dive into python3

CHAPTER -1. WHATS NEW IN DIVE INTO PYTHON3

Isnt this where we came in?

Pink Floyd, The Wall

-1.1. A.K.A. THE MINUS LEVEL

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www.princexml.comThis document was created with Prince, a great way of getting web content onto paper.

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CHAPTER 0. INSTALLING PYTHON

Tempora mutantur nos et mutamur in illis. (Times change, and we change with them.)

ancient Roman proverb

0.1. DIVING IN

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0.3. INSTALLING ONMICROSOFTWINDOWS

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0.4. INSTALLING ONMAC OS X

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0.5. INSTALLING ON UBUNTU LINUX

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0.6. INSTALLING ON OTHER PLATFORMS

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0.7. USING THE PYTHON SHELL

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IFMQ

5ZQFIFMQGPSJOUFSBDUJWFIFMQPSIFMQPCKFDUGPSIFMQBCPVUPCKFDU

0A>K> :K> MPH FH=>L H? A>EI 5HN M A>EI :;HNM : LBG@E> H;C>MNKGL RHN MH MA> ,RMAHG /A>EE IKHFIM 5HN GM>K help mode PA>K> BGLM>:= H?>O:EN:MBG@ ,RMAHG >QIK>LLBHGL RHN CNLM MRI> D>RPHK=L HK L :G= BM PBEE IKBGM HNM PA:M>O>K BMDGHPL :;HNM MA:M GM>K MA> BGM>K: A>EI FH=> MRI> IFMQ :G= IK>LL &/5&3

IFMQ

8FMDPNFUP1ZUIPO5IJTJTUIFPOMJOFIFMQVUJMJUZ

*GUIJTJTZPVSGJSTUUJNFVTJOH1ZUIPOZPVTIPVMEEFGJOJUFMZDIFDLPVU

UIFUVUPSJBMPOUIF*OUFSOFUBUIUUQEPDTQZUIPOPSHUVUPSJBM

&OUFSUIFOBNFPGBOZNPEVMFLFZXPSEPSUPQJDUPHFUIFMQPOXSJUJOH

1ZUIPOQSPHSBNTBOEVTJOH1ZUIPONPEVMFT5PRVJUUIJTIFMQVUJMJUZBOE

SFUVSOUPUIFJOUFSQSFUFSKVTUUZQFRVJU

5PHFUBMJTUPGBWBJMBCMFNPEVMFTLFZXPSETPSUPQJDTUZQFNPEVMFT

LFZXPSETPSUPQJDT&BDINPEVMFBMTPDPNFTXJUIBPOFMJOFTVNNBSZ

PGXIBUJUEPFTUPMJTUUIFNPEVMFTXIPTFTVNNBSJFTDPOUBJOBHJWFOXPSE

TVDIBTTQBNUZQFNPEVMFTTQBN

IFMQ

*HM> AHP MA> IKHFIM L ?KHF MH IFMQ 0ABL K>FBG=L RHN MA:M RHN^K> BG MA> BGM>K: A>EIFH=> *HP RHN GM>K :GR D>RPHK= G:F> ?NG [ IK>MMR FNKLM:G=L [ :G= K>:= =HGM:MBHG HG BM

IFMQ QSJOU )FMQPOCVJMUJOGVODUJPOQSJOUJONPEVMFCVJMUJOT

QSJOU

QSJOUWBMVFTFQFOE=OGJMFTZTTUEPVU

1SJOUTUIFWBMVFTUPBTUSFBNPSUPTZTTUEPVUCZEFGBVMU

0QUJPOBMLFZXPSEBSHVNFOUT

GJMFBGJMFMJLFPCKFDUTUSFBNEFGBVMUTUPUIFDVSSFOUTZTTUEPVU

TFQTUSJOHJOTFSUFECFUXFFOWBMVFTEFGBVMUBTQBDF

FOETUSJOHBQQFOEFEBGUFSUIFMBTUWBMVFEFGBVMUBOFXMJOF

IFMQ 1BQBZB8IJQ OP1ZUIPOEPDVNFOUBUJPOGPVOEGPS1BQBZB8IJQ

IFMQ RVJU

:PVBSFOPXMFBWJOHIFMQBOESFUVSOJOHUPUIF1ZUIPOJOUFSQSFUFS

*GZPVXBOUUPBTLGPSIFMQPOBQBSUJDVMBSPCKFDUEJSFDUMZGSPNUIF

JOUFSQSFUFSZPVDBOUZQFIFMQPCKFDU&YFDVUJOHIFMQTUSJOH

IBTUIFTBNFFGGFDUBTUZQJOHBQBSUJDVMBSTUSJOHBUUIFIFMQQSPNQU

0H @>M =HGM:MBHG HG MA> QSJOU ?NG QSJOU :G= IK>LL &/5&3 0A> BGM>K: A>EI FH=>

PBEE =BLIE:R LHF>MABG@ :DBG MH : F:G I:@> MA> ?NG : ;KB>? LRGHILBL MA> ?NGGML :G=MA>BK =>?:NEM O:EN>L :G= LH HG %? MA> =HGM:MBHG L>>FL HI:JN> MH RHN =HG^M I:GB< 5HN^EE E>:KG FHK>:;HNM :EE MA>L> G>QM ?>P KL

+? MA> BGM>K: A>EI FH=> =H>LG^M DGHP >O>KRMABG@ %? RHN MRI> LHF>MABG@ MA:M BLG^M : ,RMAHG ?NGK ;NBEMBG D>RPHK= MA> BGM>K: A>EI FH=> PBEE CNLM LAKN@ BML OBKMN:ELAHNE=>KL

0H JNBM MA> BGM>K: A>EI FH=> MRI> RVJU :G= IK>LL &/5&3

0A> IKHFIM L ;: E>?M MA> BGM>K: A>EI FH=> :G= K>MNKG>= MH MA>,RMAHG /A>EE

% (! MA> @K:IABEE :ELH BGL : ,RMAHG:P:K> M>QM >=BMHK

0.8. PYTHON EDITORS AND IDES

% (! BL GHM MA> HGER @:F> BG MHPG PA>G BM L MH PKBMBG@ IKH@K:FL BG ,RMAHG 3ABE> BM^L NL>?NE MH @>MLM:KM>= PBMA E>:KGBG@ MA> E:G@N:@> BML>E? F:GR =>O>EHI>KL IK>?>K HMA>K M>QM >=BMHKL HK %GM>@K:M>= >O>EHIF>GM !GOBKHGF>GML % !L % PHG^M K MA>F A>K> ;NM MA> ,RMAHG >=BMHKL MA:M KL : PB=> K:G@> H? LNIIHKM>= IE:M?HKFL :G= LH?MP:K> EBGL>L

5HN FB@AM :ELH P:GM MH EBLM H? ,RMAHG:P:K> % !L :EMAHN@A ?>P H? MA>F LNIIHKM ,RMAHG R>M+G> MA:M =H>L BL ,R >O : IEN@BG ?HK ! MA:M MNKGL ! BGMH : ?NEE?E>=@>= ,RMAHG % ! HMA !:G= ,R >O :K> G LHNK

+G MA> KK> BL /M:M>^L 'HFH=H % ! %M A:L I>KNL>K EBGLBG@ ;NM LMN=>GML M: =BL> MBF>EBFBM>= MKB:E O>KLBHG BL :O:BE:;E>

%^O> ;>>G IKH@K:FFBG@ BG ,RMAHG ?HK GBG> R>:KL :G= % >=BM FR ,RMAHG IKH@K:FL BG #*1 !F:;N@MA>F BG MA> ,RMAHG /A>EE 0A>K>^L GH KB@AM HK PKHG@ P:R MH =>O>EHI BG ,RMAHG "BG= : P:RMA:M PHKDL ?HK RHN

CHAPTER 1. YOUR FIRST PYTHON PROGRAM

Dont bury your burden in saintly silence. You have a problem? Great. Rejoice, dive in, and investigate.

Ven. Henepola Gunaratana

1.1. DIVING IN

C HGO>GMBHG =BL MA:M % LAHNE= ;HK> RHN PBMA MA> ?NG=:F>GM:E ;NBE=BG@ ;EHMABG@ NL>?NE (>M^L LDBI :EE MA:M $>K> BL : M> PHKDBG@ ,RMAHGIKH@K:F %M IKH;:;ER F:D>L :;LHENM>ER GH L>GL> MH RHN HG^M PHKKR :;HNM MA:M ;> RHN^K> @HBG@ MH=BLL> ;R EBG> NM K>:= MAKHN@A BM ?BKLM :G= L>> PA:M B? :GRMABG@ RHN H? BM

46''*9&4\

^

EFGBQQSPYJNBUF@TJ[FTJ[FB@LJMPCZUF@JT@@CZUFT5SVF

$POWFSUBGJMFTJ[FUPIVNBOSFBEBCMFGPSN

,FZXPSEBSHVNFOUT

TJ[FGJMFTJ[FJOCZUFT

B@LJMPCZUF@JT@@CZUFTJG5SVFEFGBVMU

VTFNVMUJQMFTPG

JG'BMTFVTFNVMUJQMFTPG

3FUVSOTTUSJOH

JGTJ[F

SBJTF7BMVF&SSPSOVNCFSNVTUCFOPOOFHBUJWF

NVMUJQMFJGB@LJMPCZUF@JT@@CZUFTFMTF

GPSTVGGJYJO46''*9&4

TJ[FNVMUJQMF

JGTJ[FNVMUJQMF

SFUVSO\G^\^GPSNBUTJ[FTVGGJY

SBJTF7BMVF&SSPSOVNCFSUPPMBSHF

JG@@OBNF@@@@NBJO@@

QSJOUBQQSPYJNBUF@TJ[F'BMTF

QSJOUBQQSPYJNBUF@TJ[F

*HP E>M^L KNG MABL IKH@K:F HG MA> +G 3BG=HPL BM PBEE EHHD LHF>MABG@ EBD> MABL

D=IPNF=EJWFJOUPQZUIPO=FYBNQMFT D=QZUIPO=QZUIPOFYFIVNBOTJ[FQZ

5#

(J#

+G ):< +/ 4 HK (BGNQ BM PHNE= EHHD LHF>MABG@ EBD> MABL

ZPV!MPDBMIPTU_EJWFJOUPQZUIPOFYBNQMFT QZUIPOIVNBOTJ[FQZ

5#

(J#

3A:M CNLM A:II>G>= 5HN >Q>= RHNK ?BKLM ,RMAHG IKH@K:F 5HN = MA> ,RMAHG BGM>KIK>M>K HG MA> :G= RHN I:LL>= MA> G:F> H? MA> L= ,RMAHG MH >Q> 0A> L?BG>L :LBG@E> ?NG BQQSPYJNBUF@TJ[F ?NGQ: LBS> BG ;RM>L :G= MMR] ;NM :IIKHQBF:M> LBS> 5HNÔ> IKH;:;ER L>>G MABL BG 3BG=HPL !QIEHK>K HK MA> ):< +/ 4 "BG=>KHK *:NMBENL HK HEIABG HK 0ANG:K HG (BGNQ %? RHN =BLIE:R : ?HE=>K H? =HGML :L : FNEMB PBMA MA> =HGM B =HGM G:F> MA> LBS> MRI> E:LMFH=B?B>= =:M> :G= LH HG%? MA> ?HE=>K ?BE> G:F>= 50%0 RHNK ?BE> F:G:@>K PHG^M =BLIE:R 50%0 CZUFT BMÊE L:RLHF>MABG@ EBD> 50%0 ,# BGLM>:= 0A:M^L PA:M MA> BQQSPYJNBUF@TJ[F ?NGL

(HHD :M MA> ;HMMHF H? MA> L> MPH L>:K> ?NGGM:DBG@ MA> K>MNKG O:EN> :G= I:LLBG@ BM LMK:B@AM HG MH MA> QSJOU ?NG QSJOU ?NGO>K L>> :G >QIEB :GRPA>K> 0A>K> :K> EHML H? ;NBEMBG ?NG ?NG L>I:K:M>= BGMH modules ,:MB>G @K:LLAHII>K

/H PAR =H>L KNGGBG@ MA> L @BO> RHN MA> L:F> HNMINM >O>KR MBF> 3>ÊE @>M MHMA:M "BKLM E>M^L EHHD :M MA:M BQQSPYJNBUF@TJ[F ?NGK E:G@N:@>L ;NM BM =H>L GHM A:O> L>I:K:M> A>:=>K ?BE>L EBD>

HKJOUFSGBDFJNQMFNFOUBUJPO L> ,:LG RHN G>>= : ?NG BM EBD> MABL


When you

need a

function,

just declare

it.

