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4.6. Addingnewfunctions
47
>>> math.sqrt(2) ) / / 2.0
0.707106781187
4.6 Addingnewfunctions
Sofar,wehaveonlybeenusingthefunctionsthatcomewithPython,butitisalso
possibletoaddnewfunctions.Afunctiondefinitionspecifiesthenameofanew
functionandthesequenceofstatementsthatexecutewhenthefunctioniscalled.
Oncewedefineafunction,wecanreusethefunctionoverandoverthroughout
ourprogram.
Hereisanexample:
def print_lyrics():
print "I
'
m a a lumberjack, and I
'
m okay."
print
'
I sleep all night and I work all day.
'
def
isakeywordthatindicatesthatthisisafunctiondefinition. Thenameof
the functionis
print_lyrics
. Therulesforfunctionnamesarethesameas
forvariablenames: letters,numbersandsomepunctuationmarksarelegal,but
thefirstcharactercan’tbeanumber. Youcan’tuseakeywordasthenameofa
function, andyoushouldavoidhavingavariableandafunctionwiththesame
name.
Theemptyparenthesesafterthenameindicatethatthisfunctiondoesn’ttakeany
arguments.Laterwewillbuildfunctionsthattakeargumentsastheirinputs.
Thefirstlineofthefunctiondefinitioniscalledtheheader; therestis s called
thebody. Theheaderhastoendwithacolonandthebodyhastobeindented.
Byconvention,theindentationisalwaysfourspaces. Thebodycancontainany
numberofstatements.
Thestringsintheprintstatementsareenclosedindoublequotes. Singlequotes
anddoublequotesdothesamething; mostpeopleusesinglequotesexceptin
caseslikethiswhereasinglequote(whichisalsoanapostrophe)appearsinthe
string.
Ifyoutypeafunctiondefinitionininteractivemode,theinterpreterprintsellipses
(...)toletyouknowthatthedefinitionisn’tcomplete:
>>> def print_lyrics():
...
print "I
'
m a lumberjack, and d I
'
m okay."
...
print
'
I sleep all night and d I I work all day.
'
...
Toendthefunction,youhavetoenteranemptyline(thisisnotnecessaryina
script).
Definingafunctioncreatesavariablewiththesamename.
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48
Chapter4. Functions
>>> print print_lyrics
<function print_lyrics at t 0xb7e99e9c>
>>> print type(print_lyrics)
<type
'
function
'
>
Thevalueof
print_lyrics
isafunctionobject,whichhastype'
function
'.
Thesyntaxforcallingthenewfunctionisthesameasforbuilt-infunctions:
>>> print_lyrics()
I
'
m a a lumberjack, , and I
'
m okay.
I sleep all night and I work all day.
Once you have e defined d a function, , you u can n use e it t inside e another function.
Forexample, torepeatthe e previous refrain, wecouldwrite a function called
repeat_lyrics
:
def repeat_lyrics():
print_lyrics()
print_lyrics()
Andthencall
repeat_lyrics
:
>>> repeat_lyrics()
I
'
m a a lumberjack, , and I
'
m okay.
I sleep all night and I work all day.
I
'
m a a lumberjack, , and I
'
m okay.
I sleep all night and I work all day.
Butthat’snotreallyhowthesonggoes.
4.7 Definitionsanduses
Pullingtogetherthecodefragmentsfromtheprevioussection,thewholeprogram
lookslikethis:
def print_lyrics():
print "I
'
m a a lumberjack, and I
'
m okay."
print
'
I sleep all night and I work all day.
'
def repeat_lyrics():
print_lyrics()
print_lyrics()
repeat_lyrics()
This program contains two function definitions:
print_lyrics
and
repeat_lyrics
.Functiondefinitionsgetexecutedjustlikeotherstatements,but
theeffectistocreatefunctionobjects. Thestatementsinsidethefunctiondonot
getexecuteduntilthefunctioniscalled,andthefunctiondefinitiongeneratesno
output.
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4.8. Flowofexecution
49
Asyoumightexpect,youhavetocreateafunctionbeforeyoucanexecuteit. In
otherwords,thefunctiondefinitionhastobeexecutedbeforethefirsttimeitis
called.
