothefunctioniseasywithoutrequiringanychangestothecodethatusesit[Zlobin13,page30].

Anotherusecaseworthmentioningisrelatedtoimprovingtheperformanceandmemoryusageofanapplication.AFactoryMethodcanimprovetheperformanceandmemoryusagebycreatingnewobjectsonlyifitisabsolutelynecessary[Zlobin13,page28].Whenwecreateobjectsusingadirectclassinstantiation,extramemoryisallocatedeverytimeanewobjectiscreated(unlesstheclassusescachinginternally,whichisusuallynotthecase).Wecanseethatinpracticeinthefollowingcode(fileid.py),itcreatestwoinstancesofthesameclassAandusestheid()functiontocomparetheirmemoryaddresses.Theaddressesarealsoprintedintheoutputsothatwecaninspectthem.Thefactthatthememoryaddressesare

http://j.mp/djangofacm

differentmeansthattwodistinctobjectsarecreatedasfollows:

classA(object):pass

if__name__=='__main__':a=A()b=A()

print(id(a)==id(b))print(a,b)

Executingid.pyonmycomputergivesthefollowingoutput:

>>python3id.pyFalse

NotethattheaddressesthatyouseeifyouexecutethefilearenotthesameasIseebecausetheydependonthecurrentmemorylayoutandallocation.Buttheresultmustbethesame:thetwoaddressesshouldbedifferent.There'soneexceptionthathappensifyouwriteandexecutethecodeinthePythonRead-Eval-PrintLoop(REPL)(interactiveprompt),butthat'saREPL-specificoptimizationwhichisnothappeningnormally.

ImplementationDatacomesinmanyforms.Therearetwomainfilecategoriesforstoring/retrievingdata:human-readablefilesandbinaryfiles.Examplesofhuman-readablefilesareXML,Atom,YAML,andJSON.Examplesofbinaryfilesarethe.sq3fileformatusedbySQLiteandthe.mp3fileformatusedtolistentomusic.

Inthisexample,wewillfocusontwopopularhuman-readableformats:XMLandJSON.Althoughhuman-readablefilesaregenerallyslowertoparsethanbinaryfiles,theymakedataexchange,inspection,andmodificationmucheasier.Forthisreason,itisadvisedtopreferworkingwithhuman-readablefiles,unlessthereareotherrestrictionsthatdonotallowit(mainlyunacceptableperformanceandproprietarybinaryformats).

Inthisproblem,wehavesomeinputdatastoredinanXMLandaJSONfile,andwewanttoparsethemandretrievesomeinformation.Atthesametime,wewanttocentralizetheclient'sconnectiontothose(andallfuture)externalservices.WewillusetheFactoryMethodtosolvethisproblem.TheexamplefocusesonlyonXMLandJSON,butaddingsupportformoreservicesshouldbestraightforward.

First,let'stakealookatthedatafiles.TheXMLfile,person.xml,isbasedontheWikipediaexample[j.mp/wikijson]andcontainsinformationaboutindividuals(firstName,lastName,gender,andsoon)asfollows:

http://j.mp/wikijson

JohnSmith25212ndStreetNewYorkNY10021212555-1234646555-4567maleJimyLiar19182ndStreetNewYorkNY10021212555-1234malePattyLiar20182ndStreetNewYorkNY10021212555-1234001452-8819female

TheJSONfile,donut.json,comesfromtheGitHubaccountofAdobe[j.mp/adobejson]andcontainsdonutinformation(type,price/unitthatis,ppu,topping,andsoon)asfollows:

http://j.mp/adobejson

[{"id":"0001","type":"donut","name":"Cake","ppu":0.55,"batters":{"batter":[{"id":"1001","type":"Regular"},{"id":"1002","type":"Chocolate"},{"id":"1003","type":"Blueberry"},{"id":"1004","type":"Devil'sFood"}]},"topping":[{"id":"5001","type":"None"},{"id":"5002","type":"Glazed"},{"id":"5005","type":"Sugar"},{"id":"5007","type":"PowderedSugar"},{"id":"5006","type":"ChocolatewithSprinkles"},{"id":"5003","type":"Chocolate"},{"id":"5004","type":"Maple"}]},{"id":"0002","type":"donut","name":"Raised","ppu":0.55,"batters":{"batter":[{"id":"1001","type":"Regular"}]},"topping":[{"id":"5001","type":"None"},{"id":"5002","type":"Glazed"},{"id":"5005","type":"Sugar"},{"id":"5003","type":"Chocolate"},{"id":"5004","type":"Maple"}]},{"id":"0003","type":"donut","name":"OldFashioned","ppu":0.55,"batters":{"batter":[{"id":"1001","type":"Regular"},{"id":"1002","type":"Chocolate"}]},"topping":[{"id":"5001","type":"None"},{"id":"5002","type":"Glazed"},{"id":"5003","type":"Chocolate"},{"id":"5004","type":"Maple"}]}

]

WewillusetwolibrariesthatarepartofthePythondistributionforworkingwithXMLandJSON:xml.etree.ElementTreeandjsonasfollows:

importxml.etree.ElementTreeasetreeimportjson

TheJSONConnectorclassparsestheJSONfileandhasaparsed_data()methodthatreturnsalldataasadictionary(dict).Thepropertydecoratorisusedtomakeparsed_data()appearasanormalvariableinsteadofamethodasfollows:

classJSONConnector:

def__init__(self,filepath):self.data=dict()withopen(filepath,mode='r',encoding='utf-8')asf:self.data=json.load(f)

@propertydefparsed_data(self):returnself.data

TheXMLConnectorclassparsestheXMLfileandhasaparsed_data()methodthatreturnsalldataasalistofxml.etree.Elementasfollows:

classXMLConnector:

def__init__(self,filepath):self.tree=etree.parse(filepath)

@propertydefparsed_data(self):returnself.tree

Theconnection_factory()functionisaFactoryMethod.ItreturnsaninstanceofJSONConnectororXMLConnectordependingontheextensionoftheinputfilepathasfollows:

defconnection_factory(filepath):iffilepath.endswith('json'):connector=JSONConnectoreliffilepath.endswith('xml'):connector=XMLConnectorelse:raiseValueError('Cannotconnectto{}'.format(filepath))returnconnector(filepath)

Theconnect_to()functionisawrapperofconnection_factory().Itaddsexceptionhandlingasfollows:

defconnect_to(filepath):

factory=Nonetry:factory=connection_factory(filepath)exceptValueErrorasve:print(ve)returnfactory

Themain()functiondemonstrateshowtheFactoryMethoddesignpatterncanbeused.Thefirstpartmakessurethatexceptionhandlingiseffectiveasfollows:

defmain():sqlite_factory=connect_to('data/person.sq3')

ThenextpartshowshowtoworkwiththeXMLfilesusingtheFactoryMethod.XPathisusedtofindallpersonelementsthathavethelastnameLiar.Foreachmatchedperson,thebasicnameandphonenumberinformationareshownasfollows:

xml_factory=connect_to('data/person.xml')xml_data=xml_factory.parsed_data()liars=xml_data.findall(".//{person}[{lastName}='{}']".format('Liar'))print('found:{}persons'.format(len(liars)))forliarinliars:print('firstname:{}'.format(liar.find('firstName').text))print('lastname:{}'.format(liar.find('lastName').text))[print('phonenumber({}):'.format(p.attrib['type']),p.text)forpinliar.find('phoneNumbers')]

ThefinalpartshowshowtoworkwiththeJSONfilesusingtheFactoryMethod.Here,there'snopatternmatching,andthereforethename,price,andtoppingofalldonutsareshownasfollows:

json_factory=connect_to('data/donut.json')json_data=json_factory.parsed_dataprint('found:{}donuts'.format(len(json_data)))fordonutinjson_data:print('name:{}'.format(donut['name']))print('price:${}'.format(donut['ppu']))[print('topping:{}{}'.format(t['id'],t['type']))fortindonut['topping']]

Forcompleteness,hereisthecompletecodeoftheFactoryMethodimplementation(factory_method.py)asfollows:

importxml.etree.ElementTreeasetreeimportjson

classJSONConnector:def__init__(self,filepath):self.data=dict()withopen(filepath,mode='r',encoding='utf-8')asf:self.data=json.load(f)

@propertydefparsed_data(self):returnself.data

classXMLConnector:def__init__(self,filepath):self.tree=etree.parse(filepath)

@propertydefparsed_data(self):returnself.tree

defconnection_factory(filepath):iffilepath.endswith('json'):connector=JSONConnectoreliffilepath.endswith('xml'):connector=XMLConnectorelse:raiseValueError('Cannotconnectto{}'.format(filepath))returnconnector(filepath)

defconnect_to(filepath):factory=Nonetry:factory=connection_factory(filepath)exceptValueErrorasve:print(ve)returnfactory

defmain():sqlite_factory=connect_to('data/person.sq3')print()

xml_factory=connect_to('data/person.xml')xml_data=xml_factory.parsed_dataliars=xml_data.findall(".//{}[{}='{}']".format('person','lastName','Liar'))print('found:{}persons'.format(len(liars)))forliarinliars:print('firstname:{}'.format(liar.find('firstName').text))print('lastname:{}'.format(liar.find('lastName').text))[print('phonenumber({}):'.format(p.attrib['type']),p.text)forpinliar.find('phoneNumbers')]print()

json_factory=connect_to('data/donut.json')json_data=json_factory.parsed_dataprint('found:{}donuts'.format(len(json_data)))fordonutinjson_data:print('name:{}'.format(donut['name']))print('price:${}'.format(donut['ppu']))[print('topping:{}{}'.format(t['id'],t['type']))fortindonut['topping']]

if__name__=='__main__':main()

Hereistheoutputofthisprogramasfollows:

>>>python3factory_method.pyCannotconnecttodata/person.sq3

found:2personsfirstname:Jimylastname:Liarphonenumber(home):212555-1234firstname:Pattylastname:Liarphonenumber(home):212555-1234phonenumber(mobile):001452-8819

found:3donutsname:Cakeprice:$0.55topping:5001Nonetopping:5002Glazedtopping:5005Sugartopping:5007PowderedSugartopping:5006ChocolatewithSprinklestopping:5003Chocolatetopping:5004Maplename:Raisedprice:$0.55topping:5001Nonetopping:5002Glazedtopping:5005Sugartopping:5003Chocolatetopping:5004Maplename:OldFashionedprice:$0.55topping:5001Nonetopping:5002Glazedtopping:5003Chocolatetopping:5004Maple

NoticethatalthoughJSONConnectorandXMLConnectorhavethesameinterfaces,whatisreturnedbyparsed_data()isnothandledinauniformway.Differentpythoncodemustbeusedtoworkwitheachconnector.Althoughitwouldbenicetobeabletousethesamecodeforallconnectors,thisisatmosttimesnotrealisticunlessweusesomekindofcommonmappingforthedatawhichisveryoftenprovidedbyexternaldataproviders.AssumingthatyoucanuseexactlythesamecodeforhandlingtheXMLandJSONfiles,whatchangesarerequiredtosupportathirdformat,forexample,SQLite?FindanSQLitefileorcreateyourownandtryit.