0A> D>RPHK= EFG LM:KML MA> ?NG= ;R MA> ?NG ?HEEHP>= ;R MA>:K@NF>GML BG I:K>GMA>L>L )NEMBIE> :K@NF>GML :K>L>I:K:M>= PBMA MA:M MA> ?NGLG^M =>?BG> : K>MNKG=:M:MRI> ,RMAHG ?NG =:M:MRI>H? MA>BK K>MNKG O:EN> MA>R =HG^M >O>G LI>MA>K HKGHM MA>R K>MNKG : O:EN> %G ?:O>KR ,RMAHG ?NGMNKGL : O:EN> B? MA> ?NGO>K >Q>L : SFUVSOLM:M>F>GM BM PBEE K>MNKG MA:M O:EN> HMA>KPBL> BM PBEEK>MNKG /POF MA> ,RMAHG GNEE O:EN>

%G LHF> E:G@N:@>L ?NGMNKG :O:EN> LM:KM PBMA GVODUJPO :G= LN;KHNMBG>LMA:M =H GHM K>MNKG : O:EN> LM:KM PBMA TVC0A>K> :K> GH LN;KHNMBG>L BG ,RMAHG !O>KRMABG@ BL : ?NG >O>G B? BM^L /POF :G= :EE ?NG BQQSPYJNBUF@TJ[F ?NGL MA> MPH :K@NF>GML [ TJ[F :G=B@LJMPCZUF@JT@@CZUFT [ ;NM G>BMA>K :K@NF>GM LI>L : =:M:MRI> %G ,RMAHG O:KB:;E>L :K> G>O>K>QIEB= ,RMAHG ?B@NK>L HNM PA:M MRI> : O:KB:;E> BL :G= D>>IL MK:KG:EER

%G &:O: :G= HMA>K LM:MB= E:G@N:@>L RHN FNLM LI> =:M:MRI> H? MA>?NGMNKG O:EN> :G= >:O>K >QIEB =:M:MRI> H? :GRMABG@ :L>= HG PA:M O:EN> RHN :LLB@G ,RMAHG D>>IL MK: =:M:MRI> BGM>KG:EER

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

1.2.1. OPTIONAL AND NAMED ARGUMENTS

,RMAHG :EEHPL ?NGGML MH A:O> =>?:NEM O:EN>L B? MA> ?NG :K@NF>GM MA>:K@NF>GM @>ML BML =>?:NEM O:EN> "NKMA>KFHK> :K@NF>GML LI>= BG :GR HK=>K ;R NLBG@ G:F>=:K@NF>GML

(>M^L M:D> :GHMA>K EHHD :M MA:M BQQSPYJNBUF@TJ[F ?NG L>GM B@LJMPCZUF@JT@@CZUFT LI>L : =>?:NEM O:EN> H? 5SVF 0ABL F>:GL MA>:K@NF>GM BL optional RHN ;HMMHF H? MA> LQIEB= 'BMTF :L MA> L>GM

0ABL BQQSPYJNBUF@TJ[F ?NG :K@NF>GM NM MA:M^L +' ;> MA> L>GM BL HIMBHG:E /BG MA> K =H>LG^M LI> L>GM =>?:NEML MH 5SVF :L =>?BG>= ;RMA> ?NG

GSPNIVNBOTJ[FJNQPSUBQQSPYJNBUF@TJ[F

BQQSPYJNBUF@TJ[FB@LJMPCZUF@JT@@CZUFT'BMTF ,#

BQQSPYJNBUF@TJ[FTJ[FB@LJMPCZUF@JT@@CZUFT'BMTF ,#

BQQSPYJNBUF@TJ[FB@LJMPCZUF@JT@@CZUFT'BMTFTJ[F ,#

BQQSPYJNBUF@TJ[FB@LJMPCZUF@JT@@CZUFT'BMTF 'JMFTUEJOMJOF

4ZOUBY&SSPSOPOLFZXPSEBSHBGUFSLFZXPSEBSH

BQQSPYJNBUF@TJ[FTJ[F'BMTF 'JMFTUEJOMJOF

4ZOUBY&SSPSOPOLFZXPSEBSHBGUFSLFZXPSEBSH

0ABL BQQSPYJNBUF@TJ[F ?NG ?BKLM :K@NF>GM TJ[F :G= 'BMTF ?HK MA>:K@NF>GM G:F>= B@LJMPCZUF@JT@@CZUFT 0A:M A:II>GL MH ;> MA> L>GM ;NM =H>LG^MF:MM>K :L RHN^EE L>> BG : FBGNM>

0ABL BQQSPYJNBUF@TJ[F ?NG :K@NF>GM G:F>= TJ[F :G= 'BMTF ?HK MA>:K@NF>GM G:F>= B@LJMPCZUF@JT@@CZUFT 0A>L> G:F>= :K@NF>GML A:II>G MH ;> BG MA> L:F> HK=>K :LMA> :K@NF>GML :K> EBLM>= BG MA> ?NGLG^M F:MM>K >BMA>K

0ABL BQQSPYJNBUF@TJ[F ?NG P:L F>:GM ?HK MA>B@LJMPCZUF@JT@@CZUFT :K@NF>GM NM ,RMAHG =H>LG^M PHKD MA:M P:R L LHHG :L RHN A:O> : G:F>=:K@NF>GM :EE :K@NF>GML MH MA> KB@AM H? MA:M G>>= MH ;> G:F>= :K@NF>GML MHH

Rohit Kondekar

Every

function

1.3. WRITING READABLE CODE

% PHG^M ;HK> RHN PBMA : EHG@ ?BG@>KP:@@BG@ LI>> BFIHKM:G H? =HGMBG@ RHNK &NLMDGHP MA:M BL PKBMM>G HG ;NM K>:= F:GR MBF>L :G= MA> FHLM BFIHKM:GM :N=B>G ?HK RHNK BLRHNKL>E? LBQ FHGMAL :?M>K PKBMBG@ BM i.e. :?M>K RHN^O> ?HK@HMM>G >O>KRMABG@ ;NM G>>= MH ?BQ LHF>MABG@ ,RMAHGF:D>L BM >:LR MH PKBM> K>:=:;E> LH M:D> :=O:GM:@> H? BM 5HN^EE MA:GD F> BG LBQ FHGMAL

1.3.1. DOCUMENTATION STRINGS

5HN

deserves a

decent

docstring.

0KBIE> JNHM>L :K> :ELH :G >:LR P:R MH =>?BG>: LMKBG@ PBMA ;HMA LBG@E> :G= =HN;E> JNHM>LEBD> RR BG ,>KE

!O>KRMABG@ ;>MP>>G MA> MKBIE> JNHM>L BL MA> ?NGQBLML FNLM ;> MA> ?BKLM MABG@ =>?BG>= BG: ?NG G>QM EBG> :?M>K MA> ?NG= MH @BO> RHNK?NG A>:K= MABL BG >O>KR IKH@K:FFBG@ >O>K M:D>G ;NM ,RMAHG @BO>L RHN :G :==>= BGGMBO> MA>EPDTUSJOH BL :O:BE:;E> :M KNGMBF> :L :G :MMKB;NM> H? MA> ?NG MA> EPDTUSJOH MH IKHOB=> QML>GLBMBO> =HGM:MBHG LHMA:M PA>G RHN MRI> : ?NG BML EPDTUSJOH :II>:KL :L : MHHEMBI 0ABL BG=B;ER A>EI?NE ;NM BM^L HGER :L @HH= :L MA> EPDTUSJOHL RHN PKBM>

1.4. THE JNQPSU SEARCH PATH

>?HK> MABL @H>L :GR ?NKMA>K % P:GM MH ;KB>?ER F>GMBHG MA> EB;K:KR L>:KO>K:E IE:LPA>G RHN MKR MH BFIHKM : FH=NE> /I>L =>?BG>= BG TZTQBUI 0ABL BL CNLM: EBLM :G= RHN :LBER OB>P BM HK FH=B?R BM PBMA LM:G=:K= EBLM F>MAH=L 5HN^EE E>:KG FHK> :;HNM EBLML BG*:MBO> :M:MRI>L

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

JNQPSUTZT

TZTQBUI

%FIHKMBG@ MA> TZT FH=NE> F:D>L :EE H? BML ?NGL :O:BE:;E> TZTQBUI BL : EBLM H? =BK>L MA:M MA> GM L>:KK>GM

=>I>G=BG@ HG RHNK HI>K:MBG@ LRLM>F PA:M O>KLBHG H? ,RMAHG RHN^K> KNGGBG@ :G= PA>K> BM P:L HKB@BG:EERBGLM:EE>= ,RMAHG PBEE EHHD MAKHN@A MA>L> =BK>L BG MABL HK=>K ?HK : QZ ?BE> PAHL> G:F> F:ML PA:MRHN^K> MKRBG@ MH BFIHKM

= MA> MKNMA BL FHK> GHM :EE FH=NE>L :K> LMHK>= :L QZ ?BE>L/HF> :K> built-in modules MA>R :K> := KB@AM BGMH ,RMAHG BML>E? NBEMBG FH=NE>L ;>A:O> CNLM EBD>K>@NE:K FH=NE>L ;NM MA>BK ,RMAHG LHNK BL GHM :O:BE:;E> ;> MA>R :K> GHM PKBMM>G BG ,RMAHG (BD>,RMAHG BML>E? MA>L> ;NBEMBG FH=NE>L :K> PKBMM>G BG

5HN P =BK>:K ;R :==BG@ MA> =BK> MH TZTQBUI:G= MA>G ,RMAHG PBEE EHHD BG MA:M =BK>EE PA>G>O>K RHN MKR MH BFIHKM : FH=NE> 0A> >??>

R NLBG@ TZTQBUIJOTFSU OFX@QBUI RHN BGL>KM>= : G>P =BK> ?BKLM BM>F H? MA> TZTQBUIEBLM :G= MA>K>?HK> :M MA> ;>@BGGBG@ H? ,RMAHG^L L>:KKLBHG H? : I:KMBO>KLBHG MABL >GLNK>L MA:M RHNK FH=NE>L PBEE ;> ?HNG= :G= NL>= BGLM>:= H? MA> FH=NE>L MA:M PBMA,RMAHG

1.5. EVERYTHING IS AN OBJECT

%G RHN FBLL>= BM % CNLM L:B= MA:M ,RMAHG ?NG :MMKB;NM>L :G= MA:M MAHL> :MMKB;NM>L :K> :O:BE:;E>:M KNGMBF> ?NG >O>KRMABG@ >EL> BG ,RMAHG BL :G H;C> BGM>K: ,RMAHG LA>EE :G= ?HEEHP :EHG@

JNQPSUIVNBOTJ[F

QSJOUIVNBOTJ[FBQQSPYJNBUF@TJ[F5SVF

,J#

QSJOUIVNBOTJ[FBQQSPYJNBUF@TJ[F@@EPD@@ $POWFSUBGJMFTJ[FUPIVNBOSFBEBCMFGPSN

,FZXPSEBSHVNFOUT

TJ[FGJMFTJ[FJOCZUFT

B@LJMPCZUF@JT@@CZUFTJG5SVFEFGBVMU

VTFNVMUJQMFTPG

JG'BMTFVTFNVMUJQMFTPG

3FUVSOTTUSJOH

0A> ?BKLM EBG> BFIHKML MA> IVNBOTJ[F IKH@K:F :L : FH=NE> [ : K:ERHK ?KHF : E:K@>K ,RMAHG IKH@K:F +G RHN BFIHKM : FH=NE> RHN ?>K>G :GR H? BML IN;EB< ?NGL )H=NE>L

,RMAHG BGM>K: LA>EE MHH 0ABL BL :G BFIHKM:GM > : EHM FHK> H? BM MAKHN@AHNM MABL;HHD

3A>G RHN P:GM MH NL> ?NG?BG>= BG BFIHKM>= FH=NE>L RHN G>>= MH BG MA> FH=NE> G:F> /HRHN IVNBOTJ[FBQQSPYJNBUF@TJ[F %? RHN^O> NL>= L BG&:O: MABL LAHNE= ?>>E O:@N>ER ?:FBEB:K

%GLM>:= H? ?NGQI>= ?HK HG> H? MA> ?NGL@@EPD@@

JNQPSU BG ,RMAHG BL EBD> SFRVJSF BG ,>KE +G RHN JNQPSU : ,RMAHG FH=NE> RHN: RHN:O>KRMABG@ :MMKB;NM>L :G= F>MAH=L EE ?NG : ;NBEMBG :MMKB;NM> @@EPD@@ PABMNKGL MA> EPDTUSJOH =>?BG>= BG MA> ?NG TZT FH=NE> BL:G H;C> = QBUI G= LH ?HKMA

/MBEE MABL =H>LG^M :GLP>K MA> FHK> ?NG=:F>GM:E JN>LMBHG PA:M BL :G H;C>K>GM IKH@K:FFBG@ E:G@N:@>L=>?BG> \H;C>K>GM P:RL %G LHF> BM F>:GL MA:M all H;C> :MMKB;NM>L :G= F>MAH=L BGHMA>KL BM F>:GL MA:M :EE H;C> LN; %G ,RMAHG MA> =>?BGBMBHG BL EHHL>K /HF> H;C>G>BMA>K :MMKB;NM>L GHK F>MAH=L but they could *HM :EE H;C> LN; NM >O>KRMABG@ BL :G H;C> L>GL> MA:M BM :LLB@G>= MH : O:KB:;E> HK I:LL>= :L :G :K@NF>GM MH : ?NG A>:K= MA> M>KF \?BKLMK IKH@K:FFBG@ QML %G ,RMAHG ?NGfirst-class objects 5HN K ?NGL :K> first-class objects5HN GMBK> FH=NE> :L :G :K@NF>GM MH : ?NGL :K> ?BKLM:M BM BG RHN FBLL>= BM MA> ?BKLM ?>P MBF>L everything in Python is anobject /MKBG@L :K> H;C> H;C>

1.6. INDENTING CODE

,RMAHG ?NG GH >QIEB MA> ?NG HGER =>EBFBM>K BL : BG=>GM:MBHG H? MA> BML>E?


