unit 5 python reference

8/2/2019 Unit 5 Python Reference

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PythonReference

cs101:Unit5(includesUnits1-5)

ArithmeticExpressions

addition: +

outputsthesumofthetwoinputnumbers

multiplication: *

outputstheproductofthetwoinputnumbers

subtraction: -

outputsthedifferencebetweenthetwoinputnumbers

division: /

outputstheresultofdividingthefirstnumberbythesecond

Note:ifbothnumbersarewholenumbers,theresultistruncatedtojustthewhole

numberpart.

modulo: %

outputstheremainderofdividingthefirstnumberbythesecond

exponentiation: **

outputstheresultofraising tothepower(multiplyingbyitself

numberoftimes).

Comparisons

equality: ==

outputsTrueifthetwoinputvaluesareequal, Falseotherwise

inequality: !=

outputsTrueifthetwoinputvaluesarenotequal, Falseotherwise

greaterthan: >

outputsTrueifNumber1isgreaterthanNumber2

lessthan: =

outputsTrueifNumber1isnotlessthanNumber2

lessthanorequalto:


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VariablesandAssignment

Names

AinPythoncanbeanysequenceofletters,numbers,andunderscores( _)thatdoesnotstart

withanumber.Weusuallyusealllowercaselettersforvariablenames,butcapitalizationmustmatch

exactly.HerearesomevalidexamplesofnamesinPython(butmostofthesewouldnotbegood

choicestoactuallyuseinyourprograms):

my_name

one2one

Dorina

this_is_a_very_long_variable_name

AssignmentStatement

Anassignmentstatementassignsavaluetoavariable:

=

Aftertheassignmentstatement,thevariable referstothevalueoftheonthe

rightsideoftheassignment.An isanyPythonconstructthathasavalue.

MultipleAssignment

Wecanputmorethanonenameontheleftsideofanassignmentstatement,andacorresponding

numberofexpressionsontherightside:

,,...=,,...

Alloftheexpressionsontherightsideareevaluatedfirst.Then,eachnameontheleftsideisassigned

toreferencethevalueofthecorrespondingexpressionontherightside.Thisishandyforswapping

variablevalues.Forexample,

s,t=t,s

wouldswapthevaluesofsandtsoaftertheassignmentstatement snowreferstothepreviousvalue

oft,andtreferstothepreviousvalueof s.

Note:whatisreallygoingonhereisabitdifferent.Themultiplevaluesarepackedinatuple(whichissimilartothelistdatatypeintroducedinUnit3,butanimmutableversionofalist),

andthenunpackedintoitscomponentswhentherearemultiplenamesontheleftside.This

distinctionisnotimportantforwhatwedoincs101,butdoesbecomeimportantinsome

contexts.


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Procedures

Aproceduretakesinputsandproducesoutputs.Itisanabstractionthatprovidesawaytousethesame

codetooperateondifferentdatabypassinginthatdataasitsinputs.

Definingaprocedure:

def():

Thearetheinputstotheprocedure.Thereisone foreachinputinorder,

separatedbycommas.Therecanbeanynumberofparameters(includingnone).

Toproduceoutputs:

return,,

Therecanbeanynumberofexpressionsfollowingthereturn(includingnone,inwhichcasetheoutput

oftheprocedureisthespecialvalue None).

Usingaprocedure:

(,,)

Thenumberofinputsmustmatchthenumberofparameters.Thevalueofeachinputisassignedtothe

valueofeachparameternameinorder,andthentheblockisevaluated.


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IfStatements

Theifstatementprovidesawaytocontrolwhatcodeexecutesbasedontheresultofatestexpression.

if:

ThecodeinonlyexecutesifthehasaTruevalue.

Alternateclauses.Wecanuseanelseclauseinanifstatementtoprovidecodethatwillrunwhenthe

hasaFalsevalue.

if:

else:

LogicalOperators

Theandandoroperatorsbehavesimilarlytologicalconjunction(and)anddisjunction(or).The

importantpropertytheyhavewhichisdifferentfromotheroperatorsisthatthesecondoperand

expressionisevaluatedonlywhennecessary.

and

IfExpression1hasaFalsevalue,theresultisFalseandExpression2isnotevaluated(so

evenifitwouldproduceanerroritdoesnotmatter).If Expression1hasaTruevalue,

theresultoftheandisthevalueofExpression2.

or

IfExpression1hasaTruevalue,theresultisTrueandExpression2isnotevaluated(so

evenifitwouldproduceanerroritdoesnotmatter).If Expression1hasaFalsevalue,

theresultoftheoristhevalueofExpression2.


