development strategies, function design › ~bbm101 › fall15 › lectures › w07-de… · slides...

DevelopmentStrategies,FunctionDesignBBM101- Introduction toProgramming I

Hacettepe UniversityFall2015

FuatAkal,AykutErdem,Erkut Erdem,Vahid Garousi

Today

• Howtodevelopaprogram– Programdevelopmentstrategy

• MoreonTestingandDebugging– Testinganddebugging– Blackboxtesting– Glassboxtesting– Integrationtestingandunittesting– Debuggingapproaches

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Today



Slides basedonmaterialpreparedbyRuthAnderson,MichaelErnstandBillHoweinthecourseCSE140University ofWashington

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Programdevelopmentmethodology:Englishfirst,thenPython

1. Definetheproblem2. Decideuponanalgorithm3. Translateitintocode

Trytodothesestepsinorder

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1. DefinetheproblemA. WriteanEnglishdescriptionoftheinputandoutput

forthewholeprogram.(Donotgivedetailsabouthowyouwillcomputetheoutput.)

B. Createtestcasesforthewholeprogram• Inputand expectedoutput

2. Decideuponanalgorithm3. Translateitintocode


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1. Definetheproblem2. Decideuponanalgorithm

A. ImplementitinEnglish• Writetherecipeor step-by-stepinstructions

B. Testitusingpaperandpencil• Usesmallbutnottrivialtestcases• Playcomputer,animatingthealgorithm• Beintrospective

– Noticewhatyoureallydo– Maybemoreorlessthanwhatyouwrotedown– Makethealgorithmmoreprecise

3. Translateitintocode


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A. ImplementitinPython• Decomposeitintologicalunits(functions)• Foreachfunction:

– Nameit(importantanddifficult!)– Writeitsdocumentationstring(itsspecification)– Writetests– Writeitscode– Testthefunction

B. Testthewholeprogram


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Trytodothesestepsinorder– It’sOK(evencommon)tobackuptoapreviousstep

whenyounoticeaproblem– Youareincrementallylearningabouttheproblem,

thealgorithm,andthecode– “Iterativedevelopment”

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TheWishfulThinkingapproachtoimplementingafunction

• Ifyouarenotsurehowtoimplementonepartofyourfunction,defineahelperfunctionthatdoesthattask– “IwishIknewhowtodotaskX”– Giveitanameandassumethatitworks– Goaheadandcompletetheimplementationofyourfunction,using thehelperfunction(andassumingitworks)

– Later,implementthehelperfunction– Thehelperfunctionshouldhaveasimpler/smallertask

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TheWishfulThinkingapproachtoimplementingafunction

• Canyoutesttheoriginalfunction?– Yes,byusingastub forthehelperfunction– Oftenalookuptable:worksforonly5inputs,crashesotherwise,ormaybejustreturnsthesamevalueeverytime

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Whyfunctions?Thereareseveralreasons:• Creatinganewfunctiongivesyouanopportunitytonameagroup

ofstatements,whichmakesyourprogrameasiertoreadanddebug.

• Functionscanmakeaprogramsmaller byeliminatingrepetitivecode.Later,ifyoumakeachange,youonlyhavetomakeitinoneplace.

• Dividingalongprogramintofunctionsallowsyoutodebugthepartsoneatatimeandthenassemblethemintoaworkingwhole.

• Well-designedfunctionsareoftenusefulformanyprograms.Onceyouwriteanddebugone,youcanreuseit.

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Today



Slides basedonmaterialpreparedbyE.Grimson, J.Guttag andC.Terman inMITx 6.00.1x 12

TestingandDebugging

• Wouldbegreatifourcodealwaysworkedproperlythefirsttimewerunit!

• Butlifeain’t perfect,soweneed:– Testingmethods

• Waysoftryingcodeonexamplestodetermineifrunningcorrectly

– Debuggingmethods• Waysoffixingaprogramthatyouknowdoesnotworkasintended

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Whenshouldyoutestanddebug?