JGTJ[F


NVMUJQMFJGB@LJMPCZUF@JT@@CZUFTFMTF

GPSTVGGJYJO46''*9&4 TJ[FNVMUJQMF

JGTJ[FNVMUJQMF

SFUVSO\G^\^GPSNBUTJ[FTVGGJY

SBJTF7BMVF&SSPSOVNCFSUPPMBSHF

H=> ;EH =>?BG>= ;R MA>BK BG=>GM:MBHG R \ ;EH:G ?NGF>GML GPS EHHILXIJMF EHHIL :G= LH ?HKMA %G=>GMBG@ LM:KML : ;EHGMBG@ >G=L BM 0A>K> :K> GH >QIEBRPHK=L 0ABL F>:GL MA:M PABM>LI: BL LB@GB?B GM %G MABL >Q:FIE> MA>?NGGM>= ?HNK LI:L %M =H>LG^M G>>= MH ;> ?HNK LI:L BM CNLM G>>=L MH ;> GM 0A>?BKLM EBG> MA:M BL GHM BG=>GM>= F:KDL MA> >G= H? MA> ?NGF>GM BL ?HEEHP>= ;R : ;EH JG >QIK>LLBHG >O:EN:M>L MH MKN> MA> BG=>GM>=;EHQ>= HMA>KPBL> BM ?:EEL MH MA> FMTF ;EH MA> E:GMA>L>L :KHNG= MA>>QIK>LLBHG

0ABL EBG> BL BGLB=> MA> JG ;EHF>GM PBEE K:BL> :G >QIMBHG H? MRI> 7BMVF&SSPS ;NMHGER B? TJ[F

0ABL BL not MA> >G= H? MA> ?NGM>ER ;E:GD EBG>L =HG^M R MA> FHK>K>:=:;E> ;NM MA>R =HG^M KL 0A> ?NG G>QM EBG>

Rohit Kondekar

0A> GPS EHHI :ELH F:KDL MA> LM:KM H? : ;EH ;EHR:K> :EE BG=>GM>= MA> L:F> :FHNGM 0ABL GPS EHHI A:L MAK>> EBG>L H? BG BM 0A>K> BL GH HMA>K LI> ;EHGM :G= @>M HG PBMA RHNK EB?>

?M>K LHF> BGBMB:E IKHM>LML :G= L>O>K:E LGB=> :G:EH@B>L MH "HKMK:G RHN PBEE F:D> I>: PBMA MABL :G= LM:KML>>BG@ BML ;>G>?BML +G> F:CHK ;>G>?BM BL MA:M :EE ,RMAHG IKH@K:FL EHHD LBFBE:K LBG BG=>GM:MBHG BL : E:G@N:@>K>JNBK>F>GM :G= GHM : F:MM>K H? LMRE> 0ABL F:D>L BM >:LB>K MH K>:= :G= NG=>KLM:G= HMA>K I>HIE>^L ,RMAHG

,RMAHG NL>L K>MNKGL MH L>I:K:M> LM:M>F>GML :G= : GM:MBHG MHL>I:K:M> ;EH L>FBI:K:M> LM:M>F>GML :G= I:K:M> ;EH >O>KRPA>K> BG ,RMAHG 2BKMN:EER >O>KR FH=NE> BG MA> LM:G=:K= ,RMAHG EB;K:KR NL>L MA>F :G=,RMAHG BML>E? PBEE K:BL> MA>F BG : EHM H? =B??>K>GM MA>F K>I>:M>=ER MAKHN@AHNM MABL;HHD

3A:M BL :G >QIMBHG 1LN:EER BM^L :G >KKHK :G BG=BMABG@ P>GM PKHG@ *HM :EE >QIMBHGL :K>>KKHKL ;NM G>O>K FBG= MA:M ?HK GHP /HF> IKH@K:FFBG@ E:G@N:@>L >G MA> NL> H? >KKHK K>MNKGL PABGL MA> NL> H? >QIMBHGL PABG :G >KKHK HEE BM IKBGML HNM LHF> =>M:BEL :;HNM MA> >QIMBHG :G= AHP BMA:II>G>= :G= MA:M^L MA:M 0ABL BL = :G unhandled >QIMBHG 3A>G MA> >QIMBHG P:L K:BL>= MA>K> P:LGH MH >QIEB:E PBMA BM LH BM ;N;;E>= BML P:R ;: MHI E>O>E H? MA> ,RMAHG/A>EE PAB =>;N@@BG@ BG?HKF:MBHG :G= LA>EE MA:ML GH ;B@ =>:E ;NM B? MA:MA:II>G>= PABE> RHNK : >GMBK> IKH@K:F PHNE= MH : L>L MA> >QIMBHG ):R;> MA:M^L PA:M RHN P:GM F:R;> BM BLG^M

Rohit Kondekar

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

NL> USZDBUDI ;EH >QIMBHGL :G= MA>UISPX LM:M>F>GM MH @>G>K:M> MA>F

0A> BQQSPYJNBUF@TJ[F ?NGL >QIMBHGL BG MPH =B??>K>GM L B? MA> @BO>G TJ[F BL E:K@>K MA:GMA> ?NGLB@G>= MH A:G=E> HK B? BM^L E>LL MA:G S>KH

JGTJ[F


0A> LRGM:Q ?HK K:BLBG@ :G >QIMBHG BL LBFIE> >GHN@A 1L> MA> SBJTF LM:M>F>GM ?HEEHP>= ;R MA> >QIMBHGG:F> :G= :G HIMBHG:E ANF:GK>:=:;E> LMKBG@ ?HK =>;N@@BG@ INKIHL>L 0A> LRGM:Q BL K>FBGBLGM H? QIMBHGL :K> BFIE>F>GM>= :L L :G= MABL SBJTF LM:M>F>GM BL : 7BMVF&SSPS LMKBG@ OVNCFS NVTU CF OPOOFHBUJWF MH BML BGBMB:EBS:MBHGF>MAH= NM P>^K> @>MMBG@ :A>:= H? HNKL>EO>L

5HN =HG^M G>>= MH A:G=E> :G >QIMBHG BG MA> ?NGL BM %? HG> ?NGLG^M A:G=E> BM MA> >QIMBHG BL I:LL>= MH MA> K A:G=E>= RHNKIKH@K:F PBEE

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

>G= H? MA:M @:BG F:R;> MA:M^L PA:M RHN P:GM BM =>I>G=L HG PA:M RHNK IKH@K:F=H>L

1.7.1. CATCHING IMPORT ERRORS

+G> H? ,RMAHG^L ;NBEMBG >QIMBHGL BL *NQPSU&SSPS PAB= PA>G RHN MKR MH BFIHKM : FH=NE> :G=?:BE 0ABL G ?HK : O:KB>MR H? K>:LHGL ;NM MA> LBFIE>LM BL PA>G MA> FH=NE> =H>LG^M >QBLM BG RHNKBFIHKM L>:K:MNK>L BG RHNK IKH@K:F "HK >Q:FIE> MA>DIBSEFU EB;K:KR IKHOB=>L GM>KA:IL RHNK IKH@K:F P:GML MH NL> MABL EB;K:KRif it exists ;NM @K:?NEER B? MA> NL>K A:LG^M BGLM:EE>= BM 5HN G H? MA> DIBSEFU FH=NE> PBMA : LBFIE> JG LM:M>F>GM

JGDIBSEFU

EPTPNFUIJOH

FMTF

DPOUJOVFBOZXBZ

GHMA>K H? MA> *NQPSU&SSPS >QIMBHG BL PA>G MPH FH=NE>L BFIE>F>GM : BL FHK> =>LBK:;E> MA:G MA> HMA>K ):R;> BM^L ?:LM>K HK BM NL>L E>LL F>FHKR 5HN FH=NE> ;NM ?:EE ;:K>GM FH=NE> B? MA> ?BKLM BFIHKM ?:BEL "HK >Q:FIE> MA> 4)( K M:EDL :;HNMMPH FH=NE>L MA:M BFIE>F>GM : &MFNFOU5SFF ,% 0A> ?BKLM MYNM BL : MABK=I:KMRFH=NE> MA:M RHN G>>= MH =HPGEH:= :G= BGLM:EE RHNKL>E? 0A> L>K ;NMBL I:KM H? MA> ,RMAHG LM:G=:K= EB;K:KR

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit KondekarHow try except can be used for importing modules.

USZ

GSPNMYNMJNQPSUFUSFF

FYDFQU*NQPSU&SSPS

JNQPSUYNMFUSFF&MFNFOU5SFFBTFUSFF

R MA> >G= H? MABL USZFYDFQU ;EH BFIHKM>= some FH=NE> :G= G:F>= BM FUSFF /BG ;HMAFH=NE>L BFIE>F>GM : K>LM H? RHNK =H>LG^M G>>= MH D>>I = G= LBG MA> FH=NE> MA:M did @>M BFIHKM>= BL :EP:RL = FUSFF MA> K>LM H? RHNK =H>LG^MG>>= MH ;> EBMM>K>= PBMA JG LM:M>F>GML MH K>GMERG:F>= FH=NE>L

1.8. UNBOUND VARIABLES

0:D> :GHMA>K EHHD :M MABL EBG> H? ?KHF MA> BQQSPYJNBUF@TJ[F ?NGO>K => MA> O:KB:;E> NVMUJQMF RHN CNLM :LLB@G : O:EN> MH BM 0A:M^L +' ;> ,RMAHG E>ML RHN=H MA:M 3A:M ,RMAHG PBEE not E>M RHN =H BL K>?>K>G : O:KB:;E> MA:M A:L G>O>K ;>>G :LLB@G>= : O:EN> 0KRBG@MH =H LH PBEE K:BL> : /BNF&SSPS >QIMBHG

Y

5SBDFCBDLNPTUSFDFOUDBMMMBTU

'JMFTUEJOMJOFJONPEVMF

/BNF&SSPSOBNFYJTOPUEFGJOFE

Y

Y

5HN PBEE MA:GD ,RMAHG ?HK MABL HG> =:R

Rohit Kondekar

1.9. EVERYTHING IS CASE-SENSITIVE

EE G:F>L BG ,RMAHG :K> L>GLBMBO> O:KB:;E> G:F>L ?NGL L FH=NE> G:F>L >QIMBHGG:F>L %? RHN M BM L>M BM

BO@JOUFHFS

BO@JOUFHFS

"/@*/5&(&3



/BNF&SSPSOBNF"/@*/5&(&3JTOPUEFGJOFE

"O@*OUFHFS



/BNF&SSPSOBNF"O@*OUFHFSJTOPUEFGJOFE

BO@JOUF(FS



/BNF&SSPSOBNFBO@JOUF(FSJTOPUEFGJOFE

G= LH HG

Rohit Kondekar

Rohit Kondekar

Everything

in Python is

an object.

1.10. RUNNING SCRIPTS

,RMAHG FH=NE>L :K> H;C> L>O>K:E NL>?NE:MMKB;NM>L 5HN MABL MH >:LBER M>LM RHNK FH=NE>L:L RHN PKBM> MA>F ;R BGL PA>G RHN KNG MA> ,RMAHG ?BE> HG MA>

0:D> MA> E:LM ?>P EBG>L H? IVNBOTJ[FQZ

JG@@OBNF@@@@NBJO@@

QSJOUBQQSPYJNBUF@TJ[F'BMTF

QSJOUBQQSPYJNBUF@TJ[F

(BD> ,RMAHG NL>L ?HK GM 1GEBD> ,RMAHG =H>LGHM LNIIHKM BGEBG> :LLB@GF>GM LH MA>K>^L GH RHN MAHN@AM RHN P>K> L MABL JG LM:M>F>GM LI>EE FH=NE>L :K> H;C>L A:O> : ;NBEMBG :MMKB;NM>@@OBNF@@ FH=NE>^L @@OBNF@@ =>I>G=L HG AHP RHN^K> NLBG@ MA> FH=NE> %? RHN JNQPSU MA> FH=NE> MA>G@@OBNF@@ BL MA> FH=NE>^L ?BE>G:F> PBMAHNM : =BK> >QM>GLBHG

JNQPSUIVNBOTJ[F

IVNBOTJ[F@@OBNF@@

IVNBOTJ[F

Rohit Kondekar

Rohit Kondekar

NM RHN FH=NE> =BK> IKH@K:F BG PAB : LI>?:NEM O:EN> @@NBJO@@ ,RMAHG PBEE >O:EN:M> MABL JG LM:M>F>GM ?BG= : MKN> >QIK>LLBHG :G= >Q> MA> JG ;EHL

D=IPNF=EJWFJOUPQZUIPO D=QZUIPO=QZUIPOFYFIVNBOTJ[FQZ

5#

(J#

G= MA:M^L RHNK ?BKLM ,RMAHG IKH@K:F

1.11. FURTHER READING

V ,!, HGMBHGL >QIE:BGL PA:M =BLMBG@NBLA>L : @HH= EPDTUSJOH ?KHF : @K>:M EPDTUSJOHV ,RMAHG 0NMHKB:E HGM:MBHG /MKBG@L :ELH MHNL HG MA> LN;C> #NB=> ?HK ,RMAHG H=> =BLL @HH= BG=>GM:MBHG LMRE>V Python Reference Manual >QIE:BGL PA:M BM F>:GL MH L:R MA:M >O>KRMABG@ BG ,RMAHG BL :G H;C> LHF>

I>HIE> :K> I>=:GML :G= EBD> MH =BL:M E>G@MA

Rohit Kondekar

CHAPTER 2. NATIVE DATATYPES

Wonder is the foundation of all philosophy, inquiry its progress, ignorance its end.