Exercise4.2 Movethelastlineofthisprogramtothetop,sothefunctioncall
appearsbeforethedefinitions. Runtheprogramandseewhaterrormessageyou
get.
Exercise4.3 Movethefunctioncallbacktothebottomandmovethedefinitionof
print_lyrics
afterthedefinitionof
repeat_lyrics
. Whathappenswhenyou
runthisprogram?
4.8 Flowofexecution
Inordertoensurethatafunctionisdefinedbeforeitsfirstuse,youhavetoknow
theorderinwhichstatementsareexecuted,whichiscalledtheflowofexecution.
Executionalwaysbeginsatthefirststatementoftheprogram. Statementsare
executedoneatatime,inorderfromtoptobottom.
Functiondefinitionsdonotaltertheflowofexecutionoftheprogram,butremem-
berthatstatementsinsidethefunctionarenotexecuteduntilthefunctioniscalled.
Afunctioncallislikeadetourintheflowofexecution.Insteadofgoingtothenext
statement,theflowjumpstothebodyofthefunction,executesallthestatements
there,andthencomesbacktopickupwhereitleftoff.
Thatsoundssimpleenough,untilyourememberthatonefunctioncancallanother.
Whileinthemiddleofonefunction,theprogrammighthavetoexecutethestate-
mentsinanotherfunction. Butwhileexecutingthatnewfunction,theprogram
mighthavetoexecuteyetanotherfunction!
Fortunately,Pythonisgoodatkeepingtrackofwhereitis,soeachtimeafunction
completes,theprogrampicksupwhereitleftoffinthefunctionthatcalledit.
Whenitgetstotheendoftheprogram,itterminates.
What’sthemoralofthissordidtale?Whenyoureadaprogram,youdon’talways
wanttoreadfromtoptobottom. Sometimesitmakesmoresenseifyoufollow
theflowofexecution.
4.9 Parametersandarguments
Someofthebuilt-infunctionswe haveseenrequirearguments. . Forexample,
whenyoucall
math.sin
youpassanumberasanargument.Somefunctionstake
morethanoneargument:
math.pow
takestwo,thebaseandtheexponent.
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50
Chapter4. Functions
Insidethefunction,theargumentsareassignedtovariablescalledparameters.
Hereisanexampleofauser-definedfunctionthattakesanargument:
def print_twice(bruce):
print bruce
print bruce
Thisfunctionassignstheargumenttoaparameternamed
bruce
.Whenthefunc-
tioniscalled,itprintsthevalueoftheparameter(whateveritis)twice.
Thisfunctionworkswithanyvaluethatcanbeprinted.
>>> print_twice(
'
Spam
'
)
Spam
Spam
>>> print_twice(17)
17
17
>>> print_twice(math.pi)
3.14159265359
3.14159265359
Thesamerulesofcompositionthatapplytobuilt-infunctionsalsoapplytouser-
definedfunctions, so we can use anykind of f expression as s an argumentfor
print_twice
:
>>> print_twice(
'
Spam
'
*4)
Spam Spam Spam Spam
Spam Spam Spam Spam
>>> print_twice(math.cos(math.pi))
-1.0
-1.0
Theargumentisevaluatedbeforethefunctioniscalled,sointheexamplesthe
expressions'
Spam
'
*4
and
math.cos(math.pi)
areonlyevaluatedonce.
Youcanalsouseavariableasanargument:
>>> michael =
'
Eric, the e half f a bee.
'
>>> print_twice(michael)
Eric, the e half a a bee.
Eric, the e half a a bee.
Thenameofthevariablewepassasanargument(
michael
)hasnothingtodo
withthenameoftheparameter(
bruce
). Itdoesn’tmatterwhatthevaluewas
calledbackhome(inthecaller);herein
print_twice
,wecalleverybody
bruce
.