• Designyourcodeforeaseoftestinganddebugging– Breakprogramintocomponentsthatcanbetestedanddebuggedindependently

– Documentconstraintsonmodules• Expectationsoninputs,onoutputs• Evenifcodedoesnotenforceconstraints,valuablefordebuggingtohavedescription

– Documentassumptionsbehindcodedesign

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Whenareyoureadytotest?

• Ensurethatcodewillactuallyrun– Removesyntaxerrors– Removestaticsemanticerrors– BothofthesearetypicallyhandledbythePythoninterpreter

• Haveasetofexpectedresults(i.e.input- outputpairings)ready

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Testing

• Goal:– Showthatbugsexist– Wouldbegreattoprovecodeisbugfree,butgenerallyhard• Usuallycan’trunonallpossibleinputstocheck• Formalmethodssometimeshelp,butusuallyonlyonsimplercode

“Program testing can be used to show the presence ofbugs, but never to show their absence!”

– Edsger Dijkstra

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Testsuite

• Wanttofindacollectionofinputsthathashighlikelihoodofrevealingbugs,yetisefficient– Partitionspaceofinputs intosubsetsthatprovideequivalentinformationaboutcorrectness• Partitiondividesasetintogroupofsubsetssuchthateachelementofsetisinexactlyonesubset

– Constructtestsuite thatcontainsoneinputfromeachelementofpartition

– Runtestsuite

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Exampleofpartitiondef isBigger(x, y):

“““Assumes x and y are intsreturns True if x is less than yelse False”””

• Inputspaceisallpairsofintegers• Possiblepartition

– xpositive,ypositive– xnegative,ynegative– xpositive,ynegative– xnegative,ypositive– x=0,y=0– x=0,y!=0– x!=0,y=0

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Whythispartition?

• Lotsofotherchoices– E.g.,xprime,ynot;yprime,xnot;bothprime;bothnot

• Spaceofinputsoftenhavenaturalboundaries– Integersarepositive,negativeorzero– Fromthisperspective,have9subsets•Splitx=0,y!=0intox=0,ypositiveandx=0,ynegative•Sameforx!=0,y=0

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Partitioning

• Whatifnonaturalpartitiontoinputspace?

– Randomtesting– probabilitythatcodeiscorrectincreaseswithnumberoftrials;butshouldbeabletousecodetodobetter

– Useheuristicsbasedonexploringpathsthroughthespecifications– black-boxtesting

– Useheuristicsbasedonexploringpathsthroughthecode– glass-boxtesting

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Black-boxtesting

• Testsuitedesignedwithoutlookingatcode– Canbedonebysomeoneotherthanimplementer– Willavoidinherentbiasesofimplementer,exposingpotentialbugsmoreeasily

– Testingdesignedwithoutknowledgeofimplementation,thuscanbereusedevenifimplementationchanged

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Pathsthroughaspecificationdef sqrt(x, eps):

“““Assumes x, eps floatsx >= 0eps > 0

returns res such thatx-eps <= res*res <= x+eps”””

• Pathsthroughspecification:– x=0– x>0

• Butclearlynotenough

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Pathsthroughaspecification

• Alsogoodtoconsiderboundarycases– Forlists:emptylist,singletonlist,manyelementlist– Fornumbers,verysmall,verylarge,“typical”

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Example• Foroursqrt case,trythese:– Firstfouraretypical• Perfectsquare• Irrationalsquareroot• Examplelessthan1

– Lastfivetestextremes• Ifbug,mightbecode,ormightbespec(e.g.don’ttrytofindrootifeps tiny)

x eps

0.0 0.0001

25.0 0.0001

.05 0.0001

2.0 0.0001

2.0 1.0/2.0**64.0

1.0/2.0**64.0 1.0/2.0**64.0

2.0**64.0 1.0/2.0**64.0

1.0/2.0**64.0 2.0**64.0

2.0**64.0 2.0**64.0

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Glass-boxTesting

• Usecodedirectlytoguidedesignoftestcases

• Glass-boxtestsuiteispath-completeifeverypotentialpaththroughthecodeistestedatleastonce– Notalwayspossibleifloopcanbeexercisedarbitrarytimes,orrecursioncanbearbitrarilydeep

• Evenpath-completesuitecanmissabug,dependingonchoiceofexamples

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Exampledef abs(x):

