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UTF-8isavariable-lengthencodingsystemforUnicode.Thatis,differentcharacters takeupadifferent

numberofbytes.For

ASCII

characters(A-Z,

c.)

UTF

-8 uses justonebytepercharacter.Infact,ituses

theexactsamebytes;thefirst128characters(0–127)in

UTF

-8 areindistinguishablefrom

ASCII

.“Extended

Latin”characterslikeñandöenduptakingtwobytes.(Thebytes arenotsimplytheUnicodecodepoint

liketheywouldbeinUTF-16;thereissomeserious bit-twiddlinginvolved.) Chinesecharacterslike中end

uptakingthreebytes.Therarely-used“astralplane”characters takefourbytes.

Disadvantages:becauseeachcharactercantakeadifferentnumberofbytes,findingthe

Nth

characterisan

O(N)operation—thatis,thelongerthestring,thelongerittakestofindaspecificcharacter.Also,thereis

bit-twiddlinginvolvedtoencodecharactersintobytesanddecodebytesintocharacters.

Advantages:super-efficientencodingofcommon

ASCII

characters.NoworsethanUTF-16forextendedLatin

characters.BetterthanUTF-32forChinesecharacters.Also(andyou’llhavetotrustmeonthis,because

I’mnotgoingtoshowyouthemath),duetotheexactnatureofthebittwiddling,therearenobyte-

orderingissues.Adocumentencodedin

UTF

-8 usestheexactsamestreamofbytesonany y computer.

⁂

4.3.D

IVING

InPython3,allstringsaresequencesofUnicodecharacters.ThereisnosuchthingasaPythonstring

encoded in

UTF

-8,oraPythonstringencodedasCP-1252.“Isthisstring

UTF

-8?”isaninvalid question.

UTF

-8 isawayofencodingcharactersasasequenceofbytes.Ifyouwanttotakeastringand d turnitintoa

sequenceofbytesinaparticularcharacterencoding,Python3canhelpyouwiththat.Ifyouwanttotakea

sequenceofbytesandturnitintoastring,Python 3canhelpyouwiththattoo.Bytesarenotcharacters;

bytesarebytes.Characters areanabstraction.Astringisasequenceofthoseabstractions.

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Strings can

bedefined

>>> s = '深入 Python'

①

>>> len(s)

②

>>> s[0]

③

'深'

>>> s + ' 3'

④

'深入 Python 3'

1. Tocreateastring,encloseitinquotes.Pythonstrings s canbedefinedwitheithersinglequotes(

)ordouble

quotes(

2. Thebuilt-in

len()

functionreturns thelengthofthestring,i.e.thenumberofcharacters.Thisis thesame

functionyouuseto

findthelengthofalist,tuple,set,ordictionary.Astringislikeatupleofcharacters.

3. Justlikegettingindividualitems s outofalist,youcangetindividualcharactersoutofastringusingindex

notation.

4. Justlikelists,youcanconcatenatestringsusingthe

operator.

⁂

4.4.F

ORMATTING

TRINGS

Let’s takeanotherlookat

humansize.py

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with either

singleor

double

quotes.

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SUFFIXES = {1000: ['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],

①

1024: ['KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB']}

def approximate_size(size, a_kilobyte_is_1024_bytes=True):

'''Convert a file size to human-readable form.

②

Keyword arguments:

size -- file size in bytes

a_kilobyte_is_1024_bytes -- if True (default), use multiples of 1024

if False, use multiples of 1000

Returns: string

'''

③

if size < 0:

raise ValueError('number must be non-negative')

④

multiple = 1024 if a_kilobyte_is_1024_bytes else 1000

for suffix in SUFFIXES[multiple]:

size /= multiple

if size < multiple:

return '{0:.1f} {1}'.format(size, suffix)

⑤

raise ValueError('number too large')

'KB'

'MB'

'GB'

…thoseareeach strings.

2. Functiondocstrings s arestrings.This docstringspansmultiplelines,soitusesthree-in-a-rowquotestostart

andendthestring.

3. Thesethree-in-a-rowquotesend d thedocstring.

4. There’sanotherstring,beingpassedtotheexceptionasahuman-readableerrormessage.

5. There’sa…whoa,whattheheckisthat?

Python3supportsformattingvaluesintostrings.Althoughthis can includeverycomplicated expressions,the

mostbasicusageistoinsertavalueintoastringwithsingleplaceholder.

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>>> username = 'mark'

>>> password = 'PapayaWhip'

①

>>> "{0}'s password is {1}".format(username, password)

②

"mark's password is PapayaWhip"

1. No,mypasswordisnotreally

PapayaWhip

2. There’salotgoingon n here.First,that’samethod callonastringliteral.Stringsareobjects,and objectshave

methods.Second,thewholeexpression evaluates toastring.Third,

{0}

and

{1}

arereplacementfields,which

arereplacedbythearguments passed tothe

format()

method.