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WhileLoops

Awhileloopprovidesawaytokeepexecutingablockofcodeaslongasatestexpressionis True.

while:

IftheevaluatestoFalse,thewhileloopisdoneandexecutioncontinueswiththe

followingstatement.IftheevaluatestoTrue,theisexecuted.Then,theloop

repeats,returningtotheandcontinuingtoevaluatethe aslongasthe

isTrue.

BreakStatement

Abreakstatementintheofawhileloop,jumpsoutofthecontainingwhileloop,continuing

executionatthefollowingstatement.

break

ForLoops

Aforloopprovidesawaytoexecuteablockonceforeachelementofacollection: forin:

Theloopgoesthrougheachelementofthecollection,assigningthatelementtothe and

evaluatingthe.ThecollectioncouldbeaString,inwhichcasetheelementsarethecharacters

ofastring;aList,inwhichcasetheelementsaretheelementsofthelist;aDictionary,inwhichcasethe

elementsarethekeysinthedictionary;ormanyothertypesinPythonthatrepresentcollectionsof

objects.


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Strings

Astringissequenceofcharacterssurroundedbyquotes.Thequotescanbeeithersingleordouble

quotes,butthequotesatbothendsofthestringmustbethesametype.Herearesomeexamplesof

stringsinPython:

"silly"

'string'

"I'mavalidstring,evenwithasinglequoteinthemiddle!"

StringConcatenation

Wecanusethe+operatoronstrings,butithasadifferentmeaningthanwhenitisusedonnumbers.

stringconcatenation: +

outputstheconcatenationofthetwoinputstrings(pastingthestringtogetherwithnospace

betweenthem)

Wecanalsousethemultiplicationoperatoronstrings:

stringmultiplication: *

outputsastringthatiscopiesoftheinputpastedtogether

IndexingStrings

Theindexingoperatorprovidesawaytoextractsubsequencesofcharactersfromastring.

stringindexing:[]

outputsasingle-characterstringcontainingthecharacteratposition oftheinput.Positionsinthestringarecountedstartingfrom 0,sos[1]wouldoutputthesecond

characterins.Iftheisnegative,positionsarecountedfromtheendofthestring:

s[-1]isthelastcharacterin s.

stringextraction: [:]

outputsastringthatisthesubsequenceoftheinputstringstartingfromposition

andendingjustbeforeposition< StopNumber>.Ifismissing,

startsfromthebeginningoftheinputstring;if< StopNumber>ismissing,goestotheendofthe

inputstring.

Length

length: len()

Outputsthenumberofcharactersin


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find

Thefindmethodprovidesawaytofindsub-sequencesofcharactersinstrings.

find: .find()

outputsanumbergivingthepositioninwherefirstappears.If

thereisnooccurrenceofin,outputs-1.

Tofindlateroccurrences,wecanalsopassinanumbertofind:

findafter: .find(,)

outputsanumbergivingthepositioninwherefirstappearsthat

isatorafterthepositiongiveby< StartNumber>.Ifthereisnooccurrenceof< TargetString>in

atorafter,outputs-1.

ConvertingbetweenNumbersandStrings

str: str()

outputsastringthatrepresentstheinputnumber.Forexample, str(23)outputsthestring '23'.

ord: ord()

outputsthenumbercorrespondingtotheinputstring.

chr: chr()

outputstheone-characterstringcorrespondingtothenumberinput.Thisfunctionisthe

inverseoford:chr(ord())= foranyone-characterstring .

SplittingStrings

split: .split()[,,]

outputsalistofstringsthatare(roughly)thewordscontainedintheinputstring.Thewords

aredeterminedbywhitespace(eitherspaces,tabs,ornewlines)inthestring.(Wedidnotcover

thisinclass,butsplitcanalsobeusedwithanoptionalinputthatisalistoftheseparator

characters,andasecondoptionalinputthatcontrolsthemaximumnumberofelementsinthe

outputlist.)