Asitisnow,thecodedoesnotforbidadirectinstantiationofaconnector.Isitpossibletodothis?Trydoingit.

TipHint:FunctionsinPythoncanhavenestedclasses.

AbstractFactoryTheAbstractFactorydesignpatternisageneralizationofFactoryMethod.Basically,anAbstractFactoryisa(logical)groupofFactoryMethods,whereeachFactoryMethodisresponsibleforgeneratingadifferentkindofobject[Eckel08,page193].

Areal-lifeexampleAbstractFactoryisusedincarmanufacturing.Thesamemachineryisusedforstampingtheparts(doors,panels,hoods,fenders,andmirrors)ofdifferentcarmodels.Themodelthatisassembledbythemachineryisconfigurableandeasytochangeatanytime.WecanseeanexampleofthecarmanufacturingAbstractFactoryinthefollowingfigure,whichisprovidedbywww.sourcemaking.com[j.mp/absfpat].

AsoftwareexampleThedjango_factorypackageisanAbstractFactoryimplementationforcreatingDjangomodelsintests.Itisusedforcreatinginstancesofmodelsthatsupporttest-specificattributes.Thisisimportantbecausethetestsbecomereadableandavoidsharingunnecessarycode[j.mp/djangoabs].

http://www.sourcemaking.comhttp://j.mp/absfpathttp://j.mp/djangoabs

UsecasesSincetheAbstractFactorypatternisageneralizationoftheFactoryMethodpattern,itoffersthesamebenefits:itmakestrackinganobjectcreationeasier,itdecouplesanobjectcreationfromanobjectusage,anditgivesusthepotentialtoimprovethememoryusageandperformanceofourapplication.

Butaquestionisraised:howdoweknowwhentousetheFactoryMethodversususinganAbstractFactory?TheansweristhatweusuallystartwiththeFactoryMethodwhichissimpler.IfwefindoutthatourapplicationrequiresmanyFactoryMethodswhichitmakessensetocombineforcreatingafamilyofobjects,weendupwithanAbstractFactory.

AbenefitoftheAbstractFactorythatisusuallynotveryvisiblefromauser'spointofviewwhenusingtheFactoryMethodisthatitgivesustheabilitytomodifythebehaviorofourapplicationdynamically(inruntime)bychangingtheactiveFactoryMethod.Theclassicexampleisgivingtheabilitytochangethelookandfeelofanapplication(forexample,Apple-like,Windows-like,andsoon)fortheuserwhiletheapplicationisinuse,withouttheneedtoterminateitandstartitagain[GOF95,page99].

ImplementationTodemonstratetheAbstractFactorypattern,Iwillreuseoneofmyfavoriteexamples,includedinPython3Patterns&Idioms,BruceEckel,[Eckel08,page193].Imaginethatwearecreatingagameorwewanttoincludeamini-gameaspartofourapplicationtoentertainourusers.Wewanttoincludeatleasttwogames,oneforchildrenandoneforadults.Wewilldecidewhichgametocreateandlaunchinruntime,basedonuserinput.AnAbstractFactorytakescareofthegamecreationpart.

Let'sstartwiththekid'sgame.ItiscalledFrogWorld.Themainheroisafrogwhoenjoyseatingbugs.Everyheroneedsagoodname,andinourcasethenameisgivenbytheuserinruntime.Theinteract_with()methodisusedtodescribetheinteractionofthefrogwithanobstacle(forexample,bug,puzzle,andotherfrog)asfollows:

classFrog:def__init__(self,name):self.name=name

def__str__(self):returnself.name

definteract_with(self,obstacle):print('{}theFrogencounters{}and{}!'.format(self,obstacle,obstacle.action()))

TherecanbemanydifferentkindsofobstaclesbutforourexampleanobstaclecanonlybeaBug.Whenthefrogencountersabug,onlyoneactionissupported:iteatsit!

classBug:

def__str__(self):return'abug'

defaction(self):return'eatsit'

TheFrogWorldclassisanAbstractFactory.Itsmainresponsibilitiesarecreatingthemaincharacterandtheobstacle(s)ofthegame.Keepingthecreationmethodsseparateandtheirnamesgeneric(forexample,make_character(),make_obstacle())allowsustodynamicallychangetheactivefactory(andthereforetheactivegame)withoutanycodechanges.Inastaticallytypedlanguage,theAbstractFactorywouldbeanabstractclass/interfacewithemptymethods,butinPythonthisisnotrequiredbecausethetypesarecheckedinruntime[Eckel08,page195],[j.mp/ginstromdp]asfollows:

classFrogWorld:def__init__(self,name):print(self)self.player_name=namedef__str__(self):return'\n\n\t------FrogWorld-------'

defmake_character(self):returnFrog(self.player_name)

defmake_obstacle(self):returnBug()

TheWizardWorldgameissimilar.Theonlydifferencesarethatthewizardbattlesagainstmonsterslikeorksinsteadofeatingbugs!

classWizard:def__init__(self,name):self.name=name

def__str__(self):returnself.name

definteract_with(self,obstacle):print('{}theWizardbattlesagainst{}and{}!'.format(self,obstacle,obstacle.action()))

classOrk:def__str__(self):return'anevilork'

defaction(self):return'killsit'

classWizardWorld:def__init__(self,name):print(self)self.player_name=name

def__str__(self):

http://j.mp/ginstromdp

return'\n\n\t------WizardWorld-------'

defmake_character(self):returnWizard(self.player_name)

defmake_obstacle(self):returnOrk()

TheGameEnvironmentisthemainentrypointofourgame.Itacceptsfactoryasaninput,andusesittocreatetheworldofthegame.Theplay()methodinitiatestheinteractionbetweenthecreatedheroandtheobstacleasfollows:

classGameEnvironment:def__init__(self,factory):self.hero=factory.make_character()self.obstacle=factory.make_obstacle()

defplay(self):self.hero.interact_with(self.obstacle)

Thevalidate_age()functionpromptstheusertogiveavalidage.Iftheageisnotvalid,itreturnsatuplewiththefirstelementsettoFalse.Iftheageisfine,thefirstelementofthetupleissettoTrueandthat'sthecasewhereweactuallycareaboutthesecondelementofthetuple,whichistheagegivenbytheuserasfollows:

defvalidate_age(name):try:age=input('Welcome{}.Howoldareyou?'.format(name))age=int(age)exceptValueErroraserr:print("Age{}isinvalid,pleasetryagain...".format(age))return(False,age)return(True,age)

Lastbutnotleastcomesthemain()function.Itasksfortheuser'snameandage,anddecideswhichgameshouldbeplayedbytheageoftheuserasfollows:

defmain():name=input("Hello.What'syourname?")valid_input=Falsewhilenotvalid_input:valid_input,age=validate_age(name)game=FrogWorldifage<18elseWizardWorldenvironment=GameEnvironment(game(name))environment.play()

AndthecompletecodeoftheAbstractFactoryimplementation(abstract_factory.py)isgivenasfollows:

classFrog:def__init__(self,name):self.name=name

def__str__(self):returnself.name

definteract_with(self,obstacle):print('{}theFrogencounters{}and{}!'.format(self,obstacle,obstacle.action()))

classBug:def__str__(self):return'abug'

defaction(self):return'eatsit'

classFrogWorld:def__init__(self,name):print(self)self.player_name=namedef__str__(self):return'\n\n\t------FrogWorld-------'

defmake_character(self):returnFrog(self.player_name)

defmake_obstacle(self):returnBug()

classWizard:def__init__(self,name):self.name=name

def__str__(self):returnself.namedefinteract_with(self,obstacle):print('{}theWizardbattlesagainst{}and{}!'.format(self,obstacle,obstacle.action()))

classOrk:def__str__(self):return'anevilork'

defaction(self):return'killsit'

classWizardWorld:def__init__(self,name):print(self)self.player_name=name

def__str__(self):return'\n\n\t------WizardWorld-------'

defmake_character(self):returnWizard(self.player_name)

defmake_obstacle(self):returnOrk()

classGameEnvironment:def__init__(self,factory):self.hero=factory.make_character()self.obstacle=factory.make_obstacle()

defplay(self):self.hero.interact_with(self.obstacle)

defvalidate_age(name):try:age=input('Welcome{}.Howoldareyou?'.format(name))age=int(age)exceptValueErroraserr:print("Age{}isinvalid,pleasetryagain...".format(age))return(False,age)return(True,age)

defmain():name=input("Hello.What'syourname?")valid_input=Falsewhilenotvalid_input:valid_input,age=validate_age(name)game=FrogWorldifage<18elseWizardWorldenvironment=GameEnvironment(game(name))environment.play()

if__name__=='__main__':main()

Asampleoutputofthisprogramisasfollows:

>>>python3abstract_factory.pyHello.What'syourname?NickWelcomeNick.Howoldareyou?17------FrogWorld-------NicktheFrogencountersabugandeatsit!