Michel de Montaigne

2.1. DIVING IN

D :M:MRI>L />M :LB=> RHNK ?BKLM ,RMAHG IKH@K:F ?HK CNLM : FBGNM> :G= E>M^L M:ED :;HNM =:M:MRI>L %G,RMAHG >O>KR O:EN> A:L : =:M:MRI> ;NM RHN =HG^M G>>= MH => MA> =:M:MRI> H? O:KB:;E>L $HP =H>L MA:MPHKD :L>= HG >:^L HKB@BG:E :LLB@GF>GM ,RMAHG ?B@NK>L HNM PA:M MRI> BM BL :G= D>>IL MK:KG:EER

,RMAHG A:L F:GR G:MBO> =:M:MRI>L $>K> :K> MA> BFIHKM:GM HG>L

Booleans :K> >BMA>K 5SVF HK 'BMTF Numbers BGM>@>KL :G= ?EH:ML :G= ?K:O>G Q

GNF;>KL Strings :K> L>JN>GL H? 1GB ?BE> Lists :K> HK=>K>= L>JN>GL H? O:EN>L Tuples :K> HK=>K>= BFFNM:;E> L>JN>GL H? O:EN>L Sets :K> NGHK=>K>= ;:@L H? O:EN>L Dictionaries :K> NGHK=>K>= ;:@L H? D>RO:EN> I:BKL

+? MA>K> :K> FHK> MRI>L MA:G MA>L> !O>KRMABG@ BL :G H;C>K> :K> MRI>L EBD>module function class method file :G= >O>G compiled code 5HN^O> :EK>:=R L>>G LHF> H? MA>L> FH=NE>L A:O>G:F>L ?NG EPDTUSJOHT &< 5HN^EE E>:KG :;HNM L BG E:LL>L & %M>K:MHKL :G= :;HNM ?BE>L BG"BE>L

You can use

virtually

any

expression

in a boolean

context.

/MKBG@L :G= ;RM>L :K> BFIHKM:GM >GHN@A [ :G= GHN@A [ MA:M MA>R @>M MA>BK HPG K (>M^LEHHD :M MA> HMA>KL ?BKLM

2.2. BOOLEANS

HHE>:GL :K> >BMA>K MKN> HK ?:EL> ,RMAHG A:L MPHKER G:F>= 5SVF :G= 'BMTF PAB= MH :LLB@G ;HHE>:G O:EN>L =BK>LLBHGL O:EN:M> MH : ;HHE>:G O:EN> %G KM:BG IE:L EBD>JG LM:M>F>GML ,RMAHG >QI>QIK>LLBHG MH>O:EN:M> MH : ;HHE>:G O:EN> 0A>L> IE:L :K> =boolean contexts 5HN OBKMN:EER :GR >QIK>LLBHG BG: ;HHE>:G QM :G= ,RMAHG PBEE MKR MH =>M>KFBG> BMLMKNMA O:EN> B??>K>GM =:M:MRI>L A:O> =B??>K>GM KNE>L:;HNM PABL :K> MKN> HK ?:EL> BG : ;HHE>:GQM 0ABL PBEE F:D> FHK> L>GL> HG RHN L>> LHF> >Q:FIE>L E:M>K BG MABL K

"HK >Q:FIE> M:D> MABL LGBII>M ?KHF IVNBOTJ[FQZ

JGTJ[F


TJ[F BL :G BGM>@>K BL :G BGM>@>K :G= BL : GNF>KBK:MHK 0A> K>LNEM H? MA> >QIK>LLBHG TJ[F BL:EP:RL : ;HHE>:G 5HN LM MABL RHNKL>E? BG MA> ,RMAHG BGM>K: LA>EE

TJ[F

TJ[F

'BMTF

TJ[F

TJ[F

'BMTF

TJ[F

TJ[F

5SVF

N> MH LHF> E>@:L E>?M HO>K ?KHF ,RMAHG ;HHE>:GL MK>:M>= :L GNF;>KL 5SVF BL 'BMTF BL

5SVF5SVF

5SVF'BMTF

5SVF'BMTF

5SVF'BMTF



;FSP%JWJTJPO&SSPSJOUEJWJTJPOPSNPEVMPCZ[FSP

!P >P >P HG^M =H MA:M "HK@>M % >O>G F>GMBHG>= BM

Rohit Kondekar

2.3. NUMBERS

*NF;>KL :K> :P>LHF> 0A>K> :K> LH F:GR MH ?KHF ,RMAHG LNIIHKML ;HMA BGM>@>KL :G= ?EH:MBG@ IHBGMGNF;>KL 0A>K>^L GH MRI> =>F ,RMAHG M>EEL MA>F :I:KM ;R MA> IK>L>G HK:;L>G H? : => UZQF ?NG H? :GR O:EN> HK O:KB:;E> L RHN FB@AM >QI> JTJOTUBODF ?NGK : O:EN> HK O:KB:;E> BL H? : @BO>G MRI> ==BG@ :G JOU MH :G JOU RB>E=L :G JOU ==BG@ :G JOU MH : GMPBU RB>E=L : GMPBU ,RMAHG KL MA> JOU BGMH : GMPBU MH I>K?HKF MA> :==BMBHG

MA>G K>MNKGL : GMPBU :L MA> K>LNEM

2.3.1. COERCING INTEGERS TO FLOATS AND VICE-VERSA

L RHN CNLM L:P LHF> HI>K:MHKL EBD> :==BMBHG PBEE K BGM>@>KL MH ?EH:MBG@ IHBGM GNF;>KL :L G>>=>=5HN F ;R RHNKL>E?

Rohit Kondekar

Rohit Kondekar

GMPBU

JOU

JOU

JOU

UZQF DMBTTJOU

5HN QIEB MKNG ?NG

g g

0A> HI>K:MHK I>K?HKFL ?EH:MBG@ IHBGM =BOBLBHG %M K>MNKGL : GMPBU >O>G B? ;HMA MA> GNF>K:MHK :G==>GHFBG:MHK :K> JOUL

0A> HI>K:MHK I>K?HKFL : JNBKDR DBG= H? BGM>@>K =BOBLBHG 3A>G MA> K>LNEM BL IHLBMBO> RHN G BGM>@>K=BOB=BG@ G>@:MBO> GNF;>KL MA> HI>K:MHK KHNG=L \NI] MH MA> G>:K>LM BGM>@>K ):MA>F:MB:DBG@ BM^L KHNG=BG@ \=HPG] LBG g BL E>LL MA:G g ;NM BM K> >QI> HI>K:MHK =H>LG^M :EP:RL K>MNKG :G BGM>@>K %? >BMA>K MA> GNF>K:MHK HK =>GHFBG:MHK BL : GMPBU BM PBEELMBEE KHNG= MH MA> G>:K>LM BGM>@>K ;NM MA> :MNKG O:EN> PBEE ;> : GMPBU

0A> HI>K:MHK F>:GL \K:BL>= MH MA> IHP>K H?] BL 0A> HI>K:MHK @BO>L MA> K>F:BG=>K :?M>K I>K?HKFBG@ BGM>@>K =BOBLBHG =BOB=>= ;R BL PBMA : K>F:BG=>K

H? LH MA> K>LNEM A>K> BL

%G ,RMAHG MA> HI>K:MHK NLN:EER F>:GM BGM>@>K =BOBLBHG ;NM RHN BM;>A:O> EBD> ?EH:MBG@ IHBGM =BOBLBHG ;R BG BG RHNK %G,RMAHG MA> HI>K:MHK :EP:RL F>:GL ?EH:MBG@ IHBGM =BOBLBHG />> ,!, ?HK=>M:BEL

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

Rohit Kondekar

2.3.3. FRACTIONS

,RMAHG BLG^M EBFBM>= MH BGM>@>KL :G= ?EH:MBG@ IHBGM GNF;>KL %M ?:G:KG>= BGAB@A L 0H =>?BG> : ?K: : 'SBDUJPO H;C> GNF>K:MHK :G= =>GHFBG:MHK 5HN K?HKF :EE MA> NLN:E F:MA>F:MBK:MBHGL PBMA ?K:K:MBHGL K>MNKG : G>P 'SBDUJPO

H;C> 'SBDUJPO H;C>GL> GHM MH :M> : ?K:KH =>GHFBG:MHK

2.3.4. TRIGONOMETRY

5HN MKR BG ,RMAHG

Zero values

are false,

and non-

zero values

are true.

JNQPSUNBUI

NBUIQJ

NBUITJONBUIQJ

NBUIUBONBUIQJ

0A> NBUI FH=NE> A:L : K:MBH H? : M>K 0A> NBUI FH=NE> A:L :EE MA> ;:LB< MKB@HGHF>MKB< ?NG AHP>O>K MA:M ,RMAHG =H>L GHM A:O> BG?BGBM> IK>MNKG GHM

2.3.5. NUMBERS IN A BOOLEAN CONTEXT

5HN GNF;>KL BG : ;HHE>:G QM LNF>GM 6>KH O:EN>L :K> ?:EL> :G= GHGS>KH O:EN>L:K> MKN>

EFGJT@JU@USVFBOZUIJOH JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVF ZFTJUTUSVF

JT@JU@USVF

ZFTJUTUSVF

JT@JU@USVF

OPJUTGBMTF


JT@JU@USVF

OPJUTGBMTF

JNQPSUGSBDUJPOT

JT@JU@USVFGSBDUJPOT'SBDUJPO

ZFTJUTUSVF

JT@JU@USVFGSBDUJPOT'SBDUJPO

OPJUTGBMTF

B= RHN DGHP RHN ?BG> RHNK HPG ?NG ,RMAHG BGM>K: LA>EE &NLM IK>LL &/5&3 :M MA> >G=

H? >: :G= &/5&3 HG : ;E:GD EBG> MH ?BGBLA

%G : ;HHE>:G QM GHGS>KH BGM>@>KL :K> MKN> BL ?:EL> *HGS>KH ?EH:MBG@ IHBGM GNF;>KL :K> MKN> BL ?:EL> > ?NE PBMA MABL HG> %? MA>K>^L MA> LEB@AM>LM

KHNG=BG@ >KKHK GHM BFIHLLB;E> :L RHN L:P BG MA> IK>OBHNL L>G ,RMAHG PBEE ;> M>LMBG@ BGLM>:= H? :G= PBEE K>MNKG 5SVF

"K:= BG : ;HHE>:G QM 'SBDUJPO O BL ?:EL> ?HK :EE O:EN>L H? O EE HMA>K?K: MKN>

2.4. LISTS

(BLML :K> ,RMAHG^L PHKDAHKL> =:M:MRI> 3A>G % L:R \EBLM] RHN FB@AM ;> MABGDBG@ \:KK:R PAHL> LBS> % A:O> MH=> BG :=O:G MA:M L:F> MRI> & FN :G :KK:R BG ,>KE %G ,>KE O:KB:;E>L MA:M LMHK> :KK:RL :EP:RLLM:KM PBMA MA> ! L G:F>= :GRMABG@ :G= ,RMAHGD>>IL MK: =:M:MRI> BGM>KG:EER

EBLM BG ,RMAHG BL FN MA:G :G :KK:R BG &:O: :EMAHN@A BM NL>= :L HG> B?MA:M^L K>:EER :EE RHN P:GM HNM H? EB?> ;>MM>K :G:EH@R PHNE= ;> MH MA> "SSBZ-JTU=

2.4.1. CREATING A LIST

K>:MBG@ : EBLM BL >:LR NL> LJN:K> ;K:ML MH PK:I : I:K:M>= EBLM H? O:EN>L

B@MJTU B@MJTU

B@MJTU B

B@MJTU FYBNQMF

B@MJTU FYBNQMF

B@MJTU NQJMHSJN

a_list[0] is

the first

item of

a_list.

"BKLM RHN =>?BG> : EBLM H? ?BO> BM>FL *HM> MA:M MA>R K>M:BG MA>BK HKB@BG:E HK=>K 0ABL BL GHM :G :K>= L>M H? BM>FL

EBLM NL>= EBD> : S>KH;:L>= :KK:R 0A> ?BKLM BM>F H? :GR GHG>FIMR EBLM BL :EP:RL B@MJTU 0A> E:LM BM>F H? MABL ?BO>BM>F EBLM BL B@MJTU ;> EBLML :K> :EP:RL S>KH;:L>= G>@:MBO> BG=>Q :L BM>FL ?KHF MA> >G= H? MA> EBLM F H? :GR GHG>FIMR

EBLM BL :EP:RL B@MJTU %? MA> G>@:MBO> BG=>Q BL =>?BG>= : EBLM RHN M :GR I:KM H? BM :L: G>P EBLM 0ABL BL = slicing MA> EBLM

B@MJTU

B@MJTU

B@MJTU

B@MJTU

B@MJTU

B@MJTU

B@MJTU

5HN M : I:KM H? : EBLM = : \LEB] ;R LI> K>MNKG O:EN> BL : G>P EBLM BM>FL H? MA> EBLM BG HK=>K LM:KMBG@ PBMA MA> ?BKLM LEB BG=>Q BG MABL B@MJTU NI MH;NM GHM BG L>Q BG MABL B@MJTU

/EB HK ;HMA H? MA> LEB BG=BL BL G>@:MBO> %? BM A>EIL RHN :=BG@MA> EBLM ?KHF E>?M MH KB@AM MA> ?BKLM LEB BG=>Q LI>L MA> ?BKLM BM>F RHN P:GM :G= MA> L>QLI>L MA> ?BKLM BM>F RHN =HG^M P:GM 0A> K>MNKG O:EN> BL >O>KRMABG@ BG ;>MP>>G

(BLML :K> S>KH;:L>= LH B@MJTU K>MNKGL MA> ?BKLM MAK>> BM>FL H? MA> EBLM LM:KMBG@ :M B@MJTU NI MH;NM GHM BG E>?M LEB BG=>Q BL RHN :O> BM HNM :G= BL BFIEB>= /H B@MJTU BL MA> L:F> :L B@MJTU;> MA> LM:KMBG@ BL BFIEB>=

/BFBE:KER B? MA> KB@AM LEB BG=>Q BL MA> E>G@MA H? MA> EBLM RHN :O> BM HNM /H B@MJTU BL MA> L:F> :LB@MJTU ;> MABL EBLM A:L ?BO> BM>FL 0A>K> BL : IE>:LBG@ LRFF>MKR A>K> %G MABL ?BO>BM>F EBLMB@MJTU K>MNKGL MA> ?BKLM BM>FL :G= B@MJTU K>MNKGL MA> E:LM MPH BM>FL %G ?:MNKG MA> ?BKLM O BM>FL :G= B@MJTU PBEE K>MNKG MA> K>LM K>@:K=E>LL H? MA> E>G@MA H? MA> EBLM

%? ;HMA LEB BG=BL :K> E>?M HNM :EE BM>FL H? MA> EBLM :K> BG= NM MABL BL GHM MA> L:F> :L MA> HKB@BG:EB@MJTU O:KB:;E> %M BL : G>P EBLM MA:M A:II>GL MH A:O> :EE MA> L:F> BM>FL B@MJTU BL LAHKMA:G= ?HK F:DBG@: M> K> :K> ?HNK P:RL MH :== BM>FL MH : EBLM

B@MJTU

B@MJTUB@MJTU

B@MJTU

B@MJTUBQQFOE5SVF B@MJTU

B@MJTUFYUFOE B@MJTU

B@MJTUJOTFSUi B@MJTU

0A> HI>K:MHK L EBLML MH :M> : G>P EBLM EBLM FL MA>K> BL GHLBS> EBFBM HMA>K MA:G :O:BE:;E> F>FHKR $HP>O>K B? F>FHKR BL : :P:K> MA:M EBLML : L>FHKR %G MABL MA:M G>P EBLM BL BFF>=B:M>ER :LLB@G>= MH MA>>QBLMBG@ O:KB:;E> B@MJTU /H MABL EBG> H? BL K>:EER : MPHLM>I IKHLL [ G:LLB@GF>GM [ PABFHKR PA>G RHN^K> =>:EBG@ PBMA E:K@> EBLML

EBLM :G= MA> BM>FL BG : LBG@E> EBLM =HG^M :EE G>>= MH ;> MA> L:F> MRI>$>K> P> A:O> : EBLM K :G= :G BGM>@>K

0A> BQQFOE F>MAH= :==L : LBG@E> BM>F MH MA> >G= H? MA> EBLM *HP P> A:O> four =B??>K>GM =:M:MRI>L BG MA>EBLM

(BLML :K> BFIE>F>GM>= :L L \K>:MBG@] : EBLM BL K>:EER BGLM:GMB:MBG@ : K:M> HG BM 0A> FYUFOE F>MAH= M:D>L HG> :K@NF>GM : EBLM :G= :II>G=L >: BM>FL H? MA>:K@NF>GM MH MA> HKB@BG:E EBLM

0A> JOTFSU F>MAH= BGL>KML : LBG@E> BM>F BGMH : EBLM 0A> ?BKLM :K@NF>GM BL MA> BG=>Q H? MA> ?BKLM BM>F BG MA>EBLM MA:M PBEE @>M ;NFI>= HNM H? IHLBMBHG (BLM BM>FL =H GHM G>>= MH ;> NGBJN> ?HK >Q:FIE> MA>K> :K> GHPMPH L>I:K:M> BM>FL PBMA MA> O:EN> i MA> ?BKLM BM>F B@MJTU :G= MA> E:LM BM>F B@MJTU

B@MJTUJOTFSU WBMVF BL EBD> MA> VOTIJGU ?NGKE %M :==L :G BM>F MHMA> ;>@BGGBG@ H? MA> EBLM :G= :EE MA> HMA>K BM>FL A:O> MA>BK IHLBMBHG:E BG=>Q ;NFI>=NI MH F:D> KHHF

(>M^L EHHD K :M MA> =B??>K>G ;>MP>>G BQQFOE :G= FYUFOE

B@MJTU

B@MJTUFYUFOE B@MJTU

MFOB@MJTU

B@MJTU

G

B@MJTUBQQFOE B@MJTU

MFOB@MJTU

B@MJTU

0A> FYUFOE F>MAH= M:D>L : LBG@E> :K@NF>GM PAB: BM>FL H? MA:M EBLMMH B@MJTU

%? RHN LM:KM PBMA : EBLM H? MAK>> BM>FL :G= >QM>G= BM PBMA : EBLM H? :GHMA>K MAK>> BM>FL RHN >G= NI PBMA : EBLMH? LBQ BM>FL

+G MA> HMA>K A:G= MA> BQQFOE F>MAH= M:D>L : LBG@E> :K@NF>GM PAB $>K> RHN^K> BQQFOE F>MAH= PBMA : EBLM H? MAK>> BM>FL

%? RHN LM:KM PBMA : EBLM H? LBQ BM>FL :G= :II>G= : EBLM HGMH BM RHN >G= NI PBMA

: EBLM H? L>O>G BM>FL 3ARL>O>G > MA> E:LM BM>F PABG=>= is itself a list (BLML PA:M RHN P:GM HK BM F:R GHM NM BM^L PA:M RHN :LD>= ?HK :G= BM^L PA:MRHN @HM

2.4.4. SEARCHING FOR VALUES IN A LIST

B@MJTU

B@MJTUDPVOUOFX

OFXJOB@MJTU 5SVF

DJOB@MJTU

'BMTF

B@MJTUJOEFYNQJMHSJN

B@MJTUJOEFYOFX

B@MJTUJOEFYD 5SBDFCBDLJOOFSNPTUMBTU

'JMFJOUFSBDUJWFJOQVUMJOFJO

7BMVF&SSPSMJTUJOEFYYYOPUJOMJTU

L RHN FB@AM >QI> DPVOU F>MAH= K>MNKGL MA> GNF;>K H? H BG : EBLM %? :EE RHN P:GM MH DGHP BL PA>MA>K : O:EN> BL BG MA> EBLM HK GHM MA> JO HI>K:MHK BL LEB@AMER ?:LM>K MA:G NLBG@

MA> DPVOU F>MAH= 0A> JO HI>K:MHK :EP:RL K>MNKGL 5SVF HK 'BMTF BM PBEE GHM M>EE RHN AHP F:GR MBF>LMA> O:EN> :II>:KL BG MA> EBLM

*>BMA>K MA> JO HI>K:MHK GHK MA> DPVOU F>MAH= PBEE M>EE RHN where BG MA> EBLM : O:EN> :II>:KL %? RHN G>>=MH DGHP PA>K> BG MA> EBLM : O:EN> BL JOEFY F>MAH= R =>?:NEM BM PBEE L>:K >GMBK> EBLM:EMAHN@A RHN GM H? MA> ;:L>= BG=>Q MH LM:KM ?KHF :G= >O>G :GHIMBHG:E MABK= :K@NF>GM H? MA> ;:L>= BG=>Q MH LMHI L>:K JOEFY F>MAH= ?BG=L MA> first H BG MA> EBLM %G MABL OFX H JOEFY F>MAH= PBEE K>MNKG HGER MA> BG=>Q H? MA> ?BKLMHQI> O:EN> BL GHM ?HNG= BG MA> EBLM MA> JOEFY F>MAH= PBEE K:BL> :G >QIMBHG

3:BM PA:M 0A:M^L KB@AM MA> JOEFY F>MAH= K:BL>L :G >QIMBHG B? BM =H>LG^M ?BG= MA> O:EN> BG MA> EBLM 0ABLBL GHM:;ER =B??>K>GM ?KHF FHLM E:G@N:@>L PABMNKG LHF> BGO:EB= BG=>Q EBD> 3ABE> MABL F:R L>>F:GGHRBG@ :M ?BKLM % MABGD RHN PBEE MH :IIK> BM %M F>:GL RHNK IKH@K:F PBEE LHNK H?

Lists never

have gaps.

MA> IKH;E>F BGLM>:= H? ?:BEBG@ LMK:G@>ER :G= LBE>GMER E:M>K .>F>F;>K BL : O:EB= EBLM BG=>Q %? MA> JOEFY

F>MAH= K>MNKG>= MA:M := MH LHF> GHMLH?NG =>;N@@BG@ L>LLBHGL

2.4.5. REMOVING ITEMS FROM A LIST

(BLML QI:G= :G= GMA> >QI:GLBHG I:KM 0A>K> :K> L>O>K:E =B??>K>GM P:RL MHK>FHO> BM>FL ?KHF : EBLM :L P>EE

B@MJTU

B@MJTU

C

EFMB@MJTU B@MJTU

B@MJTU OFX

5HN MA> EFM LM:M>F>GM MH =>E>M> : LI>F ?KHF : EBLM Q :?M>K =>E>MBG@ BG=>Q =H>L not K>LNEM BG :G >KKHK EE BM>FL :?M>K MA> =>E>M>= BM>F LAB?M

MA>BK IHLBMBHG:E BG=>Q MH \?BEE MA> @:I] :M>= ;R =>E>MBG@ MA> BM>F

HG^M DGHP MA> IHLBMBHG:E BG=>Q *HM : IKH;E>F RHN FHO> BM>FL ;R O:EN> BGLM>:=

B@MJTUSFNPWFOFX B@MJTU

B@MJTUSFNPWFOFX B@MJTU

B@MJTUSFNPWFOFX



7BMVF&SSPSMJTUSFNPWFYYOPUJOMJTU

5HN FHO> :G BM>F ?KHF : EBLM PBMA MA> SFNPWF F>MAH= 0A> SFNPWF F>MAH= M:D>L : value:G= K>FHO>L MA> ?BKLM H ?KHF MA> EBLM @:BG :EE BM>FL :?M>K MA> =>E>M>= BM>F PBEE A:O>MA>BK IHLBMBHG:E BG=BL ;NFI>= =HPG MH \?BEE MA> @:I] (BLML G>O>K A:O> @:IL

5HN MAH= :L H?M>G :L RHN EBD> ;NM BM PBEE K:BL> :G >QIMBHG B? RHN MKR MH K>FHO> :O:EN> MA:M BLG^M BG MA> EBLM

2.4.6. REMOVING ITEMS FROM A LIST: BONUS ROUND

GHMA>K BGM>K>LMBG@ EBLM F>MAH= BL QPQ

0A> QPQ F>MAH= BL R>M :GHMA>K P:R MH K>FHO> BM>FL ?KHF : EBLM;NM PBMA : MPBLM

B@MJTU

B@MJTUQPQ NQJMHSJN

B@MJTU

B@MJTUQPQ C

B@MJTU

B@MJTUQPQ

OFX

B@MJTUQPQ

B

B@MJTUQPQ 5SBDFCBDLNPTUSFDFOUDBMMMBTU


*OEFY&SSPSQPQGSPNFNQUZMJTU

3A>G = PBMAHNM :K@NF>GML MA> QPQ EBLM F>MAH= K>FHO>L MA> E:LM BM>F BG MA> EBLM and returns the valueit removed

5HN FL ?KHF : EBLM &NLM I:LL : IHLBMBHG:E BG=>Q MH MA> QPQ F>MAH= %M PBEE K>FHO>MA:M BM>F LAB?M :EE MA> BM>FL :?M>K BM MH \?BEE MA> @:I] :G= K>MNKG MA> O:EN> BM K>FHO>=

:EEBG@ QPQ HG :G >FIMR EBLM K:BL>L :G >QIMBHG

:EEBG@ MA> QPQ EBLM F>MAH= PBMAHNM :G :K@NF>GM BL EBD> MA> QPQ ?NGKE%M K>FHO>L MA> E:LM BM>F ?KHF MA> EBLM :G= K>MNKGL MA> O:EN> H? MA> K>FHO>= BM>F,>KE A:L :GHMA>K ?NGL MA> ?BKLM BM>F :G= K>MNKGL BMLO:EN> BG ,RMAHG MABL BL >JNBO:E>GM MH B@MJTUQPQ

Empty lists

are false; all

other lists

are true.

2.4.7. LISTS IN A BOOLEAN CONTEXT

5HN : EBLM BG : ;HHE>:G QM LNF>GM

EFGJT@JU@USVFBOZUIJOH

JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVF OPJUTGBMTF


JT@JU@USVF

B@UVQMFBCNQJMHSJN[FYBNQMF B@UVQMF

BCNQJMHSJN[FYBNQMF

B@UVQMF B

B@UVQMF FYBNQMF

B@UVQMF CNQJMHSJN

MNIE> BL =>?BG>= BG MA> L:F> P:R :L : EBLM >QIM MA:M MA> PAHE> L>M H? >E>F>GML BL >G= BG I:K>GMA>L>LBGLM>:= H? LJN:K> ;K:ML

0A> >E>F>GML H? : MNIE> A:O> : =>?BG>= HK=>K CNLM EBD> : EBLM 0NIE> BG=BL :K> S>KH;:L>= CNLM EBD> : EBLM LHMA> ?BKLM >E>F>GM H? : GHG>FIMR MNIE> BL :EP:RL B@UVQMF

*>@:MBO> BG=BL >G= H? MA> MNIE> CNLM EBD> : EBLM /EB : EBLM 3A>G RHN LEB : EBLM RHN @>M : G>P EBLM PA>G RHN LEB : MNIE> RHN @>M :

G>P MNIE>

0A> F:CHK =B??>K>G ;>MP>>G MNIE>L :G= EBLML BL MA:M MNIE>L = %G M>L:K> BFFNM:;E> %G IK:R A:O> GH F>MAH=L MA:M PHNE= :EEHP RHN MH MA>F (BLML A:O>F>MAH=L EBD> BQQFOE FYUFOE JOTFSU SFNPWF :G= QPQ

0NIE>L A:O> GHG> H? MA>L> F>MAH=L5HN ;> MA:M :M>L : G>P MNIE> :G= RHN K : MNIE> MA:M =H>LG^M MA> MNIE> :G=Y MA:M^L :;HNM BM

DPOUJOVFEGSPNUIFQSFWJPVTFYBNQMF

B@UVQMF

BCNQJMHSJN[FYBNQMF

B@UVQMFBQQFOEOFX 5SBDFCBDLJOOFSNPTUMBTU


"UUSJCVUF&SSPSUVQMFPCKFDUIBTOPBUUSJCVUFBQQFOE

B@UVQMFSFNPWF[ 5SBDFCBDLJOOFSNPTUMBTU


"UUSJCVUF&SSPSUVQMFPCKFDUIBTOPBUUSJCVUFSFNPWF

B@UVQMFJOEFYFYBNQMF

[JOB@UVQMF 5SVF

5HN E>F>GML MH : MNIE> 0NIE>L A:O> GH BQQFOE HK FYUFOE F>MAH= 5HN FHO> >E>F>GML ?KHF : MNIE> 0NIE>L A:O> GH SFNPWF HK QPQ F>MAH= 5HN can ?BG= >E>F>GML BG : MNIE> LBG MABL =H>LG^M MA> MNIE> 5HN MA> JO HI>K:MHK MH E>F>GM >QBLML BG MA> MNIE>

/H PA:M :K> MNIE>L @HH= ?HK

V 0NIE>L :K> ?:LM>K MA:G EBLML %? RHN^K> =>?BGBG@ : M H? O:EN>L :G= :EE RHN^K> >O>K @HBG@ MH =H PBMA BMBL BM>K:M> MAKHN@A BM NL> : MNIE> BGLM>:= H? : EBLM

V %M F:D>L RHNK L:?>K B? RHN \PKBM>IKHM>LG^M G>>= MH ;> = 1LBG@ : MNIE> BGLM>:=H? : EBLM BL EBD> A:OBG@ :G BFIEB>= BTTFSU LM:M>F>GM MA:M LAHPL MABL =:M: BL = MH HO>KKB=> MA:M

V /HF> MNIE>L NL>= :L =BRL LI> LMKBG@LGNF;>KL :G= HMA>K MNIE>L (BLML O>K ;> NL>= :L =BRL ;> EBLML :K> GHM BFFNM:;E>

0NIE>L KM>= BGMH EBLML :G= OBO>KL: 0A> ;NBEMBG UVQMF ?NGL: EBLM :G= K>MNKGL : MNIE> PBMA MA> L:F> >E>F>GML :G= MA> MJTU ?NGL :MNIE> :G= K>MNKGL : EBLM %G >??>>S>L : EBLM :G= MJTU MA:PL : MNIE>

2.5.1. TUPLES IN A BOOLEAN CONTEXT

5HN MNIE>L BG : ;HHE>:G QM LNF>GM


JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVF

OPJUTGBMTF

JT@JU@USVFBC

ZFTJUTUSVF

JT@JU@USVF'BMTF

ZFTJUTUSVF

UZQF'BMTF

DMBTTCPPM

UZQF'BMTF

DMBTTUVQMF

%G : ;HHE>:G QM :G >FIMR MNIE> BL ?:EL> GR MNIE> PBMA :M E>:LM HG> BM>F BL MKN> GR MNIE> PBMA :M E>:LM HG> BM>F BL MKN> 0A> O:EN> H? MA> BM>FL BL BKK>E>O:GM NM PA:M^L MA:M K> 0H :M> : MNIE> H? HG> BM>F RHN G>>= : K MA> O:EN> 3BMAHNM MA> L

RHN A:O> :G >QMK: I:BK H? I:K>GMA>L>L PABLL ;NM BM =H>LG^M :M> : MNIE>

2.5.2. ASSIGNING MULTIPLE VALUES AT ONCE

$>K>^L : O:EN>L :M HG

WB5SVF

YZ[W Y

B

Z

[

5SVF

W BL : MNIE> H? MAK>> >E>F>GML :G= Y Z [ BL : MNIE> H? MAK>> O:KB:;E>L LLB@GBG@ HG> MH MA> HMA>K:LLB@GL >: O:EN>L H? W MH >: O:KB:;E>L BG HK=>K

0ABL A:L :EE DBG=L H? NL>L /NIIHL> RHN P:GM MH :LLB@G G:F>L MH : K:G@> H? O:EN>L 5HN MA> ;NBEMBGSBOHF ?NG :LLB@GF>GM MH JNBL

.0/%":56&4%":8&%/&4%":5)634%":'3*%":4"563%":46/%":SBOHF

.0/%":

56&4%":

46/%":

0A> ;NBEMBG SBOHF ?NG@>KL 0>K:MHK GHM : EBLM HK : MNIE> ;NM RHN^EE E>:KG :;HNM MA:M =BLMBGK .0/%": 56&4%": 8&%/&4%":5)634%": '3*%": 4"563%": :G= 46/%": :K> MA> O:KB:;E>L RHN^K> =>?BGBG@ 0ABL >Q:FIE> ?KHF MA>DBMFOEBS FH=NE> : ?NG EBMME> FH=NE> MA:M IKBGML G=:KL EBD> MA> 1*%4 IKH@K:F DBM 0A> DBMFOEBSFH=NE> =>?BG>L BGM>@>K P>>D

*HP >: A:L BML O:EN> .0/%": BL 56&4%": BL :G= LH ?HKMA

5HN FNEMBO:KB:;E> :LLB@GF>GM MH ;NBE= ?NGMNKG FNEMBIE> O:EN>L LBFIER ;R K>MNKGBG@: MNIE> H? :EE MA> O:EN>L 0A> K :M BM :L : LBG@E> MNIE> HK BM O:EN>L MH BG=BOB=N:EO:KB:;E>L ):GR LM:G=:K= ,RMAHG EB;K:KB>L =H MABL BG PT FH=NE> PAB:KG :;HNM BG MA>G>QM K

2.6. SETS

L>M BL :G NGHK=>K>= \;:@] H? NGBJN> O:EN>L LBG@E> L>M =:M:MRI>+G RHN A:O> MPH L>ML RHN M HI>K:MBHGL EBD> NGBHG BGM>KL>M =B??>K>G

2.6.1. CREATING A SET

"BKLM MABG@L ?BKLM K>:MBG@ : L>M BL >:LR

B@TFU\^ B@TFU

\^

UZQFB@TFU DMBTTTFU

B@TFU\^ B@TFU

\^

0H :M> : L>M PBMA HG> O:EN> INM MA> O:EN> BG ML :K> :F>GM>= :L L ;NM =HG^M PHKKR :;HNM MA:M ?HK GHP 0H :M> : L>M PBMA FNEMBIE> O:EN>L L>I:K:M> MA> O:EN>L PBMA

B@MJTU

N> MH ABLMHKBK ?KHF ,RMAHG RHN :G >FIMR L>M PBMA MPH FIMR =BFIMR L>M

2.6.2. MODIFYING A SET

0A>K> :K> MPH =B??>K>GM P:RL MH :== O:EN>L MH :G >QBLMBG@ L>M MA> BEE F>MAH= :G= MA> VQEBUF F>MAH=

B@TFU\^

B@TFUBEE B@TFU

\^

MFOB@TFU

B@TFUBEE B@TFU

\^

MFOB@TFU

0A> BEE F>MAH= M:D>L : LBG@E> :K@NF>GM PAB :G= :==L MA> @BO>G O:EN> MH MA> L>M 0ABL L>M GHP A:L F>F;>KL />ML :K> ;:@L H? unique values %? RHN MKR MH :== : O:EN> MA:M :EK>:=R >QBLML BG MA> L>M BM PBEE =H GHMABG@ %M

PHG^M K:BL> :G >KKHK BM^L CNLM : GHHI 0ABL L>M still A:L F>F;>KL

B@TFU\^

B@TFU

\^

B@TFUVQEBUF\^

B@TFU \^

B@TFUVQEBUF\^\^ B@TFU

\^

B@TFUVQEBUF B@TFU

\^

0A> VQEBUF F>MAH= M:D>L HG> :K@NF>GM : L>M :G= :==L :EE BML F>F;>KL MH MA> HKB@BG:E L>M %M^L :L B? RHN= MA> BEE F>MAH= PBMA >:F;>K H? MA> L>M

NIEB O:EN>L :K> B@GHK>= LBG L>ML G = PBMA MPH L>ML MA>

VQEBUF F>MAH= :==L :EE MA> F>F;>KL H? >:M MH MA> HKB@BG:E L>M =KHIIBG@ =NIEBL 0A> VQEBUF F>MAH= H;C>K H? =B??>K>GM =:M:MRI>L BGG = PBMA

: EBLM MA> VQEBUF F>MAH= :==L :EE MA> BM>FL H? MA> EBLM MH MA> HKB@BG:E L>M

2.6.3. REMOVING ITEMS FROM A SET

0A>K> :K> MAK>> P:RL MH K>FHO> BG=BOB=N:E O:EN>L ?KHF : L>M 0A> ?BKLM MPH EJTDBSE :G= SFNPWF A:O>HG> LN;ME> =B??>K>G

B@TFU\^

B@TFU

\^

B@TFUEJTDBSE B@TFU

\^

B@TFUEJTDBSE B@TFU

\^

B@TFUSFNPWF B@TFU

\^

B@TFUSFNPWF 5SBDFCBDLNPTUSFDFOUDBMMMBTU


,FZ&SSPS

0A> EJTDBSE F>MAH= M:D>L : LBG@E> O:EN> :L :G :K@NF>GM :G= K>FHO>L MA:M O:EN> ?KHF MA> L>M %? RHN EJTDBSE F>MAH= PBMA : O:EN> MA:M =H>LG^M >QBLM BG MA> L>M BM =H>L GHMABG@ *H >KKHK BM^L

CNLM : GHHI 0A> SFNPWF F>MAH= :ELH M:D>L : LBG@E> O:EN> :L :G :K@NF>GM :G= BM :ELH K>FHO>L MA:M O:EN> ?KHF MA> L>M $>K>^L MA> =B??>K>G B? MA> O:EN> =H>LG^M >QBLM BG MA> L>M MA> SFNPWF F>MAH= K:BL>L : ,FZ&SSPS

>QIMBHG

(BD> EBLML L>ML A:O> : QPQ F>MAH=

B@TFU\^

B@TFUQPQ

B@TFUQPQ

B@TFUQPQ

B@TFU

\^

B@TFUDMFBS B@TFU

TFU

B@TFUQPQ 5SBDFCBDLNPTUSFDFOUDBMMMBTU


,FZ&SSPSQPQGSPNBOFNQUZTFU

0A> QPQ F>MAH= K>FHO>L : LBG@E> O:EN> ?KHF : L>M :G= K>MNKGL MA> O:EN> $HP>O>K LBG L>ML :K>NGHK=>K>= MA>K> BL GH \E:LM] O:EN> BG : L>M LH MA>K> BL GH P:R MH ML K>FHO>= %M BLM>ER :K;BMK:KR

0A> DMFBS F>MAH= K>FHO>L all O:EN>L ?KHF : L>M E>:OBG@ RHN PBMA :G >FIMR L>M 0ABL BL >JNBO:E>GM MHB@TFU TFU PAB : G>P >FIMR L>M :G= HO>KPKBM> MA> IK>OBHNL O:EN> H? MA> B@TFUO:KB:;E>

MM>FIMBG@ MH IHI : O:EN> ?KHF :G >FIMR L>M PBEE K:BL> : ,FZ&SSPS >QIMBHG

2.6.4. COMMON SET OPERATIONS

,RMAHG^L TFU MRI> LNIIHKML L>O>K:E M HI>K:MBHGL

B@TFU\^

JOB@TFU 5SVF

JOB@TFU

'BMTF

C@TFU\^

B@TFUVOJPOC@TFU \^

B@TFUJOUFSTFDUJPOC@TFU \^

B@TFUEJGGFSFODFC@TFU \^

B@TFUTZNNFUSJD@EJGGFSFODFC@TFU \^

0H M>LM PA>MA>K : O:EN> BL : F>F;>K H? : L>M NL> MA> JO HI>K:MHK 0ABL PHKDL MA> L:F> :L EBLML 0A> VOJPO F>MAH= K>MNKGL : G>P L>M >E>F>GML MA:M :K> BG either L>M 0A> JOUFSTFDUJPO F>MAH= K>MNKGL : G>P L>M >E>F>GML MA:M :K> BG both L>ML 0A> EJGGFSFODF F>MAH= K>MNKGL : G>P L>M >E>F>GML MA:M :K> BG B@TFU ;NM GHM C@TFU 0A> TZNNFUSJD@EJGGFSFODF F>MAH= K>MNKGL : G>P L>M >E>F>GML MA:M :K> BG exactly one

H? MA> L>ML

0AK>> H? MA>L> F>MAH=L :K> LRFF>MKBML BG : ;HHE>:G QM LNF>GM


JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVFTFU

OPJUTGBMTF

JT@JU@USVF\B^ ZFTJUTUSVF

JT@JU@USVF\'BMTF^ ZFTJUTUSVF

%G : ;HHE>:G QM :G >FIMR L>M BL ?:EL> GR L>M PBMA :M E>:LM HG> BM>F BL MKN> GR L>M PBMA :M E>:LM HG> BM>F BL MKN> 0A> O:EN> H? MA> BM>FL BL BKK>E>O:GM

2.7. DICTIONARIES

=BK>= L>M H? D>RO:EN> I:BKL 3A>G RHN :== : D>R MH : =B ?HK MA:M D>R 5HN O:EN> E:M>K ,RMAHG =BL :K> HIMBFBS>= ?HK K>MKB>OBG@MA> O:EN> PA>G RHN DGHP MA> D>R ;NM GHM MA> HMA>K P:R :KHNG=

=B : A:LA BG ,>KE %G ,>KE O:KB:;E>L MA:M LMHK> A:LA>L:EP:RL LM:KM PBMA : L G:F>= :GRMABG@ :G=,RMAHG D>>IL MK: =:M:MRI> BGM>KG:EER

2.7.1. CREATING A DICTIONARY

K>:MBG@ : =B:LR 0A> LRGM:Q BL LBFBE:K MH L>ML ;NM BGLM>:= H? O:EN>L RHN A:O> D>RO:EN> I:BKL+G RHN A:O> : =BBK D>R

B@EJDU\TFSWFSECEJWFJOUPQZUIPOPSHEBUBCBTFNZTRM^ B@EJDU

\TFSWFSECEJWFJOUPQZUIPOPSHEBUBCBTFNZTRM^

B@EJDU ECEJWFJOUPQZUIPOPSH

B@EJDU NZTRM

B@EJDU 5SBDFCBDLNPTUSFDFOUDBMMMBTU


,FZ&SSPSECEJWFJOUPQZUIPOPSH

"BKLM RHN :M> : G>P =BFL :G= :LLB@G BM MH MA> O:KB:;E> B@EJDU !:F BL : D>RO:EN> I:BK :G= MA> PAHE> L>M H? BM>FL BL >G= BG R :G= BML :LLH= O:EN> K>?>K>G= ;R B@EJDU BLECEJWFJOUPQZUIPOPSH

EBUBCBTF BL : D>R :G= BML :LLH= O:EN> K>?>K>G= ;R B@EJDU BL NZTRM 5HN M O:EN>L ;R D>R ;NM RHN M D>RL ;R O:EN> /H B@EJDU BL

ECEJWFJOUPQZUIPOPSH ;NM B@EJDU K:BL>L :G >QIMBHG ;>ECEJWFJOUPQZUIPOPSH BL GHM : D>R

2.7.2. MODIFYING A DICTIONARY

BL =H GHM A:O> :GR IK>=>?BG>= LBS> EBFBM 5HN P D>RO:EN> I:BKL MH : =B HK RHN O:EN> H? :G >QBLMBG@ D>R HGMBGNBG@ ?KHF MA> IK>OBHNL >Q:FIE>

B@EJDU

\TFSWFSECEJWFJOUPQZUIPOPSHEBUBCBTFNZTRM^

B@EJDUCMPH B@EJDU

\TFSWFSECEJWFJOUPQZUIPOPSHEBUBCBTFCMPH^

B@EJDUNBSL

B@EJDU \TFSWFSECEJWFJOUPQZUIPOPSHVTFSNBSLEBUBCBTFCMPH^

B@EJDUEPSB B@EJDU

\TFSWFSECEJWFJOUPQZUIPOPSHVTFSEPSBEBUBCBTFCMPH^

B@EJDUNBSL B@EJDU

\6TFSNBSLTFSWFSECEJWFJOUPQZUIPOPSHVTFSEPSBEBUBCBTFCMPH^

5HN =NIEB D>RL BG : =B MH :G >QBLMBG@ D>R PBEE PBI> HNM MA> HE=O:EN>

5HN P D>RO:EN> I:BKL :M :GR MBF> 0ABL LRGM:Q BL B=>GMBQBLMBG@ O:EN>L 0A> G>P =BF D>R VTFS O:EN> NBSL :II>:KL MH ;> BG MA> FB==E> %G ?:FL :II>:K>= MH ;> BG HK=>K BG MA> ?BKLM >Q:FIE> BM BL CNLM :L FN:K MH ;> HNM H? HK=>K GHP

LLB@GBG@ : O:EN> MH :G >QBLMBG@ =BR LBFIER K>IE:L MA> HE= O:EN> PBMA MA> G>P HG> 3BEE MABL MA> O:EN> H? MA> VTFS D>R ;: D>R ER [ MA:M^L : L>GLBMBO> LH MABL LM:M>F>GM BL :MBG@ : G>P D>RO:EN> I:BK GHM

HO>KPKBMBG@ :G >QBLMBG@ HG> %M F:R EHHD LBFBE:K MH RHN ;NM :L ?:K :L ,RMAHG BL = BM^L M>ER=B??>K>GM

2.7.3. MIXED-VALUE DICTIONARIES

BL :K>G^M CNLM ?HK LMKBG@L BL :GR =:M:MRI> BG@>KL ;HHE>:GL:K;BMK:KR H;C>O>G HMA>K =BL G= PBMABG : LBG@E> =B O:EN>L =HG^M :EE G>>= MH ;>MA> L:F> MRI> RHN =>= BRL :K> FHK> K>LMKB= ;NM MA>R LMKBG@L BGM>@>KL :G= : ?>P HMA>K MRI>L 5HN L PBMABG : =B

%G ?: :EK>:=R L>>G : =BRL :G= O:EN>L BG RHNK ?BKLM ,RMAHG IKH@K:F

46''*9&4\

^

(>ML M>:K MA:M :I:KM BG MA> BGM>K: LA>EE

46''*9&4\

^

MFO46''*9&4

JO46''*9&4 5SVF

46''*9&4

46''*9&4

46''*9&4 5#

(BD> EBLML :G= L>ML MA> MFO ?NGL RHN MA> GNF;>K H? D>RL BG : =B EBLML :G= L>ML RHN MA> JO HI>K:MHK MH M>LM PA>MA>K : LI>R BL =>?BG>= BG : =BR BG MA> 46''*9&4 =B BL : EBLM H? >B@AM BM>FL >B@AM LMKBG@L MH ;> IK> /BFBE:KER BL : D>R BG MA> 46''*9&4 =B BL :ELH : EBLM H? >B@AM BM>FL /BG 46''*9&4 BL : EBLM RHN LL BG=BOB=N:E BM>FL BG MA> EBLM ;R MA>BK ;:L>= BG=>Q

Empty

dictionaries

are false; all

other

dictionaries

are true.

2.7.4. DICTIONARIES IN A BOOLEAN CONTEXT

5HN : =B:G QM LNF>GM


JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVF\^ OPJUTGBMTF

JT@JU@USVF\B^ ZFTJUTUSVF

%G : ;HHE>:G QM :G >FIMR =B GR =B:LM HG> D>RO:EN> I:BK BL MKN>

2.8. /POF

/POF BL : LI> L:F> :L 'BMTF /POF BL GHM /POF BLGHM :G >FIMR LMKBG@ HFI:KBG@ /POF MH :GRMABG@ HMA>K MA:G /POF PBEE :EP:RL K>MNKG 'BMTF

/POF BL MA> HGER GNEE O:EN> %M A:L BML HPG =:M:MRI> /POF5ZQF 5HN ;NM RHN HMA>K /POF5ZQF H;C>L PAHL> O:EN> BL /POF :K> >JN:E MH >:K

UZQF/POF

DMBTT/POF5ZQF

/POF'BMTF

'BMTF

/POF

'BMTF

/POF

'BMTF

/POF/POF

5SVF

Y/POF

Y/POF

5SVF

Z/POF

YZ

5SVF

2.8.1. /POF IN A BOOLEAN CONTEXT

%G : ;HHE>:G QM /POF BL ?:EL> :G= OPU /POF BL MKN>


JGBOZUIJOH

QSJOUZFTJUTUSVF

FMTF

QSJOUOPJUTGBMTF

JT@JU@USVF/POF

OPJUTGBMTF

JT@JU@USVFOPU/POF

ZFTJUTUSVF

2.9. FURTHER READING

V HHE>:G HI>K:MBHGLV *NF>KB< MRI>LV />JN>G MRI>LV />M MRI>LV ):IIBG@ MRI>LV GSBDUJPOT FH=NE>V NBUI FH=NE>V ,!, 1GB?RBG@ (HG@ %GM>@>KL :G= %GM>@>KLV ,!, A:G@BG@ MA> BOBLBHG +I>K:MHK

CHAPTER 3. COMPREHENSIONS

Our imagination is stretched to the utmost, not, as in fiction, to imagine things which are not really there, but just

to comprehend those things which are.

Richard Feynman

3.1. DIVING IN

E O>KR IKH@K:FFBG@ E:G@N:@> A:L MA:M HG> ?>:MNK> : GMBHG:EER F:=> LBFIE> %?RHN^K> K E:G@N:@> RHN :LBER FBLL BM ;> RHNK HE= E:G@N:@> =B=G^M F:D> MA:MMABG@ LBFIE> ;> BM P:L ;NLR F:DBG@ LHF>MABG@ >EL> LBFIE> BGLM>:= 0ABL K PBEE M>:GLBHGL =BGLBHGL :G= L>M A>GLBHGL MAK>> K>E:M>= K>=:KHNG= HG> O>KR IHP>K?NE M> NM ?BKLM % P:GM MH M:D> : EBMME> =>MHNK BGMH MPH FH=NE>L MA:M PBEE A>EIRHN G:OB@:M> RHNK EH LRLM>F

3.2. WORKINGWITH FILES AND DIRECTORIES

,RMAHG L PBMA : FH=NE> = PT PABK:MBG@ LRLM>F] 0A> PT FH=NE> MAHK: H? ?NGM BG?HKF:MBHG HG [ :G= BG LHF> L MH F:GBINE:M> [ EHL ?BE>LIKHLL>L :G= >GOBKHGF>GM O:KB:;E>L ,RMAHG =H>L BML ;>LM MH H??>K : NGB?B>= , % :=HI>K:MBG@ LRLM>FL LH RHNK IKH@K:FL IE:M?HKFLI>

There is

always a

current

working

directory.

3.2.1. THE CURRENT WORKING DIRECTORY

3A>G RHN^K> CNLM @>MMBG@ LM:KM>= PBMA ,RMAHG RHN^K> @HBG@ MH LI>G= : EHM H? MBF> BG MA> ,RMAHG /A>EE0AKHN@AHNM MABL ;HHD RHN PBEE L>> >Q:FIE>L MA:M @H EBD> MABL

%FIHKM HG> H? MA> FH=NE>L BG MA> FYBNQMFT ?HE=>K :EE : ?NG !QIE:BG MA> K>LNEM

%? RHN =HG^M DGHP :;HNM MA> GM PHKDBG@ =BK>I PBEE IKH;:;ER ?:BE PBMA :G *NQPSU&SSPS 3AR> ,RMAHG PBEE EHHD ?HK MA> >Q:FIE> FH=NE> BG MA>BFIHKM L>:K MA>FYBNQMFT ?HE=>K BLG^M HG> H? MA> =BK>L BG MA>L>:KM I:LM MABL RHN H? MPHMABG@L

== MA> FYBNQMFT ?HE=>K MH MA> BFIHKM L>:K MA> GM PHKDBG@ =BK> FYBNQMFT

?HE=>K

0A> GM PHKDBG@ =BK> IKHI>KMRMA:M ,RMAHG AHE=L BG F>FHKR :M :EE MBF>L 0A>K> BL:EP:RL : GM PHKDBG@ =BK>MA>K RHN^K> BGMA> ,RMAHG /A>EE KNGGBG@ RHNK HPG ,RMAHG L HK KNGGBG@ : ,RMAHG #% L; L>KO>K LHF>PA>K>

0A> PT FH=NE> GM PHKDBG@ =BK>

JNQPSUPT

QSJOUPTHFUDXE

$=1ZUIPO

PTDIEJS6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFT

QSJOUPTHFUDXE

$=6TFST=QJMHSJN=EJWFJOUPQZUIPO=FYBNQMFT

0A> PT FH=NE> L PBMA ,RMAHG RHN :GRPA>K> 1L> MA> PTHFUDXE ?NGM MA> GM PHKDBG@ =BK>G RHN KNG MA> @K:IABEE MA> GM PHKDBG@ =BK> =BK>K> MA> ,RMAHG /A>EE >Q> BL +G3BG=HPL MABL =>I>G=L HG PA>K> RHN BGLM:EE>= ,RMAHG MA> =>?:NEM =BK>,RMAHG /A>EE ?KHF MA> MA> GM PHKDBG@ =BK> =BK>K> BGPA>G RHN K:G QZUIPO

1L> MA> PTDIEJS ?NG GM PHKDBG@ =BK>G % = MA> PTDIEJS ?NG= : (BGNQLMRE> I:MAG:F> ?HKP:K= LE:LA>L GH =KBO> E>MM>K >O>G

MAHN@A %^F HG 3BG=HPL 0ABL BL HG> H? MA> IE:L PA>K> ,RMAHG MKB>L MH I:I>K HO>K MA> =B??>K>GL ;>MP>>GHI>K:MBG@ LRLM>FL

3.2.2. WORKING WITH FILENAMES AND DIRECTORY NAMES

3ABE> P>^K> HG MA> LN;C>L % P:GM MH IHBGM HNM MA> PTQBUI FH=NE> PTQBUI L :G= =BK>L

JNQPSUPT

QSJOUPTQBUIKPJO6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

QSJOUPTQBUIKPJO6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFT=IVNBOTJ[FQZ

QSJOUPTQBUIFYQBOEVTFS_

D=6TFST=QJMHSJN

QSJOUPTQBUIKPJOPTQBUIFYQBOEVTFS_

EJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

D=6TFST=QJMHSJN=EJWFJOUPQZUIPO=FYBNQMFT=IVNBOTJ[FQZ

0A> PTQBUIKPJO ?NG I:KMB:E I:MAG:F>L %G MABL BMLBFIER L LMKBG@L

%G MABL LEB@AMER E>LL MKBOB:E PTQBUIKPJO ?NGQMK: LE:LA MH MA> I:MAG:F>;>?HK> CHBGBG@ BM MH MA> ?BE>G:F> %M^L : ;::= H? : ?HKP:K= LE:LA ;> % Q:FIE> HG 3BG=HPL %? RHN K>IEB MABL >Q:FIE> HG (BGNQ HK ):< +/ 4 RHN^EE L>> : ?HKP:K= LE:LABGLM>:= HG^M ?NLL PBMA LE:LA>L :EP:RL NL> PTQBUIKPJO :G= E>M ,RMAHG =H MA> KB@AM MABG@

0A> PTQBUIFYQBOEVTFS ?NGQI:G= : I:MAG:F> MA:M NL>L _ MH K>IK>L>GM MA> GM NL>K^LAHF> =BK>K> NL>KL A:O> : AHF> =BK>MNKG>= I:MA =H>L GHM A:O> : MK:BEBG@ LE:LA ;NM MA> PTQBUIKPJO ?NGLG^MFBG=

HF;BGBG@ MA>L> M>L RHN :LBER L :G= ?BE>L BG MA> NL>K^L AHF>=BK> PTQBUIKPJO ?NGK H? :K@NF>GML % P:L HO>KCHR>= PA>G %=BLK>= MABL LBG BEE4MBTI*G/FDFTTBSZ BL HG> H? MA> LMNIB= EBMME> ?NG>= MH PKBM>PA>G ;NBE=BG@ NI FR MHHE;HQ BG : G>P E:G@N:@> Do not PKBM> MABL LMNIB= EBMME> ?NGHIE> A:O> :EK>:=R M:D>G H? BM ?HK RHN

PTQBUI :ELH L :G= ?BE>G:F>L BGMH MA>BK GMI:KML

QBUIOBNF6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

PTQBUITQMJUQBUIOBNF 6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFTIVNBOTJ[FQZ

EJSOBNFGJMFOBNFPTQBUITQMJUQBUIOBNF

EJSOBNF 6TFSTQJMHSJNEJWFJOUPQZUIPOFYBNQMFT

GJMFOBNF IVNBOTJ[FQZ

TIPSUOBNFFYUFOTJPOPTQBUITQMJUFYUGJMFOBNF TIPSUOBNF

IVNBOTJ[F

FYUFOTJPO

QZ

0A> TQMJU ?NG :G= K>MNKGL : MNIE> I:MA :G= ?BE>G:F>

The HMPCmodule uses

shell-like

wildcards.

.>F>F;>K PA>G % L:B= RHN FNEMBO:KB:;E> :LLB@GF>GM MH K>MNKG FNEMBIE> O:EN>L ?KHF : ?NG PTQBUITQMJU ?NGL >Q: K>MNKG O:EN> H? MA> TQMJU ?NG H? MPH O:KB:;E>L !: K>BO>L MA> O:EN> H? MA> LIHG=BG@ >E>F>GM H? MA> K>MNKG>= MNIE>

0A> ?BKLM O:KB:;E> EJSOBNF K>BO>L MA> O:EN> H? MA> ?BKLM >E>F>GM H? MA> MNIE> K>MNKG>= ?KHF MA>PTQBUITQMJU ?NG ?BE> I:MA

0A> L> GJMFOBNF K>BO>L MA> O:EN> H? MA> L>E>F>GM H? MA> MNIE> K>MNKG>= ?KHF MA>PTQBUITQMJU ?NG ?BE>G:F>

PTQBUI :ELH PTQBUITQMJUFYU ?NG :G= K>MNKGL : MNIE> ?BE>G:F> :G= MA> ?BE> >QM>GLBHG 5HN NL> MA> L:F> M> MH :LLB@G >:F MHL>I:K:M> O:KB:;E>L

3.2.3. LISTING DIRECTORIES

0A> HMPC FH=NE> BL :GHMA>K MHHE BG MA> ,RMAHG LM:G=:K= EB;K:KR %M^L :G >:LR P:R MH @>M MA> GML H? :=BK> LHKM H? PBE=:=R ;> ?:FBEB:K PBMA ?KHFPHKDBG@ HG MA>

PTDIEJS6TFSTQJMHSJNEJWFJOUPQZUIPO

JNQPSUHMPC

HMPCHMPCFYBNQMFTYNM

PTDIEJSFYBNQMFT

HMPCHMPCUFTUQZ

0A> HMPC FH=NE> M:D>L : PBE=MNKGL MA> I:MA H? :EE ?BE>L :G= =BK>L F:M PBE=Q:FIE> MA> PBE= GM PHKDBG@ =BK> FYBNQMFT LN;=BK> PTDIEJS ?NGE:MBO> I:MAG:F>L

5HN PBE=KG 0ABL >Q:FIE> ?BG=L :EE MA> ?BE>L BG MA> GMPHKDBG@ =BK>G= BG : QZ >QM>GLBHG :G= PHK= UFTU :GRPA>K> BG MA>BK ?BE>G:F>

3.2.4. GETTING FILE METADATA

!O>KR FH=>KG ?BE> LRLM>F LMHK>L F>M:=:M: :;HNM >: :MBHG =:M> E:LMFH=B?B>= =:M> ?BE> LBS> :G= LHHG ,RMAHG IKHOB=>L : LBG@E> , % MH :M:=:M: 5HN =HG^M G>>= MH HI>G MA> ?BE> :EE RHN G>>= BLMA> ?BE>G:F>

JNQPSUPT

QSJOUPTHFUDXE

D=6TFST=QJMHSJN=EJWFJOUPQZUIPO=FYBNQMFT

NFUBEBUBPTTUBUGFFEYNM

NFUBEBUBTU@NUJNF

JNQPSUUJNF

UJNFMPDBMUJNFNFUBEBUBTU@NUJNF UJNFTUSVDU@UJNFUN@ZFBSUN@NPOUN@NEBZUN@IPVS

UN@NJOUN@TFDUN@XEBZUN@ZEBZUN@JTETU

0A> GM PHKDBG@ =BK> FYBNQMFT ?HE=>K GFFEYNM BL : ?BE> BG MA> FYBNQMFT ?HE=>K :EEBG@ MA> PTTUBU ?NGMNKGL :G H;C>K:E =B??>K>GM MRI>L H? F>M:=:M: :;HNM MA> ?BE> TU@NUJNF BL MA> FH=B?B ;NM BM^L BG : ?HKF:M MA:M BLG^M M>KKB;ER NL>?NE 0>K

H? L> !IH= :L MA> ?BKLM L>KBHNLER 0A> UJNF FH=NE> BL I:KM H? MA> ,RMAHG LM:G=:K= EB;K:KR %M >G =B??>K>GM

MBF> K>IK>L>GM:MBHGL ?HKF:M MBF> O:EN>L BGMH LMKBG@L :G= ?B==E> PBMA MBF>SHG>L 0A> UJNFMPDBMUJNF ?NG O:EN> ?KHF L>!IH TU@NUJNF

IKHI>KMR K>MNKG>= ?KHF MA> PTTUBU ?NG NL>?NE LMKN H? R>:K FHGMA =:R AHNKFBGNM> L> P:L E:LM FH=B?B>= HG &NER :M :KHNG= ,)

DPOUJOVFEGSPNUIFQSFWJPVTFYBNQMF

NFUBEBUBTU@TJ[F

JNQPSUIVNBOTJ[F

IVNBOTJ[FBQQSPYJNBUF@TJ[FNFUBEBUBTU@TJ[F ,J#

0A> PTTUBU ?NGMNKGL MA> LBS> H? : ?BE> BG MA> TU@TJ[F IKHI>KMR 0A> ?BE> GFFEYNM BL ;RM>L

5HN TU@TJ[F IKHI>KMR MH MA> BQQSPYJNBUF@TJ[F ?NG IK>OBHNL L> HMPCHMPC ?NGMNKG>= : EBLM H? K>E:MBO> I:MAG:F>L 0A> ?BKLM >Q:FIE>A:= I:MAG:F>L EBD> FYBNQMFT=GFFEYNM :G= MA> L>Q:FIE> A:= >O>G LAHKM>K K>E:MBO> I:MAG:F>L EBD>SPNBOUFTUQZ L EHG@ :L RHN LM:R BG MA> L:F> GM PHKDBG@ =BK>L> K>E:MBO> I:MAG:F>L PBEEPHKD ?HK HI>GBG@ ?BE>L HK @>MMBG@ ?BE> F>M:=:M: NM B? RHN P:GM MH [ i.e. HG>MA:M BGL :EE MA> =BK>L ;: KHHM =BK> E>MM>K [ MA>G RHN^EE G>>= MA>PTQBUISFBMQBUI ?NG

You can use

any Python

expression

in a list

comprehension.

3.3. LIST COMPREHENSIONS

EBLM A>GLBHG IKHOB=>L : E>F>GML H? MA> EBLM

B@MJTU

B@MJTU

B@MJTU B@MJTU

0H F:D> L>GL> H? MABL EHHD :M BM ?KHF KB@AM MH E>?M B@MJTU BL MA> EBLM RHN^K> F:IIBG@ 0A> ,RMAHGBGM>KIK>M>K EHHIL MAKHN@A B@MJTU HG> >E>F>GM :M : MBF> M>FIHK:KBER :LLB@GBG@ MA> O:EN> H? >:E>F>GM MHMA> O:KB:;E> FMFN ,RMAHG MA>G :IIEB>L MA> ?NGG=L MA:M K>LNEM MH MA> K>MNKG>= EBLM

EBLM A>GLBHG :M>L : G>P EBLM BM =H>L GHM MA> HKB@BG:E EBLM %M BL L:?> MH :LLB@G MA> K>LNEM H? : EBLM A>GLBHG MH MA> O:KB:;E> MA:M RHN^K> F:IIBG@ ,RMAHG P EBLM BG F>FHKR :G= PA>G MA> EBLM A>GLBHG BL M> BM :LLB@GL MA> K>LNEM MH MA> HKB@BG:EO:KB:;E>

5HN :GR ,RMAHG >QIK>LLBHG BG : EBLM A>GLBHG BG ?NG PT FH=NE> ?HKF:GBINE:MBG@ ?BE>L :G= =BK>L

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(BLM A>GLBHGL K BM>FL IKH=NLNEM MA:M LF:EE>K MA:G MA> HKB@BG:E EBLM

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3.4. DICTIONARY COMPREHENSIONS

=BGLBHG BL EBD> : EBLM A>GLBHG ;NM BM

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NFUBEBUB

NFUBEBUB BMQIBNFUJDTUFTUQZOUTUBU@SFTVMUTU@NPEFTU@JOPTU@EFW

TU@OMJOLTU@VJETU@HJETU@TJ[FTU@BUJNF

TU@NUJNFTU@DUJNF

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MJTUNFUBEBUB@EJDULFZT

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0ABL BL GHM : =BGLBHG BM^L : EBLM A>GLBHG %M ?BG=L :EE QZ ?BE>L PBMA UFTU BG MA>BKG:F> MA

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