The Python Guide for Beginners...1 Preface Python has become one of the fastest-growing programming...

ThePythonGuideforBeginnersRenanMoura

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ThePythonGuideforBeginners1Preface2IntroductiontoPython3InstallingPython34RunningCode5Syntax6Comments7Variables8Types9Typecasting10UserInput11Operators12Conditionals13Lists14Tuples15Sets16Dictionaries17whileLoops18forLoops19Functions20Scope21ListComprehensions22LambdaFunctions23Modules

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24if__name__=='__main__'25Files26ClassesandObjects27Inheritance28Exceptions29Conclusion

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1PrefacePythonhasbecomeoneof the fastest-growingprogramming languagesoverthepastfewyears.

Notonlyitiswidelyused,itisalsoanawesomelanguagetotackleifyouwanttogetintotheworldofprogramming.

This Python Guide for Beginners allows you to learn the core of thelanguageinamatterofhoursinsteadofweeks.

The intention of this book is not to be an exhaustive manual oneverythingPythonhastoofferasoneofthemajorlanguagesinmodernprogramming.

Ifocusonwhatyouwillneedtousemostofthetimetosolvemostoftheproblemsasabeginner.

I deeply believe that you should be able to learn the core of anyprogramminglanguageandthengofromtheretodiveintospecificsanddetailsasneeded.

I'm Renan Moura and I write about Software Development onrenanmf.com.

Youcanalsofindmeas@renanmourafon:

Twitter:https://twitter.com/renanmouraf

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https://renanmf.com

https://twitter.com/renanmouraf

Linkedin:https://www.linkedin.com/in/renanmourafInstagram:https://www.instagram.com/renanmouraf

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https://www.linkedin.com/in/renanmouraf

https://www.instagram.com/renanmouraf

2IntroductiontoPythonPythonwascreatedin1990byGuidoVanRossuminHolland.

One of the objectives of the language was to be accessible to non-programmers.

Pythonwasalsodesignedtobeasecondlanguageforprogrammerstolearnduetoitslowlearningcurveandeaseofuse.

PythonrunsonMac,Linux,Windows,andmanyotherplatforms.

Pythonis:

Interpreted: it can execute at runtime, and changes in a programare instantly perceptible. To be very technical, Python has acompiler. The difference when compared to Java or C++ is howtransparentandautomaticitis.WithPython,wedon'thavetoworryaboutthecompilationstepasit'sdoneinreal-time.Thetradeoffisthatinterpretedlanguagesareusuallyslowerthancompiledones.

SemanticallyDynamic:youdon'thavetospecifytypesforvariablesandthereisnothingthatmakesyoudoit.

Object-Oriented: everything in Python is an object. But you canchoose to write code in an object-oriented, procedural, or evenfunctionalway.

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Highlevel:youdon'thavetodealwithlow-levelmachinedetails.

Pythonhasbeengrowingalotrecentlypartlybecauseof itsmanyusesinthefollowingareas:

System scripting: it's a great tool to automate everyday repetitivetasks.

Data Analysis: it is a great language to experiment with and hastonsof librariesand tools tohandledata,createmodels,visualizeresults and even deploy solutions. This is used in areas likeFinance,E-commerce,andResearch.

Web Development: frameworks like Django and Flask allow thedevelopmentofwebapplications,API's,andwebsites.

MachineLearning:TensorflowandPytorcharesomeofthelibrariesthatallowscientistsandtheindustrytodevelopanddeployArtificialIntelligence solutions in Image Recognition, Health, Self-drivingcars,andmanyotherfields.

Youcaneasilyorganizeyourcodeinmodulesandreusethemorsharethemwithothers.

Finally, we have to keep in mind that Python had breaking changesbetweenversions2and3.AndsincePython2supportended in2020,thisarticleissolelybasedonPython3.

Solet'sgetstarted.

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3InstallingPython3If you use a Mac or Linux you already have Python installed. ButWindowsdoesn'tcomewithPythoninstalledbydefault.

YoualsomighthavePython2,andwearegoingtousePython3.SoyoushouldchecktoseeifyouhavePython3first.

Typethefollowinginyourterminal.

python3-V

NoticetheuppercaseV.

If your result is something similar to 'Python3.x.y', for instance,Python3.8.1,thenyouarereadytogo.

Ifnot,followthenextinstructionsaccordingtoyourOperatingSystem.

3.1InstallingPython3onWindows

Gotohttps://www.python.org/downloads/.

Downloadthelatestversion.

Afterthedownload,double-clicktheinstaller.

Onthefirstscreen,checktheboxindicatingto"AddPython3.xtoPATH"andthenclickon"InstallNow".

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https://www.python.org/downloads/

Wait for the installation process to finish until the next screenwith themessage"Setupwassuccessful".

Clickon"Close".

3.2InstallingPython3onMac

InstallXCodefromtheAppStore.

Installthecommandlinetoolsbyrunningthefollowinginyourterminal.

xcode-select--install

I recommend using Homebrew. Go to https://brew.sh/ and follow theinstructionsonthefirstpagetoinstallit.

After installing Homebrew, run the following brew commands to installPython3.

brewupdate

brewinstallpython3

HomebrewalreadyaddsPython3tothePATH,soyoudon'thavetodoanythingelse.

3.3InstallingPython3onLinux

Toinstallusingapt,availableinUbuntuandDebian,enterthefollowing:

sudoaptinstallpython3

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https://itunes.apple.com/br/app/xcode/id497799835

https://brew.sh/

Toinstallusingyum,availableinRedHatandCentOS,enterthefollowing:

sudoyuminstallpython3

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4RunningCodeYou can runPython codedirectly in the terminal as commandsor youcansavethecodeinafilewiththe.pyextensionandrunthePythonfile.

4.1Terminal

Running commands directly in the terminal is recommendedwhen youwanttorunsomethingsimple.

Openthecommandlineandtypepython3:

renan@mypc:~$python3

Youshouldseesomethinglikethisinyourterminalindicatingtheversion(inmycase,Python3.6.9), theoperatingsystem(I'musingLinux),andsomebasiccommandstohelpyou.

The>>>tellsusweareinthePythonconsole.

Python3.6.9(default,Nov72019,10:44:02)

[GCC8.3.0]onlinux

Type"help","copyright","credits"or"license"formoreinformation.

>>>

Let's test itby runningour firstprogramtoperformbasicmathandaddtwonumbers.

>>>2+2

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Theoutputis:

4

ToexitthePythonconsolesimplytypeexit().

>>>exit()

4.2Running.pyfiles

If you have a complex program, with many lines of code, the Pythonconsoleisn'tthebestoption.

Thealternative issimply toopena texteditor, type thecode,andsavethefilewitha.pyextension.

Let's do that, create a file called second_program.py with the followingcontent.

print('SecondProgram')

Theprint()functionprintsamessageonthescreen.

Themessage goes inside the parentheseswith either single quotes ordoublequotes,bothworkthesame.

Toruntheprogram,onyourterminaldothefollowing:

renan@mypc:~$python3second_program.py

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Theoutputis:

SecondProgram

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5SyntaxPythonisknownforitscleansyntax.

The language avoids using unnecessary characters to indicate somespecificity.

5.1Semicolons

Pythonmakesnouseofsemicolonstofinishlines.Anewlineisenoughtotelltheinterpreterthatanewcommandisbeginning.

Theprint()functionwilldisplaysomething.

Inthisexample,wehavetwocommandsthatwilldisplaythemessagesinsidethesinglequotes.

print('Firstcommand')

print('Secondcommand')

Output:

Firstcommand

Secondcommand

Thisiswrongduetothesemicolonsintheend:

print('Firstcommand');

print('Secondcommand');

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5.2Indentation

Manylanguagesusecurly-bracketstodefinescopes.

Python's interpreterusesonly indentation todefinewhenascopeendsandanotheronestarts.

Thismeans youhave to beawareofwhite spacesat the beginningofeachline--theyhavemeaningandmightbreakyourcodeifmisplaced.

Thisdefinitionofafunctionworks:

defmy_function():


Thisdoesn'tworkbecausetheindentationofthesecondlineismissingandwillthrowanerror:

defmy_function():


5.3Casesensitivityandvariables

Pythoniscasesensitive.SothevariablesnameandNamearenotthesamethingandstoredifferentvalues.

name='Renan'

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Name='Moura'

Asyoucouldsee,variablesareeasilycreatedbyjustassigningvaluestothemusingthe=symbol.

Thismeansnamestores'Renan'andNamestores'Moura'.

5.4Comments

Finally,tocommentsomethinginyourcode,usethehashmark#.

Thecommentedpartdoesnotinfluencetheprogramflow.

#thisfunctionprintssomething

defmy_function():


Thiswasanoverview,minordetailsoneachofthesewillbecomemoreclearinthenextchapterswithexamplesandbroaderexplanations.

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6CommentsThepurposeofcommentsistoexplainwhatishappeninginthecode.

Comments are written along with your code but do not influence yourprogramflow.

Whenyouworkbyyourself,maybecommentsdon't feel likesomethingyoushouldwrite.Afterall,at themoment,youknow thewhysofeverysinglelineofcode.

Butwhatifnewpeoplecomeonboardyourprojectafterayearandtheprojecthas3modules,eachwith10,000linesofcode?