Tryextendingthegametomakeitmorecomplete.Youcangoasfarasyouwant:manyobstacles,manyenemies,andwhateverelseyoulike.

SummaryInthischapter,wehaveseenhowtousetheFactoryMethodandtheAbstractFactorydesignpatterns.Bothpatternsareusedwhenwewantto(a)trackanobjectcreation,(b)decoupleanobjectcreationfromanobjectusage,oreven(c)improvetheperformanceandresourceusageofanapplication.Case(c)wasnotdemonstratedinthechapter.Youmightconsideritasagoodexercise.

TheFactoryMethoddesignpatternisimplementedasasinglefunctionthatdoesn'tbelongtoanyclass,andisresponsibleforthecreationofasinglekindofobject(ashape,aconnectionpoint,andsoon).WesawhowtheFactoryMethodrelatestotoyconstruction,mentionedhowitisusedbyDjangoforcreatingdifferentformfields,anddiscussedotherpossibleusecasesforit.Asanexample,weimplementedaFactoryMethodthatprovidesaccesstotheXMLandJSONfiles.

TheAbstractFactorydesignpatternisimplementedasanumberofFactoryMethodsthatbelongtoasingleclassandareusedtocreateafamilyofrelatedobjects(thepartsofacar,theenvironmentofagame,andsoforth).WementionedhowtheAbstractFactoryisrelatedwithcarmanufacturing,howthedjango_factoryDjangopackagemakesuseofittocreatecleantests,andcoveredtheusecasesofit.TheimplementationoftheAbstractFactoryisamini-gamethatshowshowwecanusemanyrelatedfactoriesinasingleclass.

Inthenextchapter,wewilltalkabouttheBuilderpattern,whichisanothercreationalpatternthatcanbeusedforfine-controllingthecreationofcomplexobjects.

Chapter2.TheBuilderPatternImaginethatwewanttocreateanobjectthatiscomposedofmultiplepartsandthecompositionneedstobedonestepbystep.Theobjectisnotcompleteunlessallitspartsarefullycreated.That'swheretheBuilderdesignpatterncanhelpus.TheBuilderpatternseparatestheconstructionofacomplexobjectfromitsrepresentation.Bykeepingtheconstructionseparatefromtherepresentation,thesameconstructioncanbeusedtocreateseveraldifferentrepresentations[GOF95,page110],[j.mp/builderpat].

ApracticalexamplecanhelpusunderstandwhatthepurposeoftheBuilderpatternis.SupposethatwewanttocreateanHTMLpagegenerator,thebasicstructure(constructionpart)ofanHTMLpageisalwaysthesame:itbeginswithandfinisheswith;insidetheHTMLsectionaretheandelements,insidetheheadsectionaretheandelements,andsoforth.Buttherepresentationofthepagecandiffer.Eachpagehasitsowntitle,itsownheadings,anddifferentcontents.Moreover,thepageisusuallybuiltinsteps:onefunctionaddsthetitle,anotheraddsthemainheading,anotherthefooter,andsoon.Onlyafterthewholestructureofapageiscompletecanitbeshowntotheclientusingafinalrenderfunction.WecantakeitevenfurtherandextendtheHTMLgeneratorsothatitcangeneratetotallydifferentHTMLpages.Onepagemightcontaintables,anotherpagemightcontainimagegalleries,yetanotherpagecontainsthecontactform,andsoon.

TheHTMLpagegenerationproblemcanbesolvedusingtheBuilderpattern.Inthispattern,therearetwomainparticipants:thebuilderandthedirector.Thebuilderisresponsibleforcreatingthevariouspartsofthecomplexobject.IntheHTMLexample,thesepartsarethetitle,heading,body,andthefooterofthepage.Thedirectorcontrolsthebuildingprocessusingabuilderinstance.TheHTMLexamplemeansforcallingthebuilder'sfunctionsforsettingthetitle,theheading,andsoon.UsingadifferentbuilderinstanceallowsustocreateadifferentHTMLpagewithouttouchinganycodeofthedirector.

Areal-lifeexampleTheBuilderdesignpatternisusedinfast-foodrestaurants.Thesameprocedureisalwaysusedtoprepareaburgerandthepackaging(boxandpaperbag),eveniftherearemanydifferentkindsofburgers(classic,cheeseburger,andmore)anddifferentpackages(small-sizedbox,medium-sizedbox,andsoforth).Thedifferencebetweenaclassicburgerandacheeseburgerisintherepresentation,andnotintheconstructionprocedure.Thedirectoristhecashierwhogivesinstructionsaboutwhatneedstobepreparedtothecrew,andthebuilderisthepersonfromthecrewthattakescareofthespecificorder.Thefollowingfigureprovidedbywww.sourcemaking.comshowsaUnifiedModelingLanguage(UML)sequencediagramofthecommunicationthattakesplacebetweenthecustomer(client),thecashier(director),andthecrew(builder)whenakid'smenuisordered[j.mp/builderpat].

http://j.mp/builderpathttp://www.sourcemaking.comhttp://j.mp/builderpat

AsoftwareexampleTheHTMLexamplethatwasmentionedatthebeginningofthechapterisactuallyusedbydjango-widgy,athird-partytreeeditorforDjangothatcanbeusedasaContentManagementSystem(CMS).Thedjango-widgyeditorcontainsapagebuilderthatcanbeusedforcreatingHTMLpageswithdifferentlayouts[j.mp/widgypb].

Thedjango-query-builderlibraryisanotherthird-partyDjangolibrarythatreliesontheBuilderpattern.Thedjango-query-builderlibrarycanbeusedforbuildingSQLqueriesdynamically.Usingthis,wecancontrolallaspectsofaqueryandcreateadifferentrangeofqueries,fromsimpletoverycomplex[j.mp/djangowidgy].

http://j.mp/widgypbhttp://j.mp/djangowidgy

UsecasesWeusetheBuilderpatternwhenweknowthatanobjectmustbecreatedinmultiplesteps,anddifferentrepresentationsofthesameconstructionarerequired.Theserequirementsexistinmanyapplicationssuchaspagegenerators(liketheHTMLpagegeneratormentionedinthischapter),documentconverters[GOF95,page110],andUserInterface(UI)formcreators[j.mp/pipbuild].

SomeresourcesmentionthattheBuilderpatterncanalsobeusedasasolutiontothetelescopicconstructorproblem[j.mp/wikibuilder].Thetelescopicconstructorproblemoccurswhenweareforcedtocreateanewconstructorforsupportingdifferentwaysofcreatinganobject.Theproblemisthatweendupwithmanyconstructorsandlongparameterlists,whicharehardtomanage.Anexampleofthetelescopicconstructorislistedatthestackoverflowwebsite[j.mp/sobuilder].Fortunately,thisproblemdoesnotexistinPython,becauseitcanbesolvedinatleasttwoways:

Withnamedparameters[j.mp/sobuipython]Withargumentlistunpacking[j.mp/arglistpy]

Atthispoint,thedistinctionbetweentheBuilderpatternandtheFactorypatternmightnotbeveryclear.ThemaindifferenceisthataFactorypatterncreatesanobjectinasinglestep,whereasaBuilderpatterncreatesanobjectinmultiplesteps,andalmostalwaysthroughtheuseofadirector.SometargetedimplementationsoftheBuilderpatternlikeJava'sStringBuilderbypasstheuseofadirector,butthat'stheexceptiontotherule.

AnotherdifferenceisthatwhileaFactorypatternreturnsacreatedobjectimmediately,intheBuilderpatterntheclientcodeexplicitlyasksthedirectortoreturnthefinalobjectwhenitneedsit[GOF95,page113],[j.mp/builderpat].

ThenewcomputeranalogymighthelptodistinguishbetweenaBuilderpatternandaFactorypattern.Assumethatyouwanttobuyanewcomputer.Ifyoudecidetobuyaspecificpreconfiguredcomputermodel,forexample,thelatestApple1.4GHzMacmini,youusetheFactorypattern.Allthehardwarespecificationsarealreadypredefinedbythemanufacturer,whoknowswhattodowithoutconsultingyou.Themanufacturertypicallyreceivesjustasingleinstruction.Code-wise,thiswouldlooklikethefollowing(apple-factory.py):

MINI14='1.4GHzMacmini'

classAppleFactory:classMacMini14:def__init__(self):self.memory=4#ingigabytesself.hdd=500#ingigabytesself.gpu='IntelHDGraphics5000'

def__str__(self):info=('Model:{}'.format(MINI14),'Memory:{}GB'.format(self.memory),'HardDisk:{}GB'.format(self.hdd),

http://j.mp/pipbuildhttp://j.mp/wikibuilderhttp://j.mp/sobuilderhttp://j.mp/sobuipythonhttp://j.mp/arglistpyhttp://j.mp/builderpat

'GraphicsCard:{}'.format(self.gpu))return'\n'.join(info)

defbuild_computer(self,model):if(model==MINI14):returnself.MacMini14()else:print("Idon'tknowhowtobuild{}".format(model))

if__name__=='__main__':afac=AppleFactory()mac_mini=afac.build_computer(MINI14)print(mac_mini)

NoteNoticethenestedMacMini14class.Thisisaneatwayofforbiddingthedirectinstantiationofaclass.