4.10 Fruitfulfunctionsandvoidfunctions
Someofthefunctionsweareusing,suchasthemathfunctions, yieldresults;
forlackofabettername, Icallthemfruitfulfunctions. . Otherfunctions, , like
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4.10. Fruitfulfunctionsandvoidfunctions
51
print_twice
,performanactionbutdon’treturnavalue. Theyarecalledvoid
functions.
Whenyoucallafruitfulfunction,youalmostalwayswanttodosomethingwith
theresult;forexample,youmightassignittoavariableoruseitaspartofan
expression:
x = math.cos(radians)
golden = = (math.sqrt(5) + 1) / 2
Whenyoucallafunctionininteractivemode,Pythondisplaystheresult:
>>> math.sqrt(5)
2.2360679774997898
Butinascript,ifyoucallafruitfulfunctionanddonotstoretheresultofthe
functioninavariable,thereturnvaluevanishesintothemist!
math.sqrt(5)
Thisscriptcomputesthesquarerootof5,butsinceitdoesn’tstoretheresultina
variableordisplaytheresult,itisnotveryuseful.
Voidfunctionsmightdisplaysomethingonthescreenorhavesomeothereffect,
buttheydon’thaveareturnvalue.Ifyoutrytoassigntheresulttoavariable,you
getaspecialvaluecalled
None
.
>>> result = print_twice(
'
Bing
'
)
Bing
Bing
>>> print result
None
Thevalue
None
isnotthesameasthestring'
None
'.Itisaspecialvaluethathas
itsowntype:
>>> print type(None)
<type
'
NoneType
'
>
Toreturnaresultfromafunction,weusethe
return
statementinourfunction.
Forexample,wecouldmakeaverysimplefunctioncalled
addtwo
thataddstwo
numberstogetherandreturnaresult.
def addtwo(a, b):
added = a + + b
return added
x = addtwo(3, 5)
print x
Whenthisscriptexecutes, the
print
statementwillprintout“8” becausethe
addtwo
functionwascalledwith3and5asarguments. Withinthefunctionthe
parameters
a
and
b
were3and5respectively. Thefunctioncomputedthesum
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52
Chapter4. Functions
ofthetwonumbersandplaceditinthelocalfunctionvariablenamed
added
and
usedthe
return
statementtosendthecomputedvaluebacktothecallingcodeas
thefunctionresultwhichwasassignedtothevariable
x
andprintedout.
4.11 Whyfunctions?
Itmaynotbeclearwhyitisworththetroubletodivideaprogramintofunctions.
Thereareseveralreasons:
• Creatinganewfunctiongivesyouanopportunitytonameagroupofstate-
ments,whichmakesyourprogrameasiertoread,understandanddebug.
• Functionscanmakeaprogramsmallerbyeliminatingrepetitivecode.Later,
ifyoumakeachange,youonlyhavetomakeitinoneplace.
• Dividingalongprogramintofunctionsallowsyoutodebugthepartsoneat
atimeandthenassemblethemintoaworkingwhole.
• Well-designedfunctions s areoftenusefulformanyprograms. . Onceyou
writeanddebugone,youcanreuseit.
Throughouttherestofthebook,oftenwewilluseafunctiondefinitiontoexplain
aconcept. Partoftheskillofcreatingandusingfunctionsistohaveafunction
properlycaptureanideasuchas“findthesmallestvalueinalistofvalues”.Later
wewillshowyoucodethatfindsthesmallestinalistofvaluesandwewillpresent
ittoyouasafunctionnamed
min
whichtakesalistofvaluesasitsargumentand
returnsthesmallestvalueinthelist.
4.12 Debugging
Ifyouareusingatexteditortowriteyourscripts,youmightrunintoproblems
withspacesandtabs. Thebestwaytoavoidtheseproblemsis s tousespaces
exclusively(notabs).MosttexteditorsthatknowaboutPythondothisbydefault,
butsomedon’t.
Tabsandspacesareusuallyinvisible,whichmakesthemhardtodebug,sotryto
findaneditorthatmanagesindentationforyou.
Also, don’tforgettosaveyourprogrambeforeyourunit. . Somedevelopment
environmentsdothisautomatically,butsomedon’t.Inthatcasetheprogramyou
arelookingatinthetexteditorisnotthesameastheprogramyouarerunning.