Thinkaboutpeoplewhodon'tknowathingaboutyourappandwhoaresuddenlyhavingtomaintainit,fixit,oraddnewfeatures.

Remember,thereisnosinglesolutionforagivenproblem.Yourwayofsolvingthingsisyoursandyoursonly.Ifyouask10peopletosolvethesameproblem,theywillcomeupwith10differentsolutions.

Ifyouwantotherstofullyunderstandyourreasoning,goodcodedesignismandatory,butcommentsareanintegralpartofanycodebase.

6.1HowtoWriteCommentsinPython

ThesyntaxofcommentsinPythonisrathereasy:justusethehashmark#symbolinfrontofthetextyouwanttobeacomment.

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#Thisisacommentanditwon'tinfluencemyprogramflow

Youcanuseacommenttoexplainwhatsomepieceofcodedoes.

#calculatesthesumofanygiventwonumbers

a+b

6.2MultilineComments

Maybe you want to comment on something very complex or describehowsomeprocessworksinyourcode.

Inthesecases,youcanusemultilinecomments.

Todothat,justuseasinglehashmark#foreachline.

#Everythingafterthehashmark#isacomment

#Thisisacommentanditwon'tinfluencemyprogramflow

#Calculatesthecostoftheprojectgivenvariablesaandb

#aisthetimeinmonthsitwilltakeuntiltheprojectisfinished

#bishowmuchmoneyitwillcostpermonth

a+b*10

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7VariablesInanyprogram,youneedtostoreandmanipulatedatatocreateafloworsomespecificlogic.

That'swhatvariablesarefor.

Youcanhaveavariabletostoreaname,anotheronetostoretheageofaperson,orevenuseamorecomplextypetostoreallofthisatoncelikeadictionary.

7.1CreatingalsoknownasDeclaring

DeclaringavariableisabasicandstraightforwardoperationinPython.

Justpickanameandattributeavaluetoitusethe=symbol.

name='Bob'

age=32

Youcanusetheprint()functiontoshowthevalueofavariable.

print(name)

print(age)

Bob

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32

NoticethatinPythonthereisnospecialwordtodeclareavariable.

Themomentyouassignavalue,thevariableiscreatedinmemory.

Pythonalsohasdynamictyping,whichmeansyoudon'thavetotell it ifyourvariableisatextoranumber,forinstance.

Theinterpreterinfersthetypingbasedonthevalueassigned.

If you need it, you can also re-declare a variable just by changing itsvalue.

#declaringnameasastring

name='Bob'

#re-declaringnameasanint

name=32

Keep inmymind, though, that this isnot recommendedsincevariablesmusthavemeaningandcontext.

IfIhaveavariablecallednameIdon'texpectittohaveanumberstoredinit.

7.2NamingConvention

Let's continue from the last section when I talked about meaning andcontext.

Don'tuserandomvariablenameslikexory.

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Sayyouwanttostorethetimeofaparty,justcallitparty_time.

Oh,didyounoticetheunderscore_?

Byconvention, if youwant touseavariablename that iscomposedoftwo or more words, you separate them by underscores. This is calledSnakeCase.

AnotheroptionwouldbeusingCamelCaseasinpartyTime.This isverycommoninother languages,butnottheconvention inPythonasstatedbefore.

Variablesarecasesensitive, soparty_time andParty_time arenot thesame.Also,keepinmindthattheconventiontellsustoalwaysuselowercase.

Remember, use names that you can recall inside your programeasily.Badnamingcancostyoualotoftimeandcauseannoyingbugs.

Insummary,variablenames:

AreCasesensitive:timeandTIMEarenotthesameHavetostartwithanunderscore_oraletter(DONOTstartwithanumber)Are allowed to have only numbers, letters and underscores. Nospecialcharacterslike:#,$,&,@,etc.

This,forinstance,isnotallowed:party#time,10partytime.

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8TypesTostoredata inPythonyouneedtouseavariable.Andeveryvariablehasitstypedependingonthevalueofthedatastored.

Python has dynamic typing, which means you don't have to explicitlydeclarethetypeofyourvariable--butifyouwantto,youcan.

Lists, Tuples, Sets, and Dictionaries are all data types and havededicated chapters later on with more details, but we'll look at thembrieflyhere.

Thisway Icanshowyou themost importantaspectsandoperationsofeachone in theirownchapterwhilekeeping this chaptermoreconciseandfocusedongivingyouabroadviewofthemaindatatypesinPython.

8.1DeterminingtheType

Firstofall,let'slearnhowtodeterminethedatatype.

Justusethetype() functionandpassthevariableofyourchoiceasanargument,liketheexamplebelow.

print(type(my_variable))

8.2Boolean

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Thebooleantypeisoneofthemostbasictypesofprogramming.

AbooleantypevariablecanonlyrepresenteitherTrueorFalse.

my_bool=True

print(type(my_bool))

my_bool=bool(1024)

print(type(my_bool))

<class'bool'>

<class'bool'>

8.3Numbers

Therearethreenumerictypes:int,float,andcomplex.

8.3.1Integer

my_int=32

print(type(my_int))

my_int=int(32)

print(type(my_int))

<class'int'>

<class'int'>

8.3.2Float

my_float=32.85

print(type(my_float))

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my_float=float(32.85)

print(type(my_float))

<class'float'>

<class'float'>

8.3.3Complex

my_complex_number=32+4j

print(type(my_complex_number))

my_complex_number=complex(32+4j)

print(type(my_complex_number))

<class'complex'>

<class'complex'>

8.4String

The text type isoneof themostcommons typesout thereand isoftencalledstringor,inPython,juststr.

my_city="NewYork"

print(type(my_city))

#Singlequoteshaveexactly

#thesameuseasdoublequotes

my_city='NewYork'


#Settingthevariabletypeexplicitly

my_city=str("NewYork")


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<class'str'>

<class'str'>

<class'str'>

Youcanusethe+operatortoconcatenatestrings.

Concatenationiswhenyouhavetwoormorestringsandyouwanttojointhemintoone.

word1='New'

word2='York'

print(word1+word2)

NewYork

Thestringtypehasmanybuilt-inmethodsthatletusmanipulatethem.Iwilldemonstratehowsomeofthesemethodswork.

Thelen()functionreturnsthelengthofastring.

print(len('NewYork'))

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Thereplace()method replacesapartof thestringwithanother.Asanexample,let'sreplace'New'for'Old'.

print('NewYork'.replace('New','Old'))

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OldYork

Theupper()methodwillreturnallcharactersasuppercase.

print('NewYork'.upper())

NEWYORK

The lower() method does the opposite, and returns all characters aslowercase.

print('NewYork'.lower())

newyork

8.5Lists

A list has its items ordered and you can add the same item asmanytimesasyouwant.Animportantdetailisthatlistsaremutable.

Mutabilitymeansyoucanchangealistafteritscreationbyaddingitems,removingthem,orevenjustchangingtheirvalues.TheseoperationswillbedemonstratedlaterinthechapterdedicatedtoLists.

my_list=["bmw","ferrari","maclaren"]

print(type(my_list))

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my_list=list(("bmw","ferrari","maclaren"))

print(type(my_list))

<class'list'>

<class'list'>

8.6Tuples

Atupleisjustlikealist:ordered,andallowsrepetitionofitems.

Thereisjustonedifference:atupleisimmutable.

Immutabilitymeansyoucan'tchangeatupleafter itscreation.Ifyoutryto add an item or update one, for instance, the Python interpreter willshowyouanerror.IwillshowthattheseerrorsoccurlaterinthechapterdedicatedtoTuples.

my_tuple=("bmw","ferrari","maclaren")

print(type(my_tuple))

my_tuple=tuple(("bmw","ferrari","maclaren"))

print(type(my_tuple))

<class'tuple'>

<class'tuple'>

8.7Sets

Setsdon'tguaranteetheorderoftheitemsandarenotindexed.

Akeypointwhenusingsets:theydon'tallowrepetitionofanitem.

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my_set={"bmw","ferrari","maclaren"}

print(type(my_set))

my_set=set(("bmw","ferrari","maclaren"))

print(type(my_set))

<class'set'>

<class'set'>

8.8Dictionaries

Adictionarydoesn'tguaranteetheorderoftheelementsandismutable.

Oneimportantcharacteristic indictionariesisthatyoucansetyourownaccesskeysforeachelement.

my_dict={"country":"France","worldcups":2}

print(type(my_dict))

my_dict=dict(country="France",worldcups=2)

print(type(my_dict))

<class'dict'>

<class'dict'>

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9TypecastingTypecastingallowsyoutoconvertbetweendifferenttypes.

Thiswayyoucanhaveanintturnedintoastr,orafloatturnedintoanint,forinstance.

9.1Explicitconversion

Tocastavariabletoastringjustusethestr()function.

#thisisjustaregularexplicitintialization

my_str=str('32')

print(my_str)

#inttostr

my_str=str(32)

print(my_str)

#floattostr

my_str=str(32.0)

print(my_str)

32

32

32.0

Tocastavariabletoanintegerjustusetheint()function.

#thisisjustaregularexplicitintialization

my_int=int(32)

print(my_int)

#floattoint:roundsdownto3

my_int=int(3.2)

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print(my_int)

#strtoint

my_int=int('32')

print(my_int)

32

3

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Tocastavariabletoafloatjustusethefloat()function.