AnotheroptionisbuyingacustomPC.Inthiscase,youusetheBuilderpattern.Youarethedirectorthatgivesorderstothemanufacturer(builder)aboutyouridealcomputerspecifications.Code-wise,thislookslikethefollowing(computer-builder.py):

classComputer:def__init__(self,serial_number):self.serial=serial_numberself.memory=None#ingigabytesself.hdd=None#ingigabytesself.gpu=None

def__str__(self):info=('Memory:{}GB'.format(self.memory),'HardDisk:{}GB'.format(self.hdd),'GraphicsCard:{}'.format(self.gpu))return'\n'.join(info)

classComputerBuilder:def__init__(self):self.computer=Computer('AG23385193')

defconfigure_memory(self,amount):self.computer.memory=amount

defconfigure_hdd(self,amount):self.computer.hdd=amount

defconfigure_gpu(self,gpu_model):self.computer.gpu=gpu_model

classHardwareEngineer:def__init__(self):self.builder=None

defconstruct_computer(self,memory,hdd,gpu):self.builder=ComputerBuilder()

[stepforstepin(self.builder.configure_memory(memory),self.builder.configure_hdd(hdd),self.builder.configure_gpu(gpu))]

@propertydefcomputer(self):returnself.builder.computer

defmain():engineer=HardwareEngineer()engineer.construct_computer(hdd=500,memory=8,gpu='GeForceGTX650Ti')computer=engineer.computerprint(computer)

if__name__=='__main__':main()

ThebasicchangesaretheintroductionofabuilderComputerBuilder,adirectorHardwareEngineer,andthestep-by-stepconstructionofacomputer,whichnowsupportsdifferentconfigurations(noticethatmemory,hdd,andgpuareparametersandnotpreconfigured).Whatdoweneedtodoifwewanttosupporttheconstructionoftablets?Implementthisasanexercise.

Youmightalsowanttochangethecomputerserial_numberintosomethingthatisdifferentforeachcomputer,becauseasitisnowitmeansthatallcomputerswillhavethesameserialnumber(whichisimpractical).

ImplementationLet'sseehowwecanusetheBuilderdesignpatterntomakeapizzaorderingapplication.Thepizzaexampleisparticularlyinterestingbecauseapizzaispreparedinstepsthatshouldfollowaspecificorder.Toaddthesauce,youfirstneedtopreparethedough.Toaddthetopping,youfirstneedtoaddthesauce.Andyoucan'tstartbakingthepizzaunlessboththesauceandthetoppingareplacedonthedough.Moreover,eachpizzausuallyrequiresadifferentbakingtime,dependingonthethicknessofitsdoughandthetoppingused.

WestartwithimportingtherequiredmodulesanddeclaringafewEnumparameters[j.mp/pytenum]plusaconstantthatareusedmanytimesintheapplication.TheSTEP_DELAYconstantisusedtoaddatimedelaybetweenthedifferentstepsofpreparingapizza(preparethedough,addthesauce,andsoon)asfollows:

fromenumimportEnum

PizzaProgress=Enum('PizzaProgress','queuedpreparationbakingready')PizzaDough=Enum('PizzaDough','thinthick')PizzaSauce=Enum('PizzaSauce','tomatocreme_fraiche')PizzaTopping=Enum('PizzaTopping','mozzarelladouble_mozzarellabaconhammushroomsred_onionoregano')STEP_DELAY=3#insecondsforthesakeoftheexample

Ourendproductisapizza,whichisdescribedbythePizzaclass.WhenusingtheBuilderpattern,theendproductdoesnothavemanyresponsibilities,sinceitisnotsupposedtobeinstantiateddirectly.Abuildercreatesaninstanceoftheendproductandmakessurethatitisproperlyprepared.That'swhythePizzaclassissominimal.Itbasicallyinitializesalldatatosanedefaultvalues.Anexceptionistheprepare_dough()method.Theprepare_dough()methodisdefinedinthePizzaclassinsteadofabuilderfortworeasons:

ToclarifythefactthattheendproductistypicallyminimaldoesnotmeanthatyoushouldneverassignitanyresponsibilitiesTopromotecodereusethroughcomposition[GOF95,page32]

classPizza:def__init__(self,name):self.name=nameself.dough=Noneself.sauce=Noneself.topping=[]

def__str__(self):returnself.name

defprepare_dough(self,dough):self.dough=doughprint('preparingthe{}doughofyour{}...'.format(self.dough.name,self))time.sleep(STEP_DELAY)

http://j.mp/pytenum

print('donewiththe{}dough'.format(self.dough.name))

Therearetwobuilders:oneforcreatingamargaritapizza(MargaritaBuilder)andanotherforcreatingacreamybaconpizza(CreamyBaconBuilder).EachbuildercreatesaPizzainstanceandcontainsmethodsthatfollowthepizza-makingprocedure:prepare_dough(),add_sauce(),add_topping(),andbake().Tobeprecise,prepare_dough()isjustawrappertotheprepare_dough()methodofthePizzaclass.Noticehoweachbuildertakescareofallthepizza-specificdetails.Forexample,thetoppingofthemargaritapizzaisdoublemozzarellaandoregano,whilethetoppingofthecreamybaconpizzaismozzarella,bacon,ham,mushrooms,redonion,andoreganoasfollows:

classMargaritaBuilder:def__init__(self):self.pizza=Pizza('margarita')self.progress=PizzaProgress.queuedself.baking_time=5#insecondsforthesakeoftheexample

defprepare_dough(self):self.progress=PizzaProgress.preparationself.pizza.prepare_dough(PizzaDough.thin)

defadd_sauce(self):print('addingthetomatosaucetoyourmargarita...')self.pizza.sauce=PizzaSauce.tomatotime.sleep(STEP_DELAY)print('donewiththetomatosauce')

defadd_topping(self):print('addingthetopping(doublemozzarella,oregano)toyourmargarita')self.pizza.topping.append([iforiin(PizzaTopping.double_mozzarella,PizzaTopping.oregano)])time.sleep(STEP_DELAY)print('donewiththetopping(doublemozzarella,oregano)')

defbake(self):self.progress=PizzaProgress.bakingprint('bakingyourmargaritafor{}seconds'.format(self.baking_time))time.sleep(self.baking_time)self.progress=PizzaProgress.readyprint('yourmargaritaisready')

classCreamyBaconBuilder:def__init__(self):self.pizza=Pizza('creamybacon')self.progress=PizzaProgress.queuedself.baking_time=7#insecondsforthesakeoftheexample

defprepare_dough(self):self.progress=PizzaProgress.preparationself.pizza.prepare_dough(PizzaDough.thick)

defadd_sauce(self):print('addingthecrèmefraîchesaucetoyourcreamybacon')

self.pizza.sauce=PizzaSauce.creme_fraichetime.sleep(STEP_DELAY)print('donewiththecrèmefraîchesauce')

defadd_topping(self):print('addingthetopping(mozzarella,bacon,ham,mushrooms,redonion,oregano)toyourcreamybacon')self.pizza.topping.append([tfortin(PizzaTopping.mozzarella,PizzaTopping.bacon,PizzaTopping.ham,PizzaTopping.mushrooms,PizzaTopping.red_onion,PizzaTopping.oregano)])time.sleep(STEP_DELAY)print('donewiththetopping(mozzarella,bacon,ham,mushrooms,redonion,oregano)')

defbake(self):self.progress=PizzaProgress.bakingprint('bakingyourcreamybaconfor{}seconds'.format(self.baking_time))time.sleep(self.baking_time)self.progress=PizzaProgress.readyprint('yourcreamybaconisready')

Thedirectorinthisexampleisthewaiter.ThecoreoftheWaiterclassistheconstruct_pizza()method,whichacceptsabuilderasaparameterandexecutesallthepizzapreparationstepsintherightorder.Choosingtheappropriatebuilder,whichcanevenbedoneinruntime,givesustheabilitytocreatedifferentpizzastyleswithoutmodifyinganycodeofthedirector(Waiter).TheWaiterclassalsocontainsthepizza()method,whichreturnstheendproduct(preparedpizza)asavariabletothecallerasfollows:

classWaiter:def__init__(self):self.builder=None

defconstruct_pizza(self,builder):self.builder=builder[step()forstepin(builder.prepare_dough,builder.add_sauce,builder.add_topping,builder.bake)]

@propertydefpizza(self):returnself.builder.pizza

Thevalidate_style()functionissimilartothevalidate_age()functionasdescribedinChapter1,TheFactoryPattern.Itisusedtomakesurethattheusergivesvalidinput,whichinthiscaseisacharacterthatismappedtoapizzabuilder.ThemcharacterusestheMargaritaBuilderclassandtheccharacterusestheCreamyBaconBuilderclass.Thesemappingsareinthebuilderparameter.Atupleisreturned,withthefirstelementsettoTrueiftheinputisvalid,orFalseifitisinvalidasfollows:

defvalidate_style(builders):try:

pizza_style=input('Whatpizzawouldyoulike,[m]argaritaor[c]reamybacon?')builder=builders[pizza_style]()valid_input=TrueexceptKeyErroraserr:print('Sorry,onlymargarita(keym)andcreamybacon(keyc)areavailable')return(False,None)return(True,builder)

Thelastpartisthemain()function.Themain()functioncontainsacodeforinstantiatingapizzabuilder.ThepizzabuilderisthenusedbytheWaiterdirectorforpreparingthepizza.Thecreatedpizzacanbedeliveredtotheclientatanylaterpoint:

defmain():builders=dict(m=MargaritaBuilder,c=CreamyBaconBuilder)valid_input=Falsewhilenotvalid_input:valid_input,builder=validate_style(builders)print()waiter=Waiter()waiter.construct_pizza(builder)pizza=waiter.pizzaprint()print('Enjoyyour{}!'.format(pizza))