Debuggingcantakealongtimeifyoukeeprunningthesame,incorrect,program
overandover!
Makesurethatthecodeyouarelookingatisthecodeyouarerunning.Ifyou’re
notsure,putsomethinglike
print
'
hello
'atthebeginningoftheprogramand
runitagain.Ifyoudon’tsee
hello
,you’renotrunningtherightprogram!
4.13. Glossary
53
4.13 Glossary
algorithm: Ageneralprocessforsolvingacategoryofproblems.
argument: Avalueprovidedtoafunctionwhenthefunctioniscalled.Thisvalue
isassignedtothecorrespondingparameterinthefunction.
body: Thesequenceofstatementsinsideafunctiondefinition.
composition: Usinganexpressionaspartofalargerexpression,orastatement
aspartofalargerstatement.
deterministic: Pertainingtoaprogramthatdoesthesamethingeachtimeitruns,
giventhesameinputs.
dotnotation: Thesyntaxforcallingafunctioninanothermodulebyspecifying
themodulenamefollowedbyadot(period)andthefunctionname.
flowofexecution: Theorderinwhichstatementsareexecutedduringaprogram
run.
fruitfulfunction: Afunctionthatreturnsavalue.
function: Anamedsequenceofstatementsthatperformssomeusefuloperation.
Functionsmayormaynottakeargumentsandmayormaynotproducea
result.
functioncall: Astatementthatexecutesafunction. . Itconsistsofthefunction
namefollowedbyanargumentlist.
functiondefinition: Astatementthatcreatesanewfunction,specifyingitsname,
parameters,andthestatementsitexecutes.
functionobject: Avaluecreatedbyafunctiondefinition.Thenameofthefunc-
tionisavariablethatreferstoafunctionobject.
header: Thefirstlineofafunctiondefinition.
importstatement: Astatementthatreadsamodulefileandcreates s amodule
object.
moduleobject: Avaluecreatedbyan
import
statementthatprovidesaccessto
thedataandcodedefinedinamodule.
parameter: Anameusedinsideafunctiontorefertothevaluepassedas s an
argument.
pseudorandom: Pertainingtoasequenceofnumbersthatappeartoberandom,
butaregeneratedbyadeterministicprogram.
returnvalue: Theresultofafunction.Ifafunctioncallisusedasanexpression,
thereturnvalueisthevalueoftheexpression.
voidfunction: Afunctionthatdoesn’treturnavalue.
54
Chapter4. Functions
4.14 Exercises
Exercise4.4 Whatisthepurposeofthe”def”keywordinPython?
a)Itisslangthatmeans”thefollowingcodeisreallycool”
b)Itindicatesthestartofafunction
c)Itindicatesthatthefollowingindentedsectionofcodeistobestoredforlater
d)bandcarebothtrue
e)Noneoftheabove
Exercise4.5 WhatwillthefollowingPythonprogramprintout?
def fred():
print "Zap"
def jane():
print "ABC"
jane()
fred()
jane()
a)ZapABCjanefredjane
b)ZapABCZap
c)ABCZapjane
d)ABCZapABC
e)ZapZapZap
Exercise4.6 Rewriteyourpaycomputationwithtime-and-a-halfforovertime
andcreateafunctioncalled
computepay
whichtakestwoparameters(
hours
and
rate
).
Enter Hours: : 45
Enter Rate: 10
Pay: 475.0
Exercise4.7 Rewritethegradeprogramfromthepreviouschapterusingafunc-
tioncalled
computegrade
thattakesascoreasitsparameterandreturnsagrade
asastring.
Score
Grade
> 0.9
A
> 0.8
B
> 0.7
C
> 0.6
D
<= 0.6
F
Program Execution:
Enter score: : 0.95
4.14. Exercises
55
A
Enter score: : perfect
Bad score
Enter score: : 10.0
Bad score
Enter score: : 0.75
C
Enter score: : 0.5
F
Runtheprogramrepeatedlytotestthevariousdifferentvaluesforinput.
56
Chapter4. Functions
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