#thisisanexplicitintialization

my_float=float(3.2)

print(my_float)

#inttofloat

my_float=float(32)

print(my_float)

#strtofloat

my_float=float('32')

print(my_float)

3.2

32.0

32.0

WhatIdidbeforeiscalledanexplicittypeconversion.

In some cases you don't need to do the conversion explicitly, sincePythoncandoitbyitself.

9.2Implicitconversion

Theexamplebelowshowsimplicitconversionwhenaddinganintanda

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float.

Notice thatmy_sum isfloat.Pythonusesfloat toavoiddata losssincetheinttypecannotrepresentthedecimaldigits.

my_int=32

my_float=3.2

my_sum=my_int+my_float

print(my_sum)

print(type(my_sum))

35.2

<class'float'>

On the other hand, in this example, when you add an int and a str,Pythonwillnotbeable tomake the implicitconversion,and theexplicittypeconversionisnecessary.

my_int=32

my_str='32'

#explicitconversionworks

my_sum=my_int+int(my_str)

print(my_sum)

#implicitconversionthrowsanerror

my_sum=my_int+my_str

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Traceback(mostrecentcalllast):

File"<stdin>",line1,in<module>

TypeError:unsupportedoperandtype(s)for+:'int'and'str'

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Thesameerroristhrownwhentryingtoaddfloatandstrtypeswithoutmakinganexplicitconversion.

my_float=3.2

my_str='32'

#explicitconversionworks

my_sum=my_float+float(my_str)

print(my_sum)

#implicitconversionthrowsanerror

my_sum=my_float+my_str

35.2



TypeError:unsupportedoperandtype(s)for+:'float'and'str'

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10UserInputIf you need to interact with a user when running your program in thecommand line (forexample, toask forapieceof information), youcanusetheinput()function.

country=input("Whatisyourcountry?")#userenters'Brazil'

print(country)

Brazil

Thecapturedvalueisalwaysstring.Justrememberthatyoumightneedtoconvertitusingtypecasting.

age=input("Howoldareyou?")#userenters'29'

print(age)

print(type(age))

age=int(age)

print(type(age))

Theoutputforeachprint()is:

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<class'str'>

<class'int'>

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Noticetheage29iscapturedasstringandthenconvertedexplicitlytoint.

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11OperatorsInaprogramminglanguage,operatorsarespecialsymbolsthatyoucanapplytoyourvariablesandvaluesinordertoperformoperationssuchasarithmetic/mathematicalandcomparison.

Pythonhas lotsofoperators thatyoucanapply toyourvariablesand Iwilldemonstratethemostusedones.

11.1ArithmeticOperators

Arithmeticoperatorsarethemostcommontypeofoperatorsandalsothemostrecognizableones.

Theyallowyoutoperformsimplemathematicaloperations.

Theyare:

+:Addition-:Subtraction*:Multiplication/:Division**:Exponentiation//:FloorDivision,roundsdowntheresultofadivision%:Modulus,givesyoutheremainderofadivision

Let'sseeaprogramthatshowshoweachofthemisused.

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print('Addition:',5+2)

print('Subtraction:',5-2)

print('Multiplication:',5*2)

print('Division:',5/2)

print('FloorDivision:',5//2)

print('Exponentiation:',5**2)

print('Modulus:',5%2)

Addition:7

Subtraction:3

Multiplication:10

Division:2.5

FloorDivision:2

Exponentiation:25

Modulus:1

11.1.1Concatenation

Concatenationiswhenyouhavetwoormorestringsandyouwanttojointhemintoone.

This isusefulwhenyouhave information inmultiplevariablesandwanttocombinethem.

For instance, in thisnextexample I combine twovariables that containmyfirstnameandmylastnamerespectivelytohavemyfullname.

The+operatorisalsousedtoconcatenate.

first_name='Renan'

last_name='Moura'

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print(first_name+last_name)

RenanMoura

Since concatenation is applied to strings, to concatenate strings withothertypes,youhavetodoanexplicittypecastusingstr().

Ihavetotypecasttheintvalue30tostringwithstr()toconcatenateitwiththerestofthestring.

age='Ihave'+str(30)+'yearsold'

print(age)

Ihave30yearsold

11.2ComparisonOperators

Usecomparisonoperatorstocomparetwovalues.

TheseoperatorsreturneitherTrueorFalse.

Theyare:

==:Equal!=:Notequal>:Greaterthan<:Lessthan>=:Greaterthanorequalto

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<=:Lessthanorequalto

Let'sseeaprogramthatshowshoweachofthemisused.

print('Equal:',5==2)

print('Notequal:',5!=2)

print('Greaterthan:',5>2)

print('Lessthan:',5<2)

print('Greaterthanorequalto:',5>=2)

print('Lessthanorequalto:',5<=2)

Equal:False

Notequal:True

Greaterthan:True

Lessthan:False

Greaterthanorequalto:True

Lessthanorequalto:False

11.3AssignmentOperators

As the name implies, these operators are used to assign values tovariables.

x=7inthefirstexampleisadirectassignmentstoringthenumber7inthevariablex.

Theassignmentoperationtakesthevalueontherightandassigns it tothevariableontheleft.

TheotheroperatorsaresimpleshorthandsfortheArithmeticOperators.

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Inthesecondexamplexstartswith7andx+=2 is justanotherwaytowritex=x+2,whichmeansthepreviousvalueofxisaddedby2andreassignedtoxthatisnowequalsto9.

=:simpleassignment

x=7

print(x)

7

+=:additionandassignment

x=7

x+=2

print(x)

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-=:subtractionandassignment

x=7

x-=2

print(x)

5

*=:multiplicationandassignment

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x=7

x*=2

print(x)

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/=:divisionandassignment

x=7

x/=2

print(x)

3.5

%=:modulusandassignment

x=7

x%=2

print(x)

1

//=:floordivisionandassignment

x=7

x//=2

print(x)

3

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**=:exponentiationandassignment

x=7

x**=2

print(x)

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11.4LogicalOperators

Logical operators are used to combine statements applying booleanalgebra.

Theyare:

and:Trueonlywhenbothstatementsaretrueor:Falseonlywhenbothxandyarefalsenot:Thenotoperatorsimplyinvertstheinput,TruebecomesFalseandviceversa.

Let'sseeaprogramthatshowshoweachoneisused.

x=5

y=2

print(x==5andy>3)

print(x==5ory>3)

print(not(x==5))

False

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True

False

11.5MembershipOperators

These operators provide an easy way to check if a certain object ispresentinasequence:string,list,tuple,set,anddictionary.

Theyare:

in:returnsTrueiftheobjectispresentnotin:returnsTrueiftheobjectisnotpresent

Let'sseeaprogramthatshowshoweachoneisused.

number_list=[1,2,4,5,6]

print(1innumber_list)

print(5notinnumber_list)

print(3notinnumber_list)

True

False

True

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12ConditionalsConditionalsareoneofthecornerstonesofanyprogramminglanguage.

They allow you to control the program flow according to specificconditionsyoucancheck.

12.1Theifstatement

Thewayyouimplementaconditionalisthroughtheifstatement.

Thegeneralformofanifstatementis:

ifexpression:

statement

The expression contains some logic that returns a boolean, and thestatementisexecutedonlyifthereturnisTrue.

Asimpleexample:

bob_age=32

sarah_age=29

ifbob_age>sarah_age:

print('BobisolderthanSarah')

BobisolderthanSarah

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We have two variables indicating the ages of Bob and Sarah. TheconditioninplainEnglishsays"ifBob'sageisgreaterthanSarah'sage,thenprintthephrase'BobisolderthanSarah'".

Since the condition returns True, the phrase will be printed on theconsole.

12.2Theifelseandelifstatements

In our last example, the program only does something if the conditionreturnsTrue.

Butwealsowant it todosomething if it returnsFalseorevencheckasecondorthirdconditionifthefirstonewasn'tmet.

Inthisexample,weswappedBob'sandSarah'sage.ThefirstconditionwillreturnFalsesinceSarahisoldernow,andthentheprogramwillprintthephraseaftertheelseinstead.

bob_age=29

sarah_age=32



else:

print('BobisyoungerthanSarah')

BobisyoungerthanSarah

Now,considertheexamplebelowwiththeelif.

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bob_age=32

sarah_age=32



elifbob_age==sarah_age:

print('BobandSarahhavethesameage')

else:


BobandSarahhavethesameage

The purpose of the elif is to provide a new condition to be checkedbeforetheelseisexecuted.

Onceagainwechangedtheiragesandnowbothare32yearsold.

Assuch,theconditionintheelifismet.Sincebothhavethesameagetheprogramwillprint"BobandSarahhavethesameage".

Notice you can have as many elifs as you want, just put them insequence.

bob_age=32

sarah_age=32



elifbob_age<sarah_age:


elifbob_age==sarah_age:

print('BobandSarahhavethesameage')

else:

print('Thisoneisneverexecuted')

BobandSarahhavethesameage

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In thisexample, theelse isneverexecutedbecauseall thepossibilitiesarecoveredinthepreviousconditionsandthuscouldberemoved.

12.3Nestedconditionals

You might need to check more than one conditional for something tohappen.

Inthiscase,youcannestyourifstatements.

