Exception Handling · The finally Clause • The finally clause is used when you need to take some...
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Exception Handling SECTION 7.4 10/17/16 63
Transcript of Exception Handling · The finally Clause • The finally clause is used when you need to take some...
Exception Handling SECTION 7.4
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Exception Handling • Therearetwoaspectstodealingwithrun-4meprogramerrors:
1) Detec4ngErrors2) HandlingErrors
• Theopenfunc4oncandetectana>empttoreadfromanon-existentfile• Theopenfunc4oncannothandletheerror• Therearemul4plealterna4ves,thefunc4ondoesnotknowwhichisthecorrectchoice
• Thefunc4onreportstheerrortoanotherpartoftheprogramtobehandled
• Excep4onhandlingprovidesaflexiblemechanismforpassingcontrolfromthepointoftheerrortoahandlerthatcandealwiththeerror
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Detecting Errors • Whatdoyoudoifsomeonetriestowithdrawtoomuchmoneyfromabankaccount?
• Youcanraiseanexcep4on• Whenyouraiseanexcep4on,execu4ondoesnotcon4nuewiththenextstatement• Ittransferstotheexcep-onhandler
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ifamount>balance:raiseValueError("Amountexceedsbalance")
Usetheraisestatementtosignalanexception
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Exception Classes (a subset) • Lookforanappropriateexcep4on
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Syntax: Raising an Exception
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Handling Exceptions • Everyexcep4onshouldbehandledsomewhereintheprogram• Thisisacomplexproblem
• Youneedtohandleeachpossibleexcep4onandreacttoitappropriately
• Handlingrecoverableerrorscanbedone:• Simply:exittheprogram• User-friendly:Asktheusertocorrecttheerror
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Handling Exceptions: Try-Except • Youhandleexcep4onswiththetry/exceptstatement
• Placethestatementintoaloca4onofyourprogramthatknowshowtohandleapar4cularexcep4on
• Thetryblockcontainsoneormorestatementsthatmaycauseanexcep4onofthekindthatyouarewillingtohandle
• Eachexceptclausecontainsthehandlerforanexcep4ontype
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Syntax: Try-Except
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Try-Except: An Example
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try:filename=input("Enterfilename:")infile=open(filename,"r")line=infile.readline()value=int(line)...exceptIOError:print("Error:filenotfound.")exceptValueErrorasexception:print("Error:",str(exception))
Executiontransfershereiffilecannotbeopened
int()canraiseaValueErrorexcep4on
open()canraiseanIOErrorexcep4on
Executiontransfershere
ifthestringcannotbeconvertedtoanint
Ifeitheroftheseexcep.onsisraised,therestoftheinstruc.onsinthetry
blockareskipped
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Example • IfanIOErrorexcep4onisraised,theexceptclausefortheIOErrorexcep4onisexecuted
• IfaValueErrorexcep4onoccurs,thensecondexceptclauseisexecuted• Ifanyotherexcep4onisraiseditwillnotbehandledbyanyoftheexceptblocks
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Output Messages • Whenthebodyofthishandlerisexecuted,itprintsthemessageincludedwiththeexcep4on
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exceptValueErrorasexception:print("Error:",str(exception))
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• Forexample,ifthestringpassedtotheint()func4onwas"35x2",thenthemessageincludedwiththeexcep4onwillbe:invalidliteralforint()withbase10:'35x2'
Output Messages (2) • Toobtainthemessage,wemusthaveaccesstotheexcep4onobjectitself
• Youcanstoretheexcep4onobjectinavariablewiththeassyntax:
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exceptValueErrorasexception:
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• WhenthehandlerforValueErrorisexecuted,excep-onissettotheexcep4onobject.Inourcode,wethenobtainthemessagestringbycallingstr(excep-on)
Source of Output Messages • Whenyouraiseanexcep4on,youcanprovideyourownmessagestring.Forexample,whenyoucall
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raiseValueError("Amountexceedsbalance")
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• Themessageoftheexcep4on,"Amountexceedsbalance",isthestringprovidedastheargumenttotheconstructor
The finally Clause • Thefinallyclauseisusedwhenyouneedtotakesomeac4onwhetherornotanexcep4onisraised
• Hereisatypicalsitua4on• Itisimportanttoalwayscloseanoutputfilewhetherornotanexcep4onwasraised(toensurethatalloutputiswri>entothefile)
• Placethecalltoclose()insideafinallyclause:
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outfile=open(filename,"w")try:writeData(outfile)finally:outfile.close()
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Syntax: The Finally Clause
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Programming Tip • Throwexcep4onsearly
• Whenamethoddetectsaproblemthatitcannotsolve,itisbe>ertothrowanexcep4onratherthantrytocomeupwithanimperfectfix
• Catchexcep4onslate• Conversely,amethodshouldonlycatchanexcep4onifitcanreallyremedythesitua4on
• Otherwise,thebestremedyissimplytohavetheexcep4onpropagatetoitscaller,allowingittobecaughtbyacompetenthandler
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Programming Tip • Donotuseexceptandfinallyinthesametryblock