“““Assumes x is an intreturns x if x>=0 and –x otherwise”””

if x < -1:return –x

else:return x

• Testsuiteof{-2,2}willbepathcomplete• Butwillmissabs(-1) whichincorrectlyreturns-1– Testingboundarycasesandtypicalcaseswouldcatchthis{-2-1,2}

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Rulesofthumbforglass-boxtesting

• Exercisebothbranchesofallifstatements• Ensureeachexceptclauseisexecuted• Foreachforloop,havetestswhere:

– Loopisnotentered– Bodyofloopexecutedexactlyonce– Bodyofloopexecutedmorethanonce

• Foreachwhileloop,– Samecasesasforloops– Casesthatcatchallwaystoexitloop

• Forrecursivefunctions,testwithnorecursivecalls,onerecursivecall,andmorethanonerecursivecall

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Conductingtests

• Startwithunittesting– Checkthateachmodule(e.g.function)workscorrectly

• Movetointegrationtesting– Checkthatsystemaswholeworkscorrectly

• Cyclebetweenthesephases

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TestDriversandStubs

• Driversarecodethat– Setupenvironmentneededtoruncode– Invokecodeonpredefinedsequenceofinputs– Saveresults,and– Report

• Driverssimulatepartsofprogramthatuseunitbeingtested

• Stubssimulatepartsofprogramusedbyunitbeingtested– Allowyoutotestunitsthatdependonsoftwarenotyetwritten

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Goodtestingpractice

• Startwithunittesting• Movetointegrationtesting• Aftercodeiscorrected,besuretodoregressiontesting:– Checkthatprogramstillpassesallthetestsitusedtopass,i.e.,thatyourcodefixhasn’tbrokensomethingthatusedtowork

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“Most people, if you describe a train of events to them, will tell you what the resultwould be. They can put those events together in their minds, and argue from them thatsomething will come to pass. There are few people, however, who, if you told them aresult, would be able to evolve from their own inner consciousness what the steps werewhich led up to that result. This power is what I mean when I talk of reasoningbackwards, or analytically.“ -- Sherlock Holmes (A Study in Scarlet, by Sir Arthur Conan Doyle)

Debugging

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Runtimebugs

• Overtvs.covert:– Overthasanobviousmanifestation– codecrashesorrunsforever

– Coverthasnoobviousmanifestation– codereturnsavalue,whichmaybeincorrectbuthardtodetermine

• Persistentvs.intermittent:– Persistentoccurseverytimecodeisrun– Intermittentonlyoccurssometimes,evenifrunonsameinput

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Categoriesofbugs

• Overtandpersistent– Obvioustodetect– Goodprogrammersusedefensiveprogrammingtotrytoensurethatiferrorismade,bugwillfallintothiscategory

• Overtandintermittent– Morefrustrating,canbehardertodebug,butifconditionsthatpromptbugcanbereproduced,canbehandled

• Covert– Highlydangerous,asusersmaynotrealizeanswersareincorrectuntilcodehasbeenrunforlongperiod

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Debuggingskills

• Treatasasearchproblem:lookingforexplanationforincorrectbehavior– Studyavailabledata– bothcorrecttestcasesandincorrectones– Formanhypothesisconsistentwiththedata– Designandrunarepeatableexperimentwithpotentialtorefute

thehypothesis– Keeprecordofexperimentsperformed:usenarrowrangeof

hypotheses

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Debuggingassearch

• Wanttonarrowdownspaceofpossiblesourcesoferror• Designexperimentsthatexposeintermediatestagesof

computation(useprint statements!),anduseresultstofurthernarrowsearch

• Binarysearchcanbeapowerfultoolforthis

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Somepragmatichints

• Lookfortheusualsuspects• Askwhythecodeisdoingwhatitis,notwhyitisnot

doingwhatyouwant• Thebugisprobablynotwhereyouthinkitis– eliminate

locations• Explaintheproblemtosomeoneelse• Don’tbelievethedocumentation• Takeabreakandcomebacktothebuglater

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development strategies, function design › ~bbm101 › fall15 › lectures › w07-de… · slides...

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