Forinstance,thesecondphrase"Bobistheoldest"isprintedonlyifbothifspass.

bob_age=32

sarah_age=28

mary_age=25



ifbob_age>mary_age:

print('Bobistheoldest')

BobisolderthanSarah

Bobistheoldest

Or,dependingonthelogic,makeitsimplerwithBooleanAlgebra.

Thisway,yourcodeissmaller,morereadableandeasiertomaintain.

bob_age=32

sarah_age=28

mary_age=25

ifbob_age>sarah_ageandbob_age>mary_age:

47

print('Bobistheoldest')

Bobistheoldest

12.4TernaryOperator

Theternaryoperatorisaone-lineifstatement.

It'sveryhandyforsimpleconditions.

Thisishowitlooks:

<expression>if<condition>else<expression>

ConsiderthefollowingPythoncode:

a=25

b=50

x=0

y=1

result=xifa>belsey

print(result)

1

Hereweusefourvariables,aandbare for thecondition,whilexandyrepresenttheexpressions.

aandbare thevalueswearecheckingagainsteachother toevaluate

48

somecondition.Inthiscase,wearecheckingifaisgreaterthanb.

Iftheexpressionholdstrue,i.e.,aisgreaterthanbthenthevalueofxwillbeattributedtoresultwhichwillbeequalto0.

However, if a is less than b, then we have the value of y assigned toresult,andresultwillholdthevalue1.

Sinceaislessthanb,25<50,resultwillhave1asfinalvaluefromy.

TheeasywaytorememberhowtheconditionisevaluatedistoreaditinplainEnglish.

Ourexamplewouldread:resultwillbexifaisgreaterthanbotherwisey.

49

13ListsAspromisedintheTypeschapter,thischapterandthenextthreeaboutTuples, Sets, and Dictionaries will have more in-depth explanations ofeachof themsincetheyarevery importantandbroadlyusedstructuresinPythontoorganizeanddealwithdata.

A list has its items ordered and you can add the same item asmanytimesasyouwant.

Animportantdetailisthatlistsaremutable.

Mutabilitymeansyoucanchangealistafteritscreationbyaddingitems,removingthem,orevenjustchangingtheirvalues.

13.0.1Initialization

13.0.1.1EmptyList

people=[]

13.0.1.2Listwithinitialvalues

people=['Bob','Mary']

50

13.0.2AddinginaList

Toaddanitemintheendofalist,useappend().


people.append('Sarah')

print(people)

['Bob','Mary','Sarah']

Tospecifythepositionforthenewitem,usetheinsert()method.


people.insert(0,'Sarah')

print(people)

['Sarah','Bob','Mary']

13.0.3UpdatinginaList

Specifythepositionoftheitemtoupdateandsetthenewvalue.


people[1]='Sarah'

print(people)

['Bob','Sarah']

51

13.0.4DeletinginaList

Usetheremove()methodtodeletetheitemgivenasanargument.


people.remove('Bob')

print(people)

['Mary']

Todeleteeverybody,usetheclear()method.


people.clear()

13.0.5RetrievinginaList

Usetheindextoreferencetheitem.

Rememberthattheindexstartsat0.

Sotoaccesstheseconditemusetheindex1.


print(people[1])

Mary

52

13.0.6CheckifagivenitemalreadyexistsinaList


if'Bob'inpeople:

print('Bobexists!')

else:

print('ThereisnoBob!')

53

14TuplesAtupleissimilartoalist:it'sordered,andallowsrepetitionofitems.

Thereisjustonedifference:atupleisimmutable.

Immutability, ifyou remember,meansyoucan'tchangea tupleafter itscreation.Ifyoutrytoaddanitemorupdateone,forinstance,thePythoninterpreterwillshowyouanerror.


14.0.1.1EmptyTuple

people=()

14.0.1.2Tuplewithinitialvalues

people=('Bob','Mary')

14.0.2AddinginaTuple

Tuplesareimmutable.Thismeansthatifyoutrytoaddanitem,youwillseeanerror.

54


people[2]='Sarah'



TypeError:'tuple'objectdoesnotsupportitemassignment

14.0.3UpdatinginaTuple

Updateanitemwillalsoreturnanerror.

Butthereisatrick:youcanconvertintoalist,changetheitem,andthenconvertitbacktoatuple.


people_list=list(people)

people_list[1]='Sarah'

people=tuple(people_list)

print(people)

('Bob','Sarah')

14.0.4DeletinginaTuple

Forthesamereasonyoucan'taddanitem,youalsocan'tdeleteanitem,sincetheyareimmutable.

14.0.5RetrievinginaTuple

Usetheindextoreferenceanitem.

55


print(people[1])

Mary

14.0.6CheckifagivenitemalreadyexistsinaTuple


if'Bob'inpeople:

print('Bobexists!')

else:


56

15SetsSetsdon'tguaranteetheorderoftheitemsandarenotindexed.

Akeypointwhenusingsets:theydon'tallowrepetitionofanitem.


15.0.1.1EmptySet

people=set()

15.0.1.2Setwithinitialvalues

people={'Bob','Mary'}

15.0.2AddinginaSet

Usetheadd()methodtoaddoneitem.

people.add('Sarah')

Usetheupdate()methoddoaddmultipleitemsatonce.

57

people.update(['Carol','Susan'])

Remember, Sets do not allow repetition, so if you add 'Mary' again,nothingchanges.


people.add('Mary')

print(people)

{'Bob','Mary'}

15.0.3UpdatinginaSet

Itemsinasetarenotmutable.Youhavetoeitheraddordeleteanitem.

15.0.4DeletinginaSet

ToremoveBobfromtheset:


people.remove('Bob')

print(people)

{'Mary'}

Todeleteeverybody:

58

people.clear()

15.0.5Checkifagivenitemalreadyexistsinaset


if'Bob'inpeople:

print('Bobexists!')

else:


59

16DictionariesThe dictionary doesn't guarantee the order of the elements and it ismutable.

One important characteristic of dictionaries is that you can set yourcustomizedaccesskeysforeachelement.

16.0.1InitializationofaDictionary

16.0.1.1EmptyDictionary

people={}

16.0.1.2Dictionarywithinitialvalues

people={'Bob':30,'Mary':25}

16.0.2AddinginaDictionary

Ifthekeydoesn'texistyet,itisappendedtothedictionary.

people['Sarah']=32

60

16.0.3UpdatingaDictionary

Ifthekeyalreadyexists,thevalueisjustupdated.

#Bob'sageis28now

people['Bob']=28

Noticethatthecodeisprettymuchthesame.

16.0.4DeletinginaDictionary

ToremoveBobfromthedictionary:

people.pop('Bob')

Todeleteeverybody:

people.clear()

16.0.5RetrievinginaDictionary

bob_age=people['Bob']

print(bob_age)

30

61

16.0.6CheckifagivenkeyalreadyexistsinaDictionary

if'Bob'inpeople:

print('Bobexists!')

else:


62

17whileLoopsLoops are used when you need to repeat a block of code a certainnumberoftimesorapplythesamelogicovereachiteminacollection.

Therearetwotypesofloops:forandwhile.

Youwilllearnaboutforloopsinthenextchapter.

17.1BasicSyntax

Thebasicsyntaxofawhileloopisasbelow.

whilecondition:

statement

TheloopwillcontinueuntiltheconditionisTrue.

17.2Thesquareofanumberis

The example below takes each value of number and calculates itssquaredvalue.

number=1

whilenumber<=5:

print(number,'squaredis',number**2)

number=number+1

1squaredis1

63

2squaredis4

3squaredis9

4squaredis16

5squaredis25

You can use any variable name, but I chose number because itmakessenseinthecontext.Acommongenericchoicewouldbesimplyi.

Theloopwillgoonuntilnumber(initializedwith1)islessthanorequalto5.

Notice that after the print() command, the variable number isincrementedby1totakethenextvalue.

If you don't do the incrementation you will have an infinite loop sincenumberwillnever reachavaluegreater than5.This isavery importantdetail!

17.3elseblock

WhentheconditionreturnsFalse,theelseblockwillbecalled.

number=1

whilenumber<=5:


number=number+1

else:

print('Nonumbersleft!')

1squaredis1

2squaredis4

3squaredis9

4squaredis16

5squaredis25

Nonumbersleft!

64

Noticethephrase'Nonumbersleft!'isprintedaftertheloopends,thatisaftertheconditionnumber<=5evaluatestoFalse.

17.4HowtobreakoutofawhileloopinPython?

Simplyusethebreakkeyword,andtheloopwillstopitsexecution.

number=1

whilenumber<=5:


number=number+1

ifnumber==4:

break

1squaredis1

2squaredis4

3squaredis9

The loop runs normally, and when number reaches 4 the if statementevaluatestoTrueandthebreakcommandiscalled.Thisfinishestheloopbeforethesquaredvalueofthenumbers4and5arecalculated.

17.5Howtoskipaniteminawhileloop

Thecontinuewilldothatforyou.

Ihadtoinverttheorderoftheifstatementandtheprint()toshowhowitworksproperly.

number=0

whilenumber<5:

65

number=number+1

ifnumber==4:

continue


1squaredis1

2squaredis4

3squaredis9

5squaredis25

Theprogramalwayschecksif4isthecurrentvalueofnumber.Ifitis,thesquare of 4 won't be calculated and the continue will skip to the nextiterationwhenthevalueofnumberis5.