Toputallthesethingstogether,here'sthecompletecodeofthisexample(builder.py):

fromenumimportEnumimporttime

PizzaProgress=Enum('PizzaProgress','queuedpreparationbakingready')PizzaDough=Enum('PizzaDough','thinthick')PizzaSauce=Enum('PizzaSauce','tomatocreme_fraiche')PizzaTopping=Enum('PizzaTopping','mozzarelladouble_mozzarellabaconhammushroomsred_onionoregano')STEP_DELAY=3#insecondsforthesakeoftheexample

classPizza:def__init__(self,name):self.name=nameself.dough=Noneself.sauce=Noneself.topping=[]

def__str__(self):returnself.name

defprepare_dough(self,dough):self.dough=doughprint('preparingthe{}doughofyour{}...'.format(self.dough.name,self))time.sleep(STEP_DELAY)print('donewiththe{}dough'.format(self.dough.name))

classMargaritaBuilder:def__init__(self):self.pizza=Pizza('margarita')self.progress=PizzaProgress.queuedself.baking_time=5#insecondsforthesakeoftheexample

defprepare_dough(self):self.progress=PizzaProgress.preparationself.pizza.prepare_dough(PizzaDough.thin)

defadd_sauce(self):print('addingthetomatosaucetoyourmargarita...')self.pizza.sauce=PizzaSauce.tomatotime.sleep(STEP_DELAY)print('donewiththetomatosauce')

defadd_topping(self):print('addingthetopping(doublemozzarella,oregano)toyourmargarita')self.pizza.topping.append([iforiin(PizzaTopping.double_mozzarella,PizzaTopping.oregano)])time.sleep(STEP_DELAY)print('donewiththetopping(doublemozzarrella,oregano)')

defbake(self):self.progress=PizzaProgress.bakingprint('bakingyourmargaritafor{}seconds'.format(self.baking_time))time.sleep(self.baking_time)self.progress=PizzaProgress.readyprint('yourmargaritaisready')

classCreamyBaconBuilder:def__init__(self):self.pizza=Pizza('creamybacon')self.progress=PizzaProgress.queuedself.baking_time=7#insecondsforthesakeoftheexample

defprepare_dough(self):self.progress=PizzaProgress.preparationself.pizza.prepare_dough(PizzaDough.thick)

defadd_sauce(self):print('addingthecrèmefraîchesaucetoyourcreamybacon')self.pizza.sauce=PizzaSauce.creme_fraichetime.sleep(STEP_DELAY)print('donewiththecrèmefraîchesauce')

defadd_topping(self):print('addingthetopping(mozzarella,bacon,ham,mushrooms,redonion,oregano)toyourcreamybacon')self.pizza.topping.append([tfortin(PizzaTopping.mozzarella,PizzaTopping.bacon,PizzaTopping.ham,PizzaTopping.mushrooms,PizzaTopping.red_onion,PizzaTopping.oregano)])time.sleep(STEP_DELAY)print('donewiththetopping(mozzarella,bacon,ham,mushrooms,redonion,oregano)')

defbake(self):self.progress=PizzaProgress.bakingprint('bakingyourcreamybaconfor{}seconds'.format(self.baking_time))time.sleep(self.baking_time)self.progress=PizzaProgress.readyprint('yourcreamybaconisready')

classWaiter:def__init__(self):self.builder=None

defconstruct_pizza(self,builder):self.builder=builder[step()forstepin(builder.prepare_dough,builder.add_sauce,builder.add_topping,builder.bake)]

@propertydefpizza(self):returnself.builder.pizza

defvalidate_style(builders):try:pizza_style=input('Whatpizzawouldyoulike,[m]argaritaor[c]reamybacon?')builder=builders[pizza_style]()valid_input=TrueexceptKeyErroraserr:print('Sorry,onlymargarita(keym)andcreamybacon(keyc)areavailable')return(False,None)return(True,builder)

defmain():builders=dict(m=MargaritaBuilder,c=CreamyBaconBuilder)valid_input=Falsewhilenotvalid_input:valid_input,builder=validate_style(builders)print()waiter=Waiter()waiter.construct_pizza(builder)pizza=waiter.pizzaprint()print('Enjoyyour{}!'.format(pizza))

if__name__=='__main__':main()

Asampleoutputofthisexampleisasfollows:

>>>python3builder.pyWhatpizzawouldyoulike,[m]argaritaor[c]reamybacon?rSorry,onlymargarita(keym)andcreamybacon(keyc)areavailableWhatpizzawouldyoulike,[m]argaritaor[c]reamybacon?m

preparingthethindoughofyourmargarita...donewiththethindoughaddingthetomatosaucetoyourmargarita...

donewiththetomatosauceaddingthetopping(doublemozzarella,oregano)toyourmargaritadonewiththetopping(doublemozzarella,oregano)bakingyourmargaritafor5secondsyourmargaritaisready

Enjoyyourmargarita!

Supportingonlytwopizzatypesisashame.ImplementaHawaiianpizzabuilder.Considerusinginheritanceafterthinkingabouttheadvantagesanddisadvantages.ChecktheingredientsofatypicalHawaiianpizzaanddecidewhichclassyouneedtoextend:MargaritaBuilderorCreamyBaconBuilder?Perhapsboth[j.mp/pymulti]?

Inthebook,EffectiveJava(2ndedition),JoshuaBlochdescribesaninterestingvariationoftheBuilderpatternwherecallstobuildermethodsarechained.Thisisaccomplishedbydefiningthebuilderitselfasaninnerclassandreturningitselffromeachofthesetter-likemethodsonit.Thebuild()methodreturnsthefinalobject.ThispatterniscalledtheFluentBuilder.Here'saPythonimplementation,whichwaskindlyprovidedbyareviewerofthebook:

classPizza:def__init__(self,builder):self.garlic=builder.garlicself.extra_cheese=builder.extra_cheese

def__str__(self):garlic='yes'ifself.garlicelse'no'cheese='yes'ifself.extra_cheeseelse'no'info=('Garlic:{}'.format(garlic),'Extracheese:{}'.format(cheese))return'\n'.join(info)

classPizzaBuilder:def__init__(self):self.extra_cheese=Falseself.garlic=False

defadd_garlic(self):self.garlic=Truereturnself

defadd_extra_cheese(self):self.extra_cheese=Truereturnself

defbuild(self):returnPizza(self)

if__name__=='__main__':pizza=Pizza.PizzaBuilder().add_garlic().add_extra_cheese().build()print(pizza)

AdaptthepizzaexampletomakeuseoftheFluentBuilderpattern.Whichversionofthetwodoyouprefer?Whataretheprosandconsofeachversion?

http://j.mp/pymulti

SummaryInthischapter,wehaveseenhowtousetheBuilderdesignpattern.WeusetheBuilderpatternforcreatinganobjectinsituationswhereusingtheFactorypattern(eitheraFactoryMethodoranAbstractFactory)isnotagoodoption.ABuilderpatternisusuallyabettercandidatethanaFactorypatternwhen:

Wewanttocreateacomplexobject(anobjectcomposedofmanypartsandcreatedindifferentstepsthatmightneedtofollowaspecificorder).Differentrepresentationsofanobjectarerequired,andwewanttokeeptheconstructionofanobjectdecoupledfromitsrepresentationWewanttocreateanobjectatonepointintimebutaccessitatalaterpoint

WesawhowtheBuilderpatternisusedinfast-foodrestaurantsforpreparingmeals,andhowtwothird-partyDjangopackages,django-widgyanddjango-query-builder,useitforgeneratingHTMLpagesanddynamicSQLqueries,respectively.WefocusedonthedifferencesbetweenaBuilderpatternandaFactorypattern,andgaveapreconfigured(Factory)versuscustomer(Builder)computerorderanalogytoclarifythem.

Intheimplementationpart,wehaveseenhowtocreateapizzaorderingapplication,whichhaspreparationdependencies.Therearemanyrecommendedinterestingexercisesinthischapter,includingimplementingaFluentBuilder.

Inthenextchapter,youwilllearnaboutthelastcreationaldesignpatterncoveredinthisbook:thePrototypepattern,whichisusedforcloninganobject.

Chapter3.ThePrototypePatternSometimes,weneedtocreateanexactcopyofanobject.Forinstance,assumethatyouwanttocreateanapplicationforstoring,sharing,andediting(suchasmodifying,addingnotes,andremoving)culinaryrecipes.UserBobfindsacakerecipeandaftermakingafewmodificationshethinksthathiscakeisdelicious,andhewantstoshareitwithhisfriend,Alice.Butwhatdoessharingarecipemean?IfBobwantstodosomefurtherexperimentationwithhisrecipeaftersharingitwithAlice,willthenewchangesalsobevisibleinAlice'srecipe?CanBobkeeptwocopiesofthecakerecipe?Hisdeliciouscakerecipeshouldremainunaffectedbyanychangesmadeintheexperimentalcakerecipe.

Suchproblemscanbesolvedbyallowingtheuserstohavemorethanoneindependentcopyofthesamerecipe.Eachcopyiscalledaclone,becauseitisanexactcopyoftheoriginalobjectataspecificpointintime.Thetimeaspectisimportant,sinceitaffectswhattheclonecontains.Forexample,ifBobsharesthecakerecipewithAlicebeforemakinghisownimprovementstoachieveperfection,AlicewillneverbeabletobakeherownversionofthedeliciouscakethatBobcreated!ShewillonlybeabletobaketheoriginalcakerecipefoundbyBob.

Notethedifferencebetweenacopyandareference.Ifwehavetworeferencestothesamecakerecipe,whateverchangesBobmakestotherecipewillbevisibletoAlice'sversionoftherecipe,andviceversa.WhatwewantisbothBobandAlicetohavetheirowncopy,sothattheycanmakeindependentchangeswithoutaffectingeachother'srecipe.Bobactuallyneedstwocopiesofthecakerecipe:thedeliciousversionandtheexperimentalversion.