• Thefinallyclauseisexecutedwheneverthetryblockisexitedinanyofthreeways:1.A]ercomple4ngthelaststatementofthetryblock2.A]ercomple4ngthelaststatementofaexceptclause,ifthistryblockcaughtanexcep4on
3.Whenanexcep4onwasraisedinthetryblockandnothandled
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try
except
finally
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Programming Tip • Itisbe>ertousetwo(nested)tryclausestocontroltheflow
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try
except
finally
try:outfile=open(filename,"w")try:#Writeoutputtooutfilefinally:out.close()#CloseresourcesexceptIOError:#Handleexception
try
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The With Statement • Becauseatry/finallystatementforopeningandclosingfilesissocommon,Pythonhasaspecialshortcut:
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withopen(filename,"w")asoutfile:Writeoutputtooutfile
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• Thiswithstatementopensthefilewiththegivenname,setsou_iletothefileobject,andclosesthefileobjectwhentheendofthestatementhasbeenreachedoranexcep4onisraised
Handling Input Errors SECTION 7.6
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Handling Input Errors • FileReadingApplica4onExample
• Goal:Readafileofdatavalues• Firstlineisthecountofvalues• Remaininglineshavevalues
• Risks:• Thefilemaynotexist
• Theopen()func4onwillraiseanexcep4onwhenthefiledoesnotexist
• Thefilemighthavedatainthewrongformat• Whentherearefewerdataitemsthanexpected,orwhenthefiledoesn’tstartwiththecountofvalues,theprogramwillraiseaValueErrorexcep4on
• Finally,whentherearemoreinputsthanexpected,aRun4meErrorexcep4onshouldberaised
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3 1.45 -2.1 0.05
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Handling Input Errors: main() • Outlineformethodwithallexcep4onhandling
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done=Falsewhilenotdone:try:#Promptuserforfilenamedata=readFile(filename)#Mayraiseexceptions#Processdatadone=true;exceptIOError:print("Filenotfound.")exceptValueError:print("Filecontentsinvalid.")exceptRuntimeErroraserror:print("Error:",str(error))
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HandlingInputErrors:readFile()• CreatesthefileobjectandcallsthereadData()func4on• Noexcep4onhandling(noexceptclauses)• finallyclauseclosesfileinallcases(excep4onornot)
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defreadFile(filename):inFile=open(filename,"r")#MaythrowexceptionstryreturnreadData(inFile)finallyin.close()
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Handling Input Errors: readData()• Noexcep4onhandling(notryorexceptclauses)• raisecreatesanValueErrorexcep4onandexits• Run4meErrorexcep4oncanoccur
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defreadData(inFile):line=inFile.readline()numberOfValues=int(line)#MayraiseaValueErrorexception.data=[]foriinrange(numberOfValues):line=inFile.readline()value=int(line)#MayraiseaValueErrorexception.data.append(value)#Makesuretherearenomorevaluesinthefile.line=inFile.readline()#Extradatainfileifline!="":raiseRuntimeError("Endoffileexpected.")returndata
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One Scenario 1. maincallsreadFile
2. readFilecallsreadData
3. readDatacallsint4. Thereisnointegerintheinput,andintraisesaValueErrorexcep4on
5. readDatahasnoexceptclause;itterminatesimmediately
6. readFilehasnoexceptclause;itterminatesimmediatelya]erexecu4ngthefinallyclauseandclosingthefile
7. TheIOErrorexceptclauseisskipped8. TheValueErrorexceptclauseisexecuted
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Example Code • Openthefileanalyzedata.py
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Summary: File Input/Output • Whenopeningafile,yousupplythenameofthefilestoredondiskandthemodeinwhichthefileistobeopened
• Closeallfileswhenyouaredoneprocessingthem
• Usethereadline()methodtoobtainlinesoftextfromafile
• Writetoafileusingthewrite()methodortheprint()func4on
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Summary: Processing Text Files • Youcaniterateoverafileobjecttoreadthelinesoftextinthefile• Usetherstrip()methodtoremovethenewlinecharacterfromalineoftext
• Usethesplit()methodtosplitastringintoindividualwords
• Readoneormorecharacterswiththeread()method
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Command Line Arguments • Programsthatstartfromthecommandlinereceivethecommandlineargumentsintheargvlistdefinedinthesysmodule
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Summary: Exceptions (1) • Tosignalanexcep4onalcondi4on,usetheraisestatementtoraiseanexcep4onobject
• Whenyouraiseanexcep4on,processingcon4nuesinanexcep4onhandler
• Placethestatementsthatcancauseanexcep4oninsideatryblock,andthehandlerinsideanexceptclause
• Onceatryblockisentered,thestatementsinafinallyclauseareguaranteedtobeexecuted,whetherornotanexcep4onisraised
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Summary: Exceptions (2) • Raiseanexcep4onassoonasaproblemisdetected
• Handleitonlywhentheproblemcanbehandled
• Whendesigningaprogram,askyourselfwhatkindsofexcep4onscanoccur
• Foreachexcep4on,youneedtodecidewhichpartofyourprogramcancompetentlyhandleit
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