66

18forLoopsfor loops are similar towhile loops in the sense that they are used torepeatblocksofcode.

The most important difference is that you can easily iterate oversequentialtypes.

18.1BasicSyntax

Thebasicsyntaxofaforloopisasbelow.

foritemincollection:

statement

18.2Loopoveralist

Toloopovera listoranyothercollection, justproceedasshownin theexamplebelow.

cars=['BMW','Ferrari','McLaren']

forcarincars:

print(car)

BMW

Ferrari

McLaren

67

Thelistofcarscontainsthreeitems.Theforloopwilliterateoverthelistandstoreeachiteminthecarvariable,andthenexecuteastatement,inthiscaseprint(car),toprinteachcarintheconsole.

18.3range()function

The range function is widely used in for loops because it gives you asimplewaytolistnumbers.

Thiscodewillloopthroughthenumbers0to5andprinteachofthem.

fornumberinrange(5):

print(number)

0

1

2

3

4

Incontrast,withouttherange()function,wewoulddosomethinglikethis.

numbers=[0,1,2,3,4]

fornumberinnumbers:

print(number)

0

1

2

3

4

68

Youcanalsodefineastartandstopusingrange().

Herewearestartingin5andstopingin10.Thenumberyousettostopisnotincluded.

fornumberinrange(5,10):

print(number)

5

6

7

8

9

Finally,itisalsopossibletosetastep.

Herewe starting in 10 and incrementing by 2 until 20, since 20 is thestop,itisnotincluded.

fornumberinrange(10,20,2):

print(number)

10

12

14

16

18

18.4elseblock

Whentheitemsinthelistareover,theelseblockwillbecalled.

69


forcarincars:

print(car)

else:

print('Nocarsleft!')

BMW

Ferrari

McLaren

Nocarsleft!

18.5HowtobreakoutofaforloopinPython

Simplyusethebreakkeyword,andtheloopwillstopitsexecution.


forcarincars:

print(car)

ifcar=='Ferrari':

break

BMW

Ferrari

Theloopwilliteratethelistandprinteachcar.

In this case, after the loop reaches 'Ferrari', the break is called and'McLaren'won'tbeprinted.

18.6Howtoskipaniteminaforloop

Inthissection,we'lllearnhowcontinuecandothisforyou.

I had to invert the order of the if statement and the continue to show

70

howitworksproperly.

Notice that Ialwayscheck if 'Ferrari' is thecurrent item. If it is, 'Ferrari'won'tbeprinted,andthecontinuewillskiptothenextitem'McLaren'.


forcarincars:

ifcar=='Ferrari':

continue

print(car)

BMW

McLaren

18.7LoopoveraLoop:NestedLoops

Sometimesyouhavemorecomplexcollections,likealistoflists.

Toiterateovertheselists,youneednestedforloops.

In thiscase, Ihave three lists:oneofBMWmodels,anotheronFerrarimodels,andfinallyonewithMcLarenmodels.

The first loop iteratesovereachbrand's list,and thesecondwill iterateoverthemodelsofeachbrand.

car_models=[

['BMWI8','BMWX3',

'BMWX1'],

['Ferrari812','FerrariF8',

'FerrariGTC4'],

['McLaren570S','McLaren570GT',

'McLaren720S']

]

71

forbrandincar_models:

formodelinbrand:

print(model)

BMWI8

BMWX3

BMWX1

Ferrari812

FerrariF8

FerrariGTC4

McLaren570S

McLaren570GT

McLaren720S

18.8LoopoverotherDataStructures

Thesamelogicshowntoapplyforloopsoveralistcanbeextendedtotheotherdatastructures:tuple,set,anddictionary.

I will briefly demonstrate how to make a basic loop over each one ofthem.

18.8.1LoopoveraTuple


forpersoninpeople:

print(person)

Bob

Mary

18.8.2LoopoveraSet

72


forpersoninpeople:

print(person)

Bob

Mary

18.8.3LoopoveraDictionary

Toprintthekeys:


forpersoninpeople:

print(person)

Bob

Mary

Toprintthevalues:


forpersoninpeople:

print(people[person])

30

25

73

19FunctionsAs the code grows the complexity also grows. And functions helporganizethecode.

Functionsareahandywaytocreateblocksofcodethatyoucanreuse.

19.1DefinitionandCalling

InPythonusethedefkeywordtodefineafunction.

Giveitanameanduseparenthesestoinform0ormorearguments.

In the line after the declaration starts, remember to indent the block ofcode.

Hereisanexampleofafunctioncalledprint_first_function()thatonlyprintsaphrase'Myfirstfunction!'.

Tocallthefunctionjustuseitsnameasdefined.

defprint_first_function():

print('Myfirstfunction!')

print_first_function()

Myfirstfunction!

19.2returnavalue

74

Usethereturnkeywordtoreturnavaluefromthefunction.

In this example the function second_function() returns the string 'Mysecondfunction!'.

Notice thatprint() is a built-in functionandour function is called frominsideit.

Thestring returnedbysecond_function() ispassedasargument to theprint()function.

defsecond_function():

return'Mysecondfunction!'

print(second_function())

Mysecondfunction!

19.3returnmultiplevalues

Functionscanalsoreturnmultiplevaluesatonce.

return_numbers()returnstwovaluessimultaneously.

defreturn_numbers():

return10,2

print(return_numbers())

(10,2)

75

19.4Arguments

Youcandefineparametersbetweentheparentheses.

When calling a function with parameters you have to pass argumentsaccordingtotheparametersdefined.

The past examples had no parameters, so there was no need forarguments. The parentheses remained emptywhen the functionswerecalled.

19.4.1OneArgument

Tospecifyoneparameter,justdefineitinsidetheparentheses.

Inthisexample,thefunctionmy_numberexpectsonenumberasargumentdefinedbytheparameternum.

The value of the argument is then accessible inside the function to beused.

defmy_number(num):

return'Mynumberis:'+str(num)

print(my_number(10))

Mynumberis:10

19.4.2TwoormoreArguments

76

Todefinemoreparameters,justuseacommatoseparatethem.

Herewehaveafunctionthataddstwonumberscalledadd,itexpectstwoargumentsdefinedbyfirst_numandsecond_num.

The arguments are added by the + operator and the result is thenreturnedbythereturn.

defadd(first_num,second_num):

returnfirst_num+second_num

print(add(10,2))

12

Thisexampleisverysimilartothelastone.Theonlydifferenceisthatwehave3parametersinsteadof2.

defadd(first_num,second_num,third_num):

returnfirst_num+second_num+third_num

print(add(10,2,3))

15

Thislogicofdefiningparametersandpassingargumentsisthesameforanynumberofparameters.

It is important topointout that theargumentshave tobepassed in thesameorderthattheparametersaredefined.

77

19.4.3Defaultvalue.

Youcansetadefaultvalueforaparameterifnoargumentisgivenusingthe=operatorandavalueofchoice.

Inthisfunction,ifnoargumentisgiven,thenumber30isassumedastheexpectedvaluebydefault.

defmy_number(my_number=30):

return'Mynumberis:'+str(my_number)

print(my_number(10))

print(my_number())

Mynumberis:10

Mynumberis:30

19.4.4KeywordorNamedArguments

Whencallinga function, theorderof theargumentshave tomatch theorderoftheparameters.

Thealternativeisifyouusekeywordornamedarguments.

Settheargumentstotheirrespectiveparametersdirectlyusingthenameoftheparametersandthe=operators.

This example flips the arguments, but the function works as expectedbecauseItellwhichvaluegoestowhichparameterbyname.

78

defmy_numbers(first_number,second_number):

return'Thenumbersare:'+str(first_number)+'and'+str(second_number)

print(my_numbers(second_number=30,first_number=10))

Thenumbersare:10and30

19.4.5Anynumberofarguments:*args

If you don't want to specify the number of parameters, just use the *before the parameter name. Then the function will take as manyargumentsasnecessary.

The parameter name could be anything like *numbers, but there is aconventioninPythontouse*argsforthisdefinitionofavariablenumberofarguments.

defmy_numbers(*args):

forarginargs:

print(arg)

my_numbers(10,2,3)

10

2

3

19.4.6AnynumberofKeyword/Namedarguments:**kwargs

Similar to *args, we can use **kwargs to pass as many keyword

79

argumentsaswewant,aslongasweuse**.

Again, the name could be anything like **numbers, but **kwargs is aconvention.

defmy_numbers(**kwargs):

forkey,valueinkwargs.items():

print(key)

print(value)

my_numbers(first_number=30,second_number=10)

first_number

30

second_number

10

19.4.7Othertypesasarguments

Thepastexamplesusedmainlynumbers,butyoucanpassanytypeasargumentandtheywillbetreatedassuchinsidethefunction.

Thisexampleistakingstringsasarguments.

defmy_sport(sport):

print('Ilike'+sport)

my_sport('football')

my_sport('swimming')

Ilikefootball

Ilikeswimming

80

Thisfunctiontakesalistasargument.

defmy_numbers(numbers):

fornumberinnumbers:

print(number)

my_numbers([30,10,64,92,105])

30

10

64

92

105

81

20ScopeThe place where a variable is created defines its availability to beaccessed and manipulated by the rest of the code. This is known asscope.

Therearetwotypesofscope:localandglobal.