Thedifferencebetweenareferenceandacopyisshowninthefollowingfigure:

Ontheleftpart,wecanseetworeferences.BothAliceandBobrefertothesamerecipe,whichessentiallymeansthattheyshareitandallmodificationsarevisiblebyboth.Ontherightpart,wecanseetwodifferentcopiesofthesamerecipe.Inthiscase,independentmodificationsareallowedandthechangesofAlicedonotaffectthechangesofBob,andviceversa.

ThePrototypedesignpatternhelpsuswithcreatingobjectclones.Initssimplestversion,thePrototypepatternisjustaclone()functionthatacceptsanobjectasaninputparameterand

returnsacloneofit.InPython,thiscanbedoneusingthecopy.deepcopy()function.Let'sseeanexample.Inthefollowingcode(fileclone.py),therearetwoclasses,AandB.AistheparentclassandBisthederivedclass.Inthemainpart,wecreateaninstanceofclassBb,andusedeepcopy()tocreateacloneofbnamedc.Theresultisthatallthemembersofthehierarchy(atthepointoftimethecloninghappens)arecopiedintheclonec.Asaninterestingexercise,youcantryusingdeepcopy()withcompositioninsteadofinheritancewhichisshowninthefollowingcode:

importcopy

classA:def__init__(self):self.x=18self.msg='Hello'

classB(A):def__init__(self):A.__init__(self)self.y=34

def__str__(self):return'{},{},{}'.format(self.x,self.msg,self.y)

if__name__=='__main__':b=B()c=copy.deepcopy(b)print([str(i)foriin(b,c)])print([iforiin(b,c)])

Whenexecutingclone.pyonmycomputer,Igetthefollowing:

>>>python3clone.py['18,Hello,34','18,Hello,34'][,]

Althoughyouroutputofthesecondlinewillmostlikelynotbethesameasmine,what'simportantistonoticethatthetwoobjectsresideintwodifferentmemoryaddresses(the0x...part).Thismeansthatthetwoobjectsaretwoindependentcopies.

IntheImplementationsection,laterinthischapter,wewillseehowtousecopy.deepcopy()withsomeextraboilerplatecodewrappedinaclass,forkeepingaregistryoftheobjectsthatarecloned.

Areal-lifeexampleThePrototypedesignpatternisallaboutcloninganobject.Mitosis,theprocessinacelldivisionbywhichthenucleusdividesresultingintwonewnuclei,eachofwhichhasexactlythesamechromosomeandDNAcontentastheoriginalcell,isanexampleofbiologicalcloning[j.mp/mmitosis].

http://j.mp/mmitosis

Thefollowingfigure,providedbywww.sourcemaking.com,showsanexampleofthemitoticdivisionofacell[j.mp/pprotpat]:

Anotherpopularexampleof(artificial)cloningisDolly,thesheep[j.mp/wikidolly].

http://www.sourcemaking.comhttp://j.mp/pprotpathttp://j.mp/wikidolly

AsoftwareexampleTherearemanyPythonapplicationsthatmakeuseofthePrototypepattern[j.mp/pythonprot],butitisalmostneverreferredtoasPrototypesincecloningobjectsisabuilt-infeatureofthelanguage.

OneapplicationthatusesPrototypeistheVisualizationToolkit(VTK)[j.mp/pyvto].VTKisanopensourcecross-platformsystemfor3Dcomputergraphics,imageprocessing,andvisualization.VTKusesPrototypeforcreatingclonesofgeometricalelementssuchaspoints,lines,hexahedrons,andsoforth[j.mp/vtkcell].

AnotherprojectthatusesPrototypeismusic21.Accordingtotheproject'spage,"music21isasetoftoolsforhelpingscholarsandotheractivelistenersanswerquestionsaboutmusicquicklyandsimply"[j.mp/pmusic21].Themusic21toolkitusesPrototypeforcopyingmusicalnotesandscores[j.mp/py21code].

http://j.mp/pythonprothttp://j.mp/pyvtohttp://j.mp/vtkcellhttp://j.mp/pmusic21http://j.mp/py21code

UsecasesThePrototypepatternisusefulwhenwehaveanexistingobjectandwewanttocreateanexactcopyofit.Acopyofanobjectisusuallyrequiredwhenweknowthatpartsoftheobjectwillbemodifiedbutwewanttokeeptheoriginalobjectuntouched.Insuchcases,itdoesn'tmakesensetorecreatetheoriginalobjectfromscratch[j.mp/protpat].

AnothercasewherePrototypecomesinhandyiswhenwewanttoduplicateacomplexobject.Byduplicatingacomplexobject,wecanthinkofanobjectthatispopulatedfromadatabaseandhasreferencestootherobjectsthatarealsopopulatedfromadatabase.Itisalotofefforttocreateanobjectclonebyqueryingthedatabase(s)multipletimesagain.UsingPrototypeforsuchcasesismoreconvenient.

Sofar,wehavecoveredonlythereferenceversuscopyissue,butacopycanbefurtherdividedintoadeepcopyversusashallowcopy.Adeepcopyiswhatwehaveseensofar:alldataoftheoriginalobjectaresimplycopiedintheclone,withoutmakinganyexceptions.Ashallowcopyreliesonreferences.Wecanintroducedatasharing,andtechniqueslikecopy-on-writetoimprovetheperformance(suchasclonecreationtime)andthememoryusage.Usingshallowcopiesmightbeworthwhileiftheavailableresourcesarelimited(suchasembeddedsystems)orperformanceiscritical(suchashigh-performancecomputing).

InPython,wecandoshallowcopiesusingthecopy.copy()function.QuotingtheofficialPythondocumentation,thedifferencesbetweenashallowcopy(copy.copy())andadeepcopy(copy.deepcopy())inPythonare[j.mp/py3copy]asfollows:

"Ashallowcopyconstructsanewcompoundobjectandthen(totheextentpossible)insertsreferencesintoittotheobjectsfoundintheoriginal.Adeepcopyconstructsanewcompoundobjectandthen,recursively,insertscopiesintoitoftheobjectsfoundintheoriginal."

Canyouthinkofanyexampleswhereusingshallowcopiesisbetterthanusingdeepcopies?

http://j.mp/protpathttp://j.mp/py3copy

ImplementationInprogramming,itisnotuncommonforabooktobeavailableinmultipleeditions.Forexample,theclassictextbookonCprogrammingTheCProgrammingLanguagebyKernighanandRitchieisavailableintwoeditions.Thefirsteditionwaspublishedin1978.Atthattime,Cwasnotstandardized.Thesecondeditionofthebookwaspublished10yearslaterandcoversthestandard(ANSI)versionofC.Whatarethedifferencesbetweenthetwoeditions?Tomentionafew,theprice,thelength(numberofpages),andthepublicationdate.Buttherearealsomanysimilarities:theauthors,thepublishers,andthetags/keywordsthatdescribethebookareexactlythesame.Thisindicatesthatcreatinganewbookfromscratchisnotalwaysthebestapproach.Ifweknowthattherearemanysimilaritiesbetweentwobookeditions,wecanusecloningandmodifyonlythedifferentpartsofthenewedition.

Let'sseehowwecanusethePrototypepatternforcreatinganapplicationthatshowsbookinformation.Webeginwiththerepresentationofabook.Apartfromtheusualinitialization,theBookclassdemonstratesaninterestingtechnique.Itshowshowwecanavoidthetelescopicconstructorproblem.Inthe__init__()method,onlythreeparametersarefixed:name,authors,andprice.Butclientscanpassmoreparametersintheformofkeywords(name=value)usingtherestvariable-lengthlist.Thelineself.__dict__.update(rest)addsthecontentsofresttotheinternaldictionaryoftheBookclasstomakethempartofit.

Butthere'sacatch.Sincewedon'tknowallthenamesoftheaddedparameters,weneedtoaccesstheinternaldictformakinguseofthemin__str__().Andsincethecontentsofadictionarydonotfollowanyspecificorder,weuseanOrderedDicttoforceanorder;otherwise,everytimetheprogramisexecuted,differentoutputswillbeshown.Ofcourse,youshouldnottakemywordsforgranted.Asanexercise,removetheusageofOrderedDictandsorted()andruntheexampletoseeifI'mright:

classBook:def__init__(self,name,authors,price,**rest):'''Examplesofrest:publisher,length,tags,publicationdate'''self.name=nameself.authors=authorsself.price=price#inUSdollarsself.__dict__.update(rest)

def__str__(self):mylist=[]ordered=OrderedDict(sorted(self.__dict__.items()))foriinordered.keys():mylist.append('{}:{}'.format(i,ordered[i]))ifi=='price':mylist.append('$')mylist.append('\n')return''.join(mylist)

ThePrototypeclassimplementsthePrototypedesignpattern.TheheartofthePrototypeclassistheclone()method,whichdoestheactualcloningusingthefamiliarcopy.deepcopy()

function.ButthePrototypeclassdoesabitmorethansupportingcloning.Itcontainstheregister()andunregister()methods,whichcanbeusedtokeeptrackoftheobjectsthatareclonedinadictionary.Notethatthisisjustaconvenience,andnotanecessity.