LoopingthroughStrings

Aforloopprovidesawaytoexecuteablockonceforeachcharacterinastring(justlikeloopingthroughtheelementsofalist): forin:

Theloopgoesthrougheachcharacterofthestringinturn,assigningthatelementtothe and

evaluatingthe.


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Lists

Alistisamutablecollectionofobjects.Theelementsinalistcanbeofanytype,includingotherlists.

Constructingalist.Alistisasequenceofzeroormoreelements,surroundedbysquarebrackets:

[,,]

Selectingelements: []

Outputsthevalueoftheelementin< List>atposition.Elementsareindexedstarting

from0.

Selectingsub-sequences: [:]

Outputsasub-sequenceof< List>startingfromposition< Start>,upto(butnotincluding)

position.

Update: []=

Modifiesthevalueoftheelementin< List>atpositiontobe.

Length: len()

Outputsthenumberof(top-level)elementsin.

Append: .append() Mutatesbyaddingtotheendofthelist.

Concatenation: +

Outputsanewlistthatistheelementsoffollowedbytheelementsof< List2>.

Popping: .pop()

Mutatesbyremovingitslastelement.Outputsthevalueofthatelement.Ifthereareno

elementsin,[].pop()producesanerror. Finding: .index()

Outputsthepositionofthefirstoccurrenceofanelementmatching< Value>in.If

isnotfoundin,producesanerror. Membership: in

OutputsTrueifoccursin.Otherwise,outputsFalse.

Non-membership: notin

OutputsFalseifoccursin.Otherwise,outputsTrue. LoopsonLists

AforloopprovidesawaytoexecuteablockonceforeachelementofaList: forin:


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Dictionaries

ADictionaryprovidesamappingbetweenkeys,whichcanbevaluesofanyimmutabletype,andvalues,

whichcanbeanyvalue.BecauseaDictionaryisimplementedusingahashtable,thetimetolookupa

valuedoesnotincrease(significantly)evenwhenthenumberofkeysincreases.

ConstructingaDictionary.ADictionaryisasetofzeroormorekey-valuepairs,surroundedbysquiggly

braces:

{:,:,}

Lookingupelements: []

outputsthevalueassociatedwithinthe.Producesanerroriftheis

notakeyinthe.

Updatingelements: []=

updatesthevalueassociatedwithinthetobe.Ifisalreadya

keyin,replacesthevalueassociatedwith;ifnot,addsanew:pairtothe.

Membership: in

outputsTrueifisakeyin,Falseotherwise.

LoopsonDictionaries

AforloopprovidesawaytoexecuteablockonceforeachkeyinaDictionary: forin:


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Eval

TheevalfunctionprovidesawaytoevaluateaninputstringasaPythonexpression:

eval()

TheinputisastringthatisaPythonexpression;thevalueisthevaluethatstringwouldevaluatetoinPython.

Time

Thetimelibraryprovidesfunctionsforobtainingandmeasuringtime.

importtime

Systemtime: time.clock()

outputstheprocessortimeinseconds(arealnumber,includingfractionalsecondswithlimited

accuracy)(Notethatwhatclockmeansdependsonyourplatform,anditmaynotprovide

accuratetimingsonallplatforms.)

Exceptions

(WeintroducedexceptionsinUnit4becauseweneededtheminourget_pageprocedure,but

youarenotexpectedtounderstandthisindetailorbeabletouseexceptionhandlersinyour

owncodeincs101.)

try:

except:

Executethecodeinthe.Ifitcompletesnormally,skiptheand

continuewiththefollowingstatement.Ifcodeintheraisesanerror,jumptothe

codeinthe.

Theexceptcanbefollowedbyanexceptiontypeandavariablename:

except,:


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Iftheexceptionraisedinthematchesthetype,evaluatethe

withthevariablereferringtotheexceptionobject.

Libraries

import

ImportstheintothePythonenvironment,enablingthefollowingcodetousethe

definitionsprovidedinthe.Pythonprovidesmanylibrariessuchasurllibthatweuse

fordownloadingwebpagesinget_page,andyoucanalsoimportyourowncode.Asprograms

getlarger,itisimportanttoorganizethemwellintoseparatefiles.

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