20.1GlobalScope

Aglobalscopeallowsyoutousethevariableanywhereinyourprogram.

Ifyourvariableisoutsideafunction,ithasglobalscopebydefault.

name='Bob'

defprint_name():

print('Mynameis'+name)

print_name()

MynameisBob

NoticethatthefunctioncouldusethevariablenameandprintMynameisBob.

20.2LocalScope

Whenyoudeclareavariable insidea function, it onlyexists inside that

82

functionandcan'tbeaccessedfromtheoutside.

defprint_name():

name="Bob"


print_name()

MynameisBob

The variable name was declared inside the function, the output is thesameasbefore.

Butthiswillthrowanerror:

defprint_name():

name='Bob'


print(name)

Theoutputofthecodeaboveis:



NameError:name'name'isnotdefined

Itriedtoprintthevariablenamefromoutsidethefunction,butthescopeofthevariablewaslocalandcouldnotbefoundinaglobalscope.

20.3MixingScopes

83

Ifyouusethesamenameforvariablesinsideandoutsideafunction,thefunctionwillusetheoneinsideitsscope.

So when you call print_name(), the name='Bob' is used to print thephrase.

On the other hand, when calling print() outside the function scope,name="Sarah"isusedbecauseofitsglobalscope.

name="Sarah"

defprint_name():

name='Bob'


print_name()

print(name)

Theoutputofthecodeaboveis:

MynameisBob

Sarah

84

21ListComprehensionsSometimes we want to perform some very simple operations over theitemsofalist.

List comprehensions give us a succinct way to work on lists as analternativetoothermethodsofiteration,suchasforloops.

21.1Basicsyntax

Tousealistcomprehensiontoreplacearegularforloop,wecanmake:

[expressionforiteminlist]

Whichisthesameasdoing:

foriteminlist:

expression

Ifwewantsomeconditionaltoapplytheexpression,wehave:

[expressionforiteminlistifconditional]

Whichisthesameasdoing:

foriteminlist:

85

ifconditional:

expression

21.2Example1:calculatingthecubeofanumber

Regularway

numbers=[1,2,3,4,5]

new_list=[]

forninnumbers:

new_list.append(n**3)

print(new_list)

[1,8,27,64,125]

Usinglistcomprehensions

numbers=[1,2,3,4,5]

new_list=[]

new_list=[n**3forninnumbers]

print(new_list)

[1,8,27,64,125]

21.3Example2:calculatingthecubeofanumberonlyifitisgreaterthan3

86

Using the conditional, we can filter only the values we want theexpressiontobeappliedto.

Regularway

numbers=[1,2,3,4,5]

new_list=[]

forninnumbers:

if(n>3):

new_list.append(n**3)

print(new_list)

[64,125]

Usinglistcomprehensions

numbers=[1,2,3,4,5]

new_list=[]

new_list=[n**3forninnumbersifn>3]

print(new_list)

[64,125]

21.4Example3:callingfunctionswithlistcomprehensions

Wecanalsocallfunctionsusingthelistcomprehensionsyntax:

87

numbers=[1,2,3,4,5]

new_list=[]

defcube(number):

returnnumber**3

new_list=[cube(n)forninnumbersifn>3]

print(new_list)

[64,125]

88

22LambdaFunctionsAPythonlambdafunctioncanonlyhaveoneexpressionandcan'thavemultiplelines.

It is supposed tomake it easier to create some small logic in one lineinsteadofawholefunctionasitisusuallydone.

Lambdafunctionsarealsoanonymous,whichmeansthereisnoneedtonamethem.

22.1BasicSyntax

Thebasicsyntaxisverysimple:justusethelambdakeyword,definetheparameters needed, use ":" to separate the parameters from theexpression.

Thegeneralformis:

lambdaarguments:expression

22.1.1Oneparameterexample

Lookatthisexampleusingonlyoneparameter.

cubic=lambdanumber:number**3

print(cubic(2))

89

8

22.1.2Multipleparameterexample

Ifyouwant,youcanalsohavemultipleparameters.

exponential=lambdamultiplier,number,exponent:multiplier*number**exponent

print(exponential(2,2,3))

16

22.1.3CallingtheLambdaFunctiondirectly

Youdon'tneedtouseavariableaswedidbefore,youcanmakeuseofparenthesisaroundthe lambdafunctionandanotherpairofparenthesisaroundthearguments.

Thedeclarationofthefunctionandtheexecutionwillhappeninthesameline.

(lambdamultiplier,number,exponent:multiplier*number**exponent)(2,2,3)

16

22.2Examplesusinglambdafunctionswithotherbuilt-infunctions

90

22.2.1Map

TheMapfunctionappliestheexpressiontoeachiteminalist.

Let'scalculatethecubicofeachnumberinthelist.

numbers=[2,5,10]

cubics=list(map(lambdanumber:number**3,numbers))

print(cubics)

[8,125,1000]

22.2.2Filter

TheFilterfunctionwillfilterthelistbasedontheexpression.

Let'sfiltertohaveonlythenumbersgreaterthan5.

numbers=[2,5,10]

filtered_list=list(filter(lambdanumber:number>5,numbers))

print(filtered_list)

[10]

91

23ModulesAfter some time your code starts to get more complex with lots offunctionsandvariables.

TomakeiteasiertoorganizethecodeweuseModules.

A well-designedModule also has the advantage of being reusable, soyouwritecodeonceandreuseiteverywhere.

Youcanwriteamodulewithall themathematicaloperationsandotherpeoplecanuseit.

And, ifyouneed,youcanusesomeoneelse'smodulestosimplifyyourcode,speedingupyourproject.

Inotherprogramminglanguages,thesearealsoreferredtoaslibraries.

23.1UsingaModule

Touseamoduleweusetheimportkeyword.

Asthenameimplieswehavetotellourprogramwhatmoduletoimport.

Afterthat,wecanuseanyfunctionavailableinthatmodule.

Let'sseeanexampleusingthemathmodule.

First,let'sseehowtohaveaccesstoaconstant,Euler'snumber.

92

importmath

math.e

2.718281828459045

Inthissecondexample,wearegoingtouseafunctionthatcalculatesthesquarerootofanumber.

Itisalsopossibletousetheaskeywordtocreateanalias.

importmathasm

m.sqrt(121)

m.sqrt(729)

11

27

Finally, using the from keyword, we can specify exactly what to importinstead of the whole module and use the function directly without themodule'sname.

This example uses the floor() function that returns the largest integerlessthanorequaltoagivennumber.

frommathimportfloor

floor(9.8923)

93

9

23.2CreatingaModule

Nowthatweknowhowtousemodules,let'sseehowtocreateone.

It isgoingtobeamodulewiththebasicmathoperationsadd,subtract,multiply,divideanditisgonnabecalledbasic_operations.

Createthebasic_operations.pyfilewiththefourfunctions.



defsubtract(first_num,second_num):

returnfirst_num-second_num

defmultiply(first_num,second_num):

returnfirst_num*second_num

defdivide(first_num,second_num):

returnfirst_num/second_num

Then,justimportthebasic_operationsmoduleandusethefunctions.

importbasic_operations

basic_operations.add(10,2)

basic_operations.subtract(10,2)

basic_operations.multiply(10,2)

basic_operations.divide(10,2)

12

8

20

5.0

94

24if__name__=='__main__'You are on the process of building a module with the basic mathoperationsadd,subtract,multiply,dividecalledbasic_operationssavedinthebasic_operations.pyfile.

Toguaranteeeverythingisfine,youcanrunsometests.









print(add(10,2))

print(subtract(10,2))

print(multiply(10,2))

print(divide(10,2))

Afterrunningthecode:

renan@pro-home:~$python3basic_operations.py

Theoutputis:

12

8

20

5.0

95

Theoutputforthosetestsarewhatweexpected.

Ourcodeisrightandreadytoshare.

Let'simportthenewmoduleandrunitinthePythonconsole.


[GCC8.3.0]onlinux


>>>importbasic_operations

12

8

20

5.0

>>>

Whenthemoduleisimportedourtestsaredisplayedonthescreeneventhoughwedidn'tdoanythingbesidesimportingbasic_operations.

Tofixthatweuseif__name__=='__main__'inthebasic_operations.pyfilelikethis:









if__name__=='__main__':

print(add(10,2))

print(subtract(10,2))

print(multiply(10,2))

96

print(divide(10,2))

Running the code directly on the terminal will continue to display thetests.Butwhenyouimportitlikeamodule,thetestswon'tshowandyoucanusethefunctionsthewayyouintended.


[GCC8.3.0]onlinux


>>>importbasic_operations

>>>basic_operations.multiply(10,2)

20

>>>

Now that you know how to use the if __name__ == '__main__', let'sunderstandhowitworks.

Theconditiontellswhentheinterpreteristreatingthecodeasamoduleorasaprogramitselfbeingexecuteddirectly.

Pythonhasthisnativevariablecalled__name__.

Thisvariablerepresentsthenameofthemodulewhichisthenameofthe.pyfile.

Createafilemy_program.pywiththefollowingandexecuteit.

print(__name__)

Theoutputwillbe:

97

__main__

This tells us that when a program is executed directly, the variable__name__isdefinedas__main__.

Butwhenitisimportedasamodule,thevalueof__name__isthenameofthemodule.