Moreover,theclone()methodusesthesametrickthat__str__()usesintheBookclass,butthistimeforadifferentreason.Usingthevariable-lengthlistattr,wecanpassonlythevariablesthatreallyneedtobemodifiedwhencloninganobjectasfollows:

classPrototype:def__init__(self):self.objects=dict()

defregister(self,identifier,obj):self.objects[identifier]=obj

defunregister(self,identifier):delself.objects[identifier]

defclone(self,identifier,**attr):found=self.objects.get(identifier)ifnotfound:raiseValueError('Incorrectobjectidentifier:{}'.format(identifier))obj=copy.deepcopy(found)obj.__dict__.update(attr)returnobj

Themain()functionshowsTheCProgrammingLanguagebookcloningexamplementionedatthebeginningofthissectioninpractice.Whencloningthefirsteditionofthebooktocreatethesecondedition,weonlyneedtopassthemodifiedvaluesoftheexistingparameters.Butwecanalsopassextraparameters.Inthiscase,editionisanewparameterthatwasnotneededinthefirstbookbutisusefulinformationfortheclone:

defmain():b1=Book('TheCProgrammingLanguage',('BrianW.Kernighan','DennisM.Ritchie'),price=118,publisher='PrenticeHall',length=228,publication_date='1978-02-22',tags=('C','programming','algorithms','datastructures'))

prototype=Prototype()cid='k&r-first'prototype.register(cid,b1)b2=prototype.clone(cid,name='TheCProgrammingLanguage(ANSI)',price=48.99,length=274,publication_date='1988-04-01',edition=2)

foriin(b1,b2):print(i)print("IDb1:{}!=IDb2:{}".format(id(b1),id(b2)))

Noticetheusageoftheid()functionwhichreturnsthememoryaddressofanobject.Whenwecloneanobjectusingadeepcopy,thememoryaddressesoftheclonemustbedifferentfromthememoryaddressesoftheoriginalobject.

Theprototype.pyfileisasfollows:

importcopyfromcollectionsimportOrderedDict

classBook:def__init__(self,name,authors,price,**rest):'''Examplesofrest:publisher,length,tags,publicationdate'''self.name=nameself.authors=authorsself.price=price#inUSdollarsself.__dict__.update(rest)

def__str__(self):mylist=[]ordered=OrderedDict(sorted(self.__dict__.items()))foriinordered.keys():mylist.append('{}:{}'.format(i,ordered[i]))ifi=='price':mylist.append('$')mylist.append('\n')return''.join(mylist)

classPrototype:def__init__(self):self.objects=dict()

defregister(self,identifier,obj):self.objects[identifier]=obj

defunregister(self,identifier):delself.objects[identifier]

defclone(self,identifier,**attr):found=self.objects.get(identifier)ifnotfound:raiseValueError('Incorrectobjectidentifier:{}'.format(identifier))obj=copy.deepcopy(found)obj.__dict__.update(attr)returnobj

defmain():b1=Book('TheCProgrammingLanguage',('BrianW.Kernighan','DennisM.Ritchie'),price=118,publisher='PrenticeHall',length=228,publication_date='1978-02-22',tags=('C','programming','algorithms','datastructures'))

prototype=Prototype()cid='k&r-first'prototype.register(cid,b1)b2=prototype.clone(cid,name='TheCProgrammingLanguage(ANSI)',price=48.99,length=274,publication_date='1988-04-01',edition=2)

foriin(b1,b2):print(i)print("IDb1:{}!=IDb2:{}".format(id(b1),id(b2)))

if__name__=='__main__':main()

Theoutputofid()dependsonthecurrentmemoryallocationofthecomputerandyoushouldexpectittodifferoneveryexecutionofthisprogram.Butnomatterwhattheactualaddressesare,theyshouldnotbethesameinanychance.

AsampleoutputwhenIexecutethisprogramonmymachineisasfollows:

>>>python3prototype.pyauthors:('BrianW.Kernighan','DennisM.Ritchie')length:228name:TheCProgrammingLanguageprice:118$publication_date:1978-02-22publisher:PrenticeHalltags:('C','programming','algorithms','datastructures')

authors:('BrianW.Kernighan','DennisM.Ritchie')edition:2length:274name:TheCProgrammingLanguage(ANSI)price:48.99$publication_date:1988-04-01publisher:PrenticeHalltags:('C','programming','algorithms','datastructures')

IDb1:140004970829304!=IDb2:140004970829472

Indeed,Prototypeworksasexpected.ThesecondeditionofTheCProgrammingLanguagebookreusesalltheinformationthatwassetinthefirstedition,andallthedifferencesthatwedefinedareonlyappliedtothesecondedition.Thefirsteditionremainsunaffected.Ourconfidencecanbeincreasedbylookingattheoutputoftheid()function:thetwoaddressesaredifferent.

Asanexercise,youcancomeupwithyourownexampleofPrototype.Afewideasareasfollows:

TherecipeexamplethatwasmentionedinthischapterThedatabase-populatedobjectthatwasmentionedinthischapterCopyinganimagesothatyoucanaddyourownmodificationswithouttouchingtheoriginal

SummaryInthischapter,wehaveseenhowtousethePrototypedesignpattern.Prototypeisusedforcreatingexactcopiesofobjects.Creatingacopyofanobjectcanactuallymeantwothings:

Relyingonreferences,whichhappenswhenashallowcopyiscreatedDuplicatingeverything,whichhappenswhenadeepcopyiscreated

Inthefirstcase,wewanttofocusonimprovingtheperformanceandthememoryusageofourapplicationbyintroducingdatasharingbetweenobjects.Butweneedtobecarefulaboutmodifyingdata,becauseallmodificationsarevisibletoallcopies.Shallowcopieswerenotintroducedinthischapter,butyoumightwanttoexperimentwiththem.

Inthesecondcase,wewanttobeabletomakemodificationstoonecopywithoutaffectingtherest.That'susefulforcaseslikethecake-recipeexamplethatwehaveseen.Here,nodatasharingisdoneandsoweneedtobecarefulabouttheresourceconsumptionandtheoverheadthatisintroducedbyourclones.

WeshowedasimpleexampleofadeepcopyingwhichinPythonisdoneusingthecopy.deepcopy()function.Wealsomentionedexamplesofcloningfoundinreallife,focusingonmitosis.

ManysoftwareprojectsusePrototype,butinPythonitisnotmentionedassuchbecauseitisabuilt-infeature.AmongthemaretheVTK,whichusesPrototypeforcreatingclonesofgeometricalelements,andmusic21,whichusesitforduplicatingmusicalscoresandnotes.

Finally,wediscussedtheusecasesofPrototypeandimplementedaprogramthatsupportscloningbookssothatallinformationthatdoesnotchangeinaneweditioncanbereused,butatthesametimemodifiedinformationcanbeupdatedandnewinformationcanbeadded.

Prototypeisthelastcreationaldesignpatterncoveredinthisbook.ThenextchapterbeginswithAdapter,astructuraldesignpatternthatcanbeusedtomaketwoincompatiblesoftwareinterfacescompatible.

Chapter4.TheAdapterPatternStructuraldesignpatternsdealwiththerelationshipsbetweentheentities(suchasclassesandobjects)ofasystem.Astructuraldesignpatternfocusesonprovidingasimplewayofcomposingobjectsforcreatingnewfunctionality[GOF95,page155],[j.mp/structpat].

Adapterisastructuraldesignpatternthathelpsusmaketwoincompatibleinterfacescompatible.First,let'sanswerwhatincompatibleinterfacesreallymean.Ifwehaveanoldcomponentandwewanttouseitinanewsystem,oranewcomponentthatwewanttouseinanoldsystem,thetwocanrarelycommunicatewithoutrequiringanycodechanges.Butchangingthecodeisnotalwayspossible,eitherbecausewedon'thaveaccesstoit(forexample,thecomponentisprovidedasanexternallibrary)orbecauseitisimpractical.Insuchcases,wecanwriteanextralayerthatmakesalltherequiredmodificationsforenablingthecommunicationbetweenthetwointerfaces.ThislayeriscalledtheAdapter.

E-commercesystemsareknownexamples.Assumethatweuseane-commercesystemthatcontainsacalculate_total(order)function.Thefunctioncalculatesthetotalamountofanorder,butonlyinDanishKroner(DKK).Itisreasonableforourcustomerstoaskustoaddsupportformorepopularcurrencies,suchasUnitedStatesDollars(USD)andEuros(EUR).IfweownthesourcecodeofthesystemwecanextenditbyaddingnewfunctionsfordoingtheconversionsfromDKKtoUSDandfromDKKtoEUR.Butwhatifwedon'thaveaccesstothesourcecodeoftheapplicationbecauseitisprovidedtousonlyasanexternallibrary?Inthiscase,wecanstillusethelibrary(forexample,callitsmethods),butwecannotmodify/extendit.Thesolutionistowriteawrapper(alsoknownasAdapter)thatconvertsthedatafromthegivenDKKformattotheexpectedUSDorEURformat.

TheAdapterpatternisnotusefulonlyfordataconversions.Ingeneral,ifyouwanttouseaninterfacethatexpectsfunction_a()butyouonlyhavefunction_b(),youcanuseanAdaptertoconvert(adapt)function_b()tofunction_a()[Eckel08,page207],[j.mp/adapterpat].Thisisnotonlytrueforfunctionsbutalsoforfunctionparameters.Anexampleisafunctionthatexpectstheparametersx,y,andzbutyouonlyhaveafunctionthatworkswiththeparametersxandyathand.WewillseehowtousetheAdapterpatternintheimplementationsection.

Areal-lifeexampleProbablyallofususetheAdapterpatterneveryday,butinhardwareinsteadofsoftware.Ifyouhaveasmartphoneoratablet,youneedtousesomething(forexample,thelightningconnectorofaniPhone)withaUSBadapterforconnectingittoyourcomputer.IfyouaretravelingfrommostEuropeancountriestotheUK,youneedtouseaplugadapterforchargingyourlaptop.ThesameistrueifyouaretravelingfromEuropetoUSA,ortheotherwayaround.Adaptersareeverywhere!

Thefollowingimage,courtesyofsourcemaking.com,showsseveralexamplesofhardwareadapters[j.mp/adapterpat]:

http://j.mp/structpathttp://j.mp/adapterpathttp://sourcemaking.comhttp://j.mp/adapterpat

AsoftwareexampleGrokisaPythonframeworkthatrunsontopofZope3andfocusesonagiledevelopment.TheGrokframeworkusesAdaptersformakingitpossibleforexistingobjectstoconformtospecificAPIswithouttheneedtomodifythem[j.mp/grokada].