4.4.1.C

OMPOUND

IELD

AMES

Thepreviousexampleshows thesimplestcase,wherethereplacementfieldsaresimplyintegers.Integer

replacementfieldsaretreated as positionalindicesintotheargumentlistofthe

format()

method.That

meansthat

{0}

isreplaced bythefirstargument(

username

inthiscase),

{1}

isreplacedbythesecond

argument(

password

c.Youcanhaveasmanypositionalindicesasyouhavearguments,andyoucanhave

asmanyargumentsasyouwant.Butreplacementfields aremuchmorepowerfulthanthat.

>>> import humansize

>>> si_suffixes = humansize.SUFFIXES[1000]

①

>>> si_suffixes

['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB']

>>> '1000{0[0]} = 1{0[1]}'.format(si_suffixes)

②

'1000KB = 1MB'

1. Ratherthancallinganyfunctioninthe

humansize

module,you’rejustgrabbingoneofthedatastructuresit

defines:thelistof“SI”(powers-of-1000)suffixes.

2. Thislookscomplicated,butit’snot.

{0}

wouldrefertothefirstargumentpassedtothe

format()

method,

si_suffixes

.But

si_suffixes

isalist.So

{0[0]}

referstothefirstitemofthelistwhichisthefirst

argumentpassedtothe

format()

method:

'KB'

.Meanwhile,

{0[1]}

referstotheseconditemofthesame

list:

'MB'

.Everythingoutsidethecurlybraces—including

1000

,theequalssign,andthespaces—is

untouched.Thefinalresultis thestring

'1000KB = = 1MB'

115

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{0} is

replaced by

the 1

format()

argument.

{1} is

replaced by

the 2

Whatthisexampleshows isthatformatspeciferscan

accessitemsandpropertiesofdatastructuresusing(almost)

Pythonsyntax.Thisis calledcompoundfieldnames.The

followingcompoundfield names “justwork”:

• Passingalist,and d accessinganitemofthelistbyindex

(asinthepreviousexample)

• Passingadictionary,and d accessingavalueofthe

dictionary bykey

• Passingamodule,and d accessingitsvariables and

functionsbyname

• Passingaclassinstance,andaccessingits s propertiesand

methods byname

• Anycombinationoftheabove

Justtoblowyourmind,here’sanexamplethat

combines alloftheabove:

>>> import humansize

>>> import sys

>>> '1MB = 1000{0.modules[humansize].SUFFIXES[1000][0]}'.format(sys)

'1MB = 1000KB'

Here’showitworks:

• The

sys

moduleholds informationaboutthecurrentlyrunningPython instance.Sinceyoujustimportedit,

youcanpassthe

sys

moduleitselfasanargumenttothe

format()

method.Sothereplacementfield

{0}

referstothe

sys

module.

•

sys.modules

isadictionaryofallthemodulesthathavebeenimportedin this Pythoninstance.Thekeysare

themodulenames as strings;thevaluesarethemoduleobjectsthemselves.Sothereplacementfield

{0.modules}

referstothedictionaryof importedmodules.

116

☞

•

sys.modules['humansize']

is the

humansize

modulewhichyoujustimported.Thereplacementfield

{0.modules[humansize]}

refers tothe

humansize

module.Notetheslightdifferenceinsyntax here.Inreal

Pythoncode,thekeysofthe

sys.modules

dictionaryarestrings;torefertothem,youneedtoputquotes

aroundthemodulename(e.g.

'humansize'

).Butwithinareplacementfield,youskipthequotes aroundthe

dictionary keyname(e.g.

humansize

).Toquote

PEP 3101:Advanced StringFormatting,“Therulesforparsing

anitemkeyareverysimple.Ifitstartswithadigit,thenitis treatedasanumber,otherwiseitisusedasa

string.”

•

sys.modules['humansize'].SUFFIXES

is thedictionarydefinedatthetopofthe

humansize

module.The

replacementfield

{0.modules[humansize].SUFFIXES}

referstothatdictionary.

•

sys.modules['humansize'].SUFFIXES[1000]

is alistof

suffixes:

['KB', 'MB', 'GB', 'TB', 'PB', 'EB',

'ZB', 'YB']

.Sothereplacementfield

{0.modules[humansize].SUFFIXES[1000]}

referstothatlist.

•

sys.modules['humansize'].SUFFIXES[1000][0]

is thefirstitemofthelistof

suffixes:

'KB'

.Therefore,the

completereplacementfield

{0.modules[humansize].SUFFIXES[1000][0]}

is replacedbythetwo-character

string

4.4.2.F

ORMAT

PECIFIERS

Butwait!There’smore!Let’stakeanotherlookatthatstrangelineofcodefrom

humansize.py

if size < multiple:

return '{0:.1f} {1}'.format(size, suffix)

{1}

isreplacedwiththesecondargumentpassed tothe

format()

method,whichis

suffix

.Butwhatis

{0:.1f}

?It’stwothings:

{0}

,whichyourecognize,and

:.1f

,whichyoudon’t.Thesecondhalf(includingand

afterthecolon) definestheformatspecifier,whichfurtherrefineshowthereplaced variableshouldbe

formatted.

Formatspecifiersallowyoutomungethereplacementtextinavarietyofuseful

ways,likethe

printf()

functioninC.Youcanaddzero- orspace-padding,align

strings,controldecimalprecision,andevenconvertnumbers tohexadecimal.

117

Withinareplacementfield,acolon(

) marksthestartoftheformatspecifier.Theformatspecifier“

”

means“roundtothenearesttenth”(i.e.displayonlyonedigitafterthedecimalpoint).Theformatspecifier

“

”means“fixed-pointnumber”(as opposedtoexponentialnotationorsomeotherdecimalrepresentation).

Thus,givena

size

698.24

and

suffix

'GB'

,theformattedstringwould be

'698.2 GB'

,because

698.24

getsroundedtoonedecimalplace,thenthesuffixisappended afterthenumber.

>>> '{0:.1f} {1}'.format(698.24, 'GB')

'698.2 GB'

Forallthegorydetailsonformatspecifiers,consultthe

FormatSpecificationMini-Languageintheofficial

Pythondocumentation.

⁂

4.5.O

THER

OMMON

TRING

ETHODS

Besidesformatting,strings candoanumberofotherusefultricks.

118

>>> s = '''Finished files are the re-

①

... sult of years of scientif-

... ic study combined with the

... experience of years.'''

>>> s.splitlines()

②

['Finished files are the re-',

'sult of years of scientif-',

'ic study combined with the',

'experience of years.']

>>> print(s.lower())

③

finished files are the re-

sult of years of scientif-

ic study combined with the

experience of years.

>>> s.lower().count('f')

④

1. You u caninputmultilinestringsinthePythoninteractiveshell.Onceyoustartamultilinestringwithtriple

quotationmarks,justhit

ENTER

andtheinteractiveshellwillpromptyoutocontinuethestring.Typingthe

closingtriplequotationmarksendsthestring,andthenext

ENTER

willexecutethecommand(inthis case,

assigningthestringto

2. The

splitlines()

methodtakesonemultilinestringand returnsalistofstrings,oneforeachlineof the

original.Notethatthecarriagereturnsattheendofeachlinearenotincluded.

3. The

lower()

methodconvertstheentirestringtolowercase.(Similarly,the

upper()

methodconvertsa

stringtouppercase.)

4. The

count()

methodcountsthenumberofoccurrencesofasubstring.Yes,therereallyaresix“f”sinthat

sentence!

Here’sanothercommoncase.Let’ssayyouhavealistofkey-valuepairsintheform

key1=value1&key2=value2

,andyouwanttosplitthemupandmakeadictionaryoftheform

{key1: value1,

key2: value2}

119

☞

>>> query = 'user=pilgrim&database=master&password=PapayaWhip'

>>> a_list = query.split('&')

①

>>> a_list

['user=pilgrim', 'database=master', 'password=PapayaWhip']

>>> a_list_of_lists = [v.split('=', 1) for v in a_list]

②

>>> a_list_of_lists

[['user', 'pilgrim'], ['database', 'master'], ['password', 'PapayaWhip']]

>>> a_dict = dict(a_list_of_lists)

③

>>> a_dict

{'password': 'PapayaWhip', 'user': 'pilgrim', 'database': 'master'}

1. The

split()

stringmethodtakesoneargument,adelimiter,andsplitastringintoalistofstringsbased on

thedelimiter.Here,thedelimiterisanampersand character,butitcouldbeanything.

2. Nowwehavealistofstrings,eachwithakey,followed d byanequals sign,followedbyavalue.Wecan use

listcomprehensiontoiterateovertheentirelistandspliteachstringintotwostringsbased on thefirst

equalssign.(Intheory,avaluecould containanequalssigntoo.Ifwejustused

'key=value=foo'.split('=')

wewouldendupwithathree-itemlist

['key', 'value', 'foo']

3. Finally,Pythoncanturnthatlist-of-listsintoadictionarysimplybypassingittothe

dict()

function.

Thepreviousexamplelooksalotlikeparsingqueryparametersina

URL

,butreal-life

URL

parsingisactuallymorecomplicatedthanthis.Ifyou’redealingwith

URL

query

parameters,you’rebetteroffusingthe

urllib.parse.parse_qs()

function,which

handlessomenon-obviousedgecases.

4.5.1.S

LICING

TRING

Onceyou’vedefinedastring,youcangetanypartofitas anewstring.Thisiscalledslicingthestring.