[GCC8.3.0]onlinux


>>>importmy_program

my_program

>>>

This is how the Python interpreter differentiates the behavior of animportedmoduleandaprogramexecuteddirectlyintheterminal.

98

25FilesCreating,deleting,reading,andmanyotherfunctionsappliedtofilesareanintegralpartofmanyprograms.

Assuch,itisveryimportanttoknowhowtoorganizeanddealwithfilesdirectlyfromyourcode.

Let'sseehowtohandlefilesinPython.

25.1Filecreate

Firstthingsfirst,create!

Wearegoingtousetheopen()function.

Thisfunctionopensafileandreturnsitscorrespondingobject.

The first argument is the nameof the fileweare handling, the secondreferstotheoperationweareusing.

Thecodebelowcreatesthefile"people.txt",thexargumentisusedwhenwejustwanttocreatethefile.Ifafilewiththesamenamealreadyexists,itwillthrowanexception.

people_file=open("people.txt","x")

Youcanalsowethewmodetocreateafile.Unlikethexmode,itwillnot

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throwanexceptionsince thismode indicates thewritingmode.Weareopeningafiletowritedataintoitand,ifthefiledoesn'texist,itiscreated.

people_file=open("people.txt","w")

The last one is the a mode which stands for append. As the nameimplies,youcanappendmoredata to the file,while thewmodesimplyoverwritesanyexistingdata.

Whenappending,ifthefiledoesn'texist,italsocreatesit.

people_file=open("people.txt","a")

25.2Filewrite

Towritedataintoafile,yousimplyopenafilewiththewmode.

Then,toadddata,youusetheobjectreturnedbytheopen()function,inthiscase, theobject iscalledpeople_file.Thencall thewrite() functionpassingthedataasargument.


people_file.write("Bob\n")

people_file.write("Mary\n")

people_file.write("Sarah\n")

people_file.close()

Weuse\nat theendtobreakthe line,otherwisethecontent in thefile

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willstayinthesamelineas"BobMarySarah".

Onemoredetailistoclose()thefile.Thisisnotonlyagoodpractice,butalsoensuresthatyourchangeswereappliedtothefile.

Rememberthatwhenusingwmode,thedatathatalreadyexistedinthefilewillbeoverwrittenby thenewdata.Toaddnewdatawithout losingwhatwasalreadythere,wehavetousetheappendmode.

25.3Fileappend

Theamodeappendsnewdatatothefile,keepingtheexistingone.

In this example, after the first writingwith wmode, we are using the amodetoappend.Theresultisthateachnamewillappeartwiceinthefile"people.txt".

#firstwrite





people_file.close()

#appendingmoredata

#keepingtheexistingdata

people_file=open("people.txt","a")




people_file.close()

25.4Fileread

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Readingthefileisalsoverystraightforward:justusethermodelikeso.

If you read the "people.txt" file created in the last example, you shouldsee6namesinyouroutput.

people_file=open("people.txt","r")

print(people_file.read())

Bob

Mary

Sarah

Bob

Mary

Sarah

Theread()functionreadsthewholefileatonceifyouusethereadline()function,youcanreadthefilelinebyline.


print(people_file.readline())



Bob

Mary

Sarah

Youcanalsoalooptoreadthelinesliketheexamplebelow.


forpersoninpeople_file:

print(person)

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Bob

Mary

Sarah

Bob

Mary

Sarah

25.5DeleteaFile

Todeleteafile,youalsoneedtheosmodule.

Usetheremove()method.

importos

os.remove('my_file.txt')

25.6CheckifaFileExists

Usetheos.path.exists()methodtochecktheexistenceofafile.

importos

ifos.path.exists('my_file.txt'):

os.remove('my_file.txt')

else:

print('Thereisnosuchfile!')

25.7CopyaFile

Forthisone,Iliketousethecopyfile()methodfromtheshutilmodule.

fromshutilimportcopyfile

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copyfile('my_file.txt','another_file.txt')

Thereareafewoptionstocopyafile,butcopyfile()isthefastestone.

25.8RenameandMoveaFile

Ifyouneedtomoveorrenameafileyoucanusethemove()methodfromtheshutilmodule.

fromshutilimportmove

move('my_file.txt','another_file.txt')

104

26ClassesandObjectsClasses and Objects are the fundamental concepts of Object-OrientedProgramming.

InPython,everythingisanobject!

Avariable(object)isjustaninstanceofitstype(class).

That'swhywhenyoucheckthetypeofavariableyoucanseetheclasskeywordrightnexttoitstype(class).

Thiscodesnippetshowsthatmy_cityisanobjectanditisaninstanceoftheclassstr.

my_city="NewYork"


<class'str'>

26.1DifferentiateClassxObject

Theclassgivesyouastandardway tocreateobjects.Aclass is likeabaseproject.

SayyouareanengineerworkingforBoeing.

Your newmission is to build the new product for the company, a new

105

model called 747-Space. This aircraft flies higher altitudes than othercommercialmodels.

Boeingneedstobuilddozensofthosetoselltoairlinesallovertheworld,andtheaircraftshavetobeallthesame.

Toguaranteethat theaircrafts(objects) followthesamestandards,youneedtohaveaproject(class)thatcanbereplicable.

Theclassisaproject,ablueprintforanobject.

Thiswayyoumaketheprojectonce,andreuseitmanytimes.

In our code example before, consider that every string has the samebehaviorand thesameattributes.So itonlymakessense forstrings tohaveaclassstrtodefinethem.

26.2AttributesandMethods

Objectshavesomebehaviorwhichisisgivenbyattributesandmethods.

Insimpleterms, in thecontextofanobject,attributesarevariablesandmethodsarefunctionsattachedtoanobject.

Forexample,astringhasmanybuilt-inmethodsthatwecanuse.

Theyworklikefunctions,youjustneedtothemfromtheobjectsusinga..

In this code snippet, I'm calling the replace() method from the string

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variablemy_citywhichisanobject,andaninstanceoftheclassstr.

The replace() method replaces a part of the string for another andreturns a new string with the change. The original string remains thesame.

Let'sreplace'New'for'Old'in'NewYork'.

my_city='NewYork'

print(my_city.replace('New','Old'))

print(my_city)

OldYork

NewYork

26.3CreatingaClass

Wehaveusedmanyobjects(instancesofclasses)likestrings,integers,lists,anddictionaries.AllofthemareinstancesofpredefinedclassesinPython.

Tocreateourownclassesweusetheclasskeyword.

Byconvention, thenameof theclassmatches thenameof the.py fileand themodulebyconsequence. It isalsoagoodpractice toorganizethecode.

Createafilevehicle.pywiththefollowingclassVehicle.

classVehicle:

107

def__init__(self,year,model,plate_number,current_speed=0):

self.year=year

self.model=model

self.plate_number=plate_number

self.current_speed=current_speed

defmove(self):

self.current_speed+=1

defaccelerate(self,value):

self.current_speed+=value

defstop(self):

self.current_speed=0

defvehicle_details(self):

returnself.model+','+str(self.year)+','+self.plate_number

Let'sbreakdowntheclasstoexplainitinparts.

TheclasskeywordisusedtospecifythenameoftheclassVehicle.

The __init__ function is a built-in function that all classes have, it iscalled when an object is created and is often used to initialize theattributes,assigningvaluestothem,similartowhatisdonetovariables.

The first parameter self in the __init__ function is a reference to theobject(instance)itself.Wecallitselfbyconventionandithastobethefirst parameter in every instancemethod, as you can see in the othermethod definitions def move(self), def accelerate(self, value), defstop(self),anddefvehicle_details(self).

Vehiclehas4attributes:year,model,plate_number,andcurrent_speed.

Inside the__init__,eachoneof them is initializedwith theparametersgivenwhentheobjectisinstantiated.

108

Notice thatcurrent_speed is initializedwith0bydefault,meaning that ifnovalueisgiven,current_speedwillbeequalto0whentheobjectisfirstinstantiated.

Finally,we have threemethods tomanipulate our vehicle regarding itsspeed:defmove(self),defaccelerate(self,value),defstop(self).

And one method to give back information about the vehicle: defvehicle_details(self).

The implementation inside the methods work the same way as infunctions.Youcanalsohaveareturntogiveyoubacksomevalueattheendofthemethodasdemonstratedbydefvehicle_details(self).

26.4UsingtheClass

Use the classona terminal, import theVehicle class from thevehiclemodule.

Create an instance calledmy_car, initializingyearwith 2009,modelwith'F8',plate_numberwith'ABC1234',andcurrent_speedwith100.

Theselfparameterisnottakenintoconsiderationwhencallingmethods.The Python interpreter infers its value as being the currentobject/instance automatically, so we just have to pass the otherargumentswheninstantiatingandcallingmethods.

Nowusethemethodstomove()thecarwhichincreasesitscurrent_speedby 1, accelerate(10) which increases its current_speed by the value

109

givenintheargument,andstop()whichsetsthecurrent_speedto0.

Remembertoprintthevalueofcurrent_speedateverycommandtoseethechanges.

To finish the test, call vehicle_details() to print the information aboutourvehicle.

>>>fromvehicleimportVehicle

>>>

>>>my_car=Vehicle(2009,'F8','ABC1234',100)

>>>print(my_car.current_speed)

100

>>>my_car.move()


101

>>>my_car.accelerate(10)


111

>>>my_car.stop()


0

>>>print(my_car.vehicle_details())

F8,2009,ABC1234

Ifwedon'tsettheinitialvalueforcurrent_speed,itwillbezerobydefaultasstatedbeforeanddemonstratedinthenextexample.

>>>fromvehicleimportVehicle

>>>

>>>my_car=Vehicle(2009,'F8','ABC1234')


0

>>>my_car.move()


1

>>>my_car.accelerate(10)


11

>>>my_car.stop()

110


0

>>>print(my_car.vehicle_details())

F8,2009,ABC1234

111

27InheritanceLet'sdefineagenericVehicleclassandsaveitinsidethevehicle.pyfile.

classVehicle:

def__init__(self,year,model,plate_number,current_speed):

self.year=year

self.model=model

self.plate_number=plate_number

self.current_speed=current_speed

defmove(self):

self.current_speed+=1

defaccelerate(self,value):

self.current_speed+=value

defstop(self):

self.current_speed=0

defvehicle_details(self):

returnself.model+','+str(self.year)+','+self.plate_number

Avehiclehasattributesyear,model,plate_number,andcurrent_speed.

Thedefinitionofvehicle intheVehicle isverygenericandmightnotbesuitablefortrucksforinstancebecauseitshouldincludeacargoattribute.

Ontheotherhand,acargoattributedoesnotmakemuchsenseforsmallvehicleslikemotorcycles.

Tosolvethiswecanuseinheritance.

Whenaclass(child)inheritsanotherclass(parent),alltheattributesandmethodsfromtoparentclassareinheritedbythechildclass.

112

27.1ParentandChild

In our case, we want a new Truck class to inherit everything from theVehicle class. Then we want it to add its own specific attributecurrent_cargotocontroltheadditionandremovalofcargofromthetruck.

TheTruckclass iscalledachildclassthat inherits fromitsparentclassVehicle.

A parent class is also called a superclass while a child class is alsoknownasasubclass.

CreatetheclassTruckandsaveitinsidethetruck.pyfile.

fromvehicleimportVehicle

classTruck(Vehicle):

def__init__(self,year,model,plate_number,current_speed,current_cargo):

super().__init__(year,model,plate_number,current_speed)

self.current_cargo=current_cargo

defadd_cargo(self,cargo):

self.current_cargo+=cargo

defremove_cargo(self,cargo):

self.current_cargo-=cargo

Let'sbreakdowntheclasstoexplainitinparts.

TheclassVehicle insidetheparentheseswhendefiningtheclassTruckindicatesthattheparentVehicleisbeinginheritedbyitschildTruck.

The__init__methodhasselfasitsfirstparameter,asusual.

113

Theparametersyear,model,plate_number,andcurrent_speedare theretomatchtheonesintheVehicleclass.

Weaddedanewparametercurrent_cargosuitedfortheTruckclass.

Inthefirstlineofthe__init__methodoftheTruckclasswehavetocallthe__init__methodoftheVehicleclass.

To do that we use super() to make a reference to the supperclassVehicle, so when super().__init__(year, model, plate_number,

current_speed)iscalledweavoidrepetitionofourcode.

Afterthat,wecanassignthevalueofcurrent_cargonormally.

Finally, we have two methods to deal with the current_cargo: defadd_cargo(self,cargo):,anddefremove_cargo(self,cargo):.

Remember that Truck inherits attributes andmethods from Vehicle, sowe also have an implicit access to the methods that manipulate thespeed:defmove(self),defaccelerate(self,value),defstop(self).

27.2UsingtheTruckclass

Use the class in your terminal, import the Truck class from the truckmodule.

Createaninstancecalledmy_truck,initializingyearwith2015,modelwith'V8', plate_number with 'XYZ1234', current_speed with 0, andcurrent_cargowith0.

114

Useadd_cargo(10) toincreasecurrent_cargoby10,remove_cargo(4), todecreasecurrent_cargoby4.

Remembertoprintthevalueofcurrent_cargoateverycommandtoseethechanges.

Byinheritance,wecanusethemethodsfromtheVehicleclasstomove()the truckwhich increases itscurrent_speed by1,accelerate(10)whichincreases its current_speed by the value given in the argument, andstop()whichsetsthecurrent_speedto0.

Remembertoprintthevalueofcurrent_speedateveryinteractiontoseethechanges.

Tofinishthetest,callvehicle_details()inheritedfromtheVehicleclasstoprinttheinformationaboutourtruck.

>>>fromtruckimportTruck

>>>

>>>my_truck=Truck(2015,'V8','XYZ1234',0,0)

>>>print(my_truck.current_cargo)

0

>>>my_truck.add_cargo(10)


10

>>>my_truck.remove_cargo(4)


6

>>>print(my_truck.current_speed)

0

>>>my_truck.accelerate(10)


10

>>>my_truck.stop()


0

>>>print(my_truck.vehicle_details())

V8,2015,XYZ1234

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28ExceptionsErrors are a part of every programmer's life, and knowing how to dealwiththemisaskillonitsown.

ThewayPythondealswitherrorsiscalled'ExceptionHandling'.

Ifsomepieceofcoderunsintoanerror,thePythoninterpreterwillraiseanexception.

28.1TypesofExceptions

Let's try to raise some exceptions on purpose and see the errors theyproduce.

TypeError

First,trytoaddastringandaninteger:

'Iamastring'+32



TypeError:mustbestr,notint

IndexError

Now,trytoaccessanindexthatdoesn'texistinalist.

117

Acommonmistake is to forget thatsequencesare0-indexed,meaningthefirstitemhasindex0,not1.

Inthisexample,thelistcar_brandsendsatindex2.

car_brands=['ford','ferrari','bmw']

print(car_brands[3])



IndexError:listindexoutofrange

NameError

Ifwetrytoprintavariablethatdoesn'texist.

print(my_variable)



NameError:name'my_variable'isnotdefined

ZeroDivisionError

Mathdoesn'tallowdivisionbyzero,tryingtodosowillraiseanerror,asexpected.

32/0

118



ZeroDivisionError:divisionbyzero

Thiswasjustasampleofthekindsofexceptionsyoumightseeonyourdailyroutineandwhatcancauseeachofthem.

28.2ExceptionHandling

Nowweknowhowtocauseerrorsthatwillcrashourcodeandshowsussomemessagesayingsomethingiswrong.

Tohandletheseexceptionsjustmakeuseofthetry/exceptstatement.

try:

32/0

except:

print('Dividingbyzero!')

Dividingbyzero!

Theexampleaboveshowstheuseofthetrystatement.

Puttheblockofcodethatmaycauseanexceptioninsidethetryscope.If everything runs alright, the except block is not invoked. But if anexceptionisraised,theblockofcodeinsidetheexceptisexecuted.

Thiswaytheprogramdoesn'tcrashandifyouhavesomecodeaftertheexception,itwillkeeprunningifyouwantto.

28.3SpecificExceptionHandling

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In the last example the except block was generic, meaning it wascatchinganything.

Goodpractice it tospecify the typeofexceptionweare trying tocatch,whichhelpsalotwhendebuggingthecodelater.

If you know a block of code can throw an IndexError, specify it in theexcept:

try:

car_brands=['ford','ferrari','bmw']

print(car_brands[3])

exceptIndexError:

print('Thereisnosuchindex!')

Thereisnosuchindex!

Youcanuseatupletospecifyasmanyexceptionstypesasyouwantinasingleexcept:

try:

print('Mycode!')

except(IndexError,ZeroDivisionError,TypeError):

print('MyExcepetion!')

28.4else

It is possible to addanelse commandat the endof thetry/except. Itrunsonlyiftherearenoexceptions.

120

my_variable='Myvariable'

try:

print(my_variable)

exceptNameError:

print('NameErrorcaught!')

else:

print('NoNameError')

Myvariable

NoNameError

28.5RaisingExceptions

Theraisecommandallowsyoutomanuallyraiseanexception.

This is particularly useful if you want to catch an exception and dosomethingwith it -- like logging theerror insomepersonalizedway likeredirecting it to a log aggregator, or ending the execution of the codesincetheerrorshouldnotallowtheprogressoftheprogram.

try:

raiseIndexError('Thisindexisnotallowed')

except:

print('Doingsomethingwiththeexception!')

raise

Doingsomethingwiththeexception!



IndexError:Thisindexisnotallowed

28.6finally

121

Thefinallyblock isexecutedindependentlyofexceptionsbeingraisedornot.

Theyareusually there to allow theprogram to cleanup resources likefiles,memory,networkconnections,etc.

try:

print(my_variable)

exceptNameError:

print('Exceptblock')

finally:

print('Finallyblock')

Exceptblock

Finallyblock

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29ConclusionThat'sit!

Congratulationsonreachingtheend.

Iwanttothankyouforreadingthisbook.

Ifyouwanttolearnmore,checkoutmyblogrenanmf.com.

Let me know if you have any suggestions by reaching out to me [email protected].

Youcanalsofindmeas@renanmourafon:

Twitter:https://twitter.com/renanmourafLinkedin:https://www.linkedin.com/in/renanmourafInstagram:https://www.instagram.com/renanmouraf

123

https://renanmf.com

mailto:[email protected]

https://twitter.com/renanmouraf

https://www.linkedin.com/in/renanmouraf

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