ThePythonTraitspackagealsousestheAdapterpatternfortransforminganobjectthatdoesnotimplementofaspecificinterface(orsetofinterfaces)toanobjectthatdoes[j.mp/pytraitsad].

http://j.mp/grokadahttp://j.mp/pytraitsad

UsecasesTheAdapterpatternisusedformakingthingsworkaftertheyhavebeenimplemented[j.mp/adapterpat].Usuallyoneofthetwoincompatibleinterfacesiseitherforeignorold/legacy.Iftheinterfaceisforeign,itmeansthatwehavenoaccesstothesourcecode.Ifitisolditisusuallyimpracticaltorefactorit.Wecantakeitevenfurtherandarguethatalteringtheimplementationofalegacycomponenttomeetourneedsisnotonlyimpractical,butitalsoviolatestheopen/closeprinciple[j.mp/adaptsimp].Theopen/closeprincipleisoneofthefundamentalprinciplesofObject-Orienteddesign(theOofSOLID).Itstatesthatasoftwareentityshouldbeopenforextension,butclosedformodification.Thatbasicallymeansthatweshouldbeabletoextendthebehaviorofanentitywithoutmakingsourcecodemodifications.Adapterrespectstheopen/closedprinciple[j.mp/openclosedp].

Therefore,usinganAdapterformakingthingsworkaftertheyhavebeenimplementedisabetterapproachbecauseit:

DoesnotrequireaccesstothesourcecodeoftheforeigninterfaceDoesnotviolatetheopen/closedprinciple

http://j.mp/adapterpathttp://j.mp/adaptsimphttp://j.mp/openclosedp

ImplementationTherearemanywaysofimplementingtheAdapterdesignpatterninPython[Eckel08,page207].AllthetechniquesdemonstratedbyBruceEckeluseinheritance,butPythonprovidesanalternative,andinmyopinion,amoreidiomaticwayofimplementinganAdapter.Thealternativetechniqueshouldbefamiliartoyou,sinceitusestheinternaldictionaryofaclass,andwehaveseenhowtodothatinChapter3,ThePrototypePattern.

Let'sbeginwiththewhatwehavepart.OurapplicationhasaComputerclassthatshowsbasicinformationaboutacomputer.Alltheclassesofthisexample,includingtheComputerclassareveryprimitive,becausewewanttofocusontheAdapterpatternandnotonhowtomakeaclassascompleteaspossible.

classComputer:def__init__(self,name):self.name=name

def__str__(self):return'the{}computer'.format(self.name)

defexecute(self):return'executesaprogram'

Inthiscase,theexecute()methodisthemainactionthatthecomputercanperform.Thismethodiscalledbytheclientcode.

Nowwemovetothewhatwewantpart.Wedecidetoenrichourapplicationwithmorefunctionality,andluckily,wefindtwointerestingclassesimplementedintwodifferentlibrariesthatareunrelatedwithourapplication:SynthesizerandHuman.IntheSynthesizerclass,themainactionisperformedbytheplay()method.IntheHumanclass,itisperformedbythespeak()method.Toindicatethatthetwoclassesareexternal,weplacetheminaseparatemodule,asshown:

classSynthesizer:def__init__(self,name):self.name=name

def__str__(self):return'the{}synthesizer'.format(self.name)

defplay(self):return'isplayinganelectronicsong'

classHuman:def__init__(self,name):self.name=name

def__str__(self):return'{}thehuman'.format(self.name)

defspeak(self):

return'sayshello'

Sofarsogood.But,wehaveaproblem.Theclientonlyknowshowtocalltheexecute()method,andithasnoideaaboutplay()orspeak().HowcanwemakethecodeworkwithoutchangingtheSynthesizerandHumanclasses?Adapterstotherescue!WecreateagenericAdapterclassthatallowsustoadaptanumberofobjectswithdifferentinterfaces,intooneunifiedinterface.Theobjargumentofthe__init__()methodistheobjectthatwewanttoadapt,andadapted_methodsisadictionarycontainingkey/valuepairsofmethodtheclientcalls/methodthatshouldbecalled.

classAdapter:def__init__(self,obj,adapted_methods):self.obj=objself.__dict__.update(adapted_methods)

def__str__(self):returnstr(self.obj)

Let'sseehowwecanusetheAdapterpattern.Anobjectslistholdsalltheobjects.ThecompatibleobjectsthatbelongtotheComputerclassneednoadaptation.Wecanaddthemdirectlytothelist.Theincompatibleobjectsarenotaddeddirectly.TheyareadaptedusingtheAdapterclass.Theresultisthattheclientcodecancontinueusingtheknownexecute()methodonallobjectswithouttheneedtobeawareofanyinterfacedifferencesbetweentheusedclasses.

defmain():objects=[Computer('Asus')]synth=Synthesizer('moog')objects.append(Adapter(synth,dict(execute=synth.play)))human=Human('Bob')objects.append(Adapter(human,dict(execute=human.speak)))

foriinobjects:print('{}{}'.format(str(i),i.execute()))

Let'sseethecompletecodeoftheAdapterpatternexample(filesexternal.pyandadapter.py)asfollows:

classSynthesizer:def__init__(self,name):self.name=name

def__str__(self):return'the{}synthesizer'.format(self.name)

defplay(self):return'isplayinganelectronicsong'

classHuman:def__init__(self,name):self.name=name

def__str__(self):return'{}thehuman'.format(self.name)

defspeak(self):return'sayshello'

fromexternalimportSynthesizer,Human

classComputer:def__init__(self,name):self.name=name

def__str__(self):return'the{}computer'.format(self.name)

defexecute(self):return'executesaprogram'

classAdapter:def__init__(self,obj,adapted_methods):self.obj=objself.__dict__.update(adapted_methods)

def__str__(self):returnstr(self.obj)

defmain():objects=[Computer('Asus')]synth=Synthesizer('moog')objects.append(Adapter(synth,dict(execute=synth.play)))human=Human('Bob')objects.append(Adapter(human,dict(execute=human.speak)))

foriinobjects:print('{}{}'.format(str(i),i.execute()))

if__name__=="__main__":main()

Theoutputwhenexecutingtheexampleis:

>>>python3adapter.pytheAsuscomputerexecutesaprogramthemoogsynthesizerisplayinganelectronicsongBobthehumansayshello

WemanagedtomaketheHumanandSynthesizerclassescompatiblewiththeinterfaceexpectedbytheclient,withoutchangingtheirsourcecode.Thisisnice.

Here'sachallengingexerciseforyou.Thereisaproblemwiththisimplementation.Whileallclasseshaveanameattribute,thefollowingcodefails:

foriinobjects:print(i.name)

Firstofall,whydoesthiscodefail?Althoughthismakessensefromacodingpointofview,itdoesnotmakesenseatallfortheclientcodewhichshouldnotbeawareofdetailssuchaswhatisadaptedandwhatisnotadapted.Wejustwanttoprovideauniforminterface.Howcanwemakethiscodework?

TipHint:Thinkofhowyoucandelegatethenon-adaptedpartstotheobjectcontainedintheAdapterclass.

SummaryThischaptercoveredtheAdapterdesignpattern.WeusetheAdapterpatternformakingtwo(ormore)incompatibleinterfacescompatible.Asamotivation,ane-commercesystemthatshouldsupportmultiplecurrencieswasmentioned.Weuseadapterseverydayforinterconnectingdevices,chargingthem,andsoon.

Adaptermakesthingsworkaftertheyhavebeenimplemented.TheGrokPythonframeworkandtheTraitspackageusetheAdapterpatternforachievingAPIconformanceandinterfacecompatibility,respectively.Theopen/closeprincipleisstronglyconnectedwiththeseaspects.

Intheimplementationsection,wesawhowtoachieveinterfaceconformanceusingtheAdapterpatternwithoutmodifyingthesourcecodeoftheincompatiblemodel.ThisisachievedthroughagenericAdapterclassthatdoestheworkforus.Althoughwecouldusesub-classing(inheritance)toimplementtheAdapterpatterninthetraditionalwayinPython,thistechniqueisagreatalternative.

Inthenextchapter,wewillseehowwecanusetheDecoratorpatterntoextendthebehaviorofanobjectwithoutusingsub-classing.

Chapter5.TheDecoratorPatternWheneverwewanttoaddextrafunctionalitytoanobject,wehaveanumberofdifferentoptions.Wecan:

Addthefunctionalitydirectlytotheclasstheobjectbelongsto,ifitmakessense(forexample,addanewmethod)UsecompositionUseinheritance

Compositionshouldgenerallybepreferredoverinheritance,becauseinheritancemakescodereuseharder,it'sstatic,andappliestoanentireclassandallinstancesofit[GOF95,page31],[j.mp/decopat].

Designpatternsofferusafourthoptionthatsupportsextendingthefunctionalityofanobjectdynamically(inruntime):Decorators.ADecoratorpatterncanaddresponsibilitiestoanobjectdynamically,andinatransparentmanner(withoutaffectingotherobjects)[GOF95,page196].

Inmanyprogramminglanguages,theDecoratorpatternisimplementedusingsub-classing(inheritance)[GOF95,page198].InPython,wecan(andshould)usethebuilt-indecoratorfeature.APythondecoratorisaspecificchangetothesyntaxofPythonthatisusedforextendingthebehaviorofaclass,method,orfunctionwithoutusinginheritance.Intermsofimplementation,aPythondecoratorisacallable(function,method,class)thatacceptsafunctionobjectfinasinput,andreturnsanotherfunctionobjectfout[j.mp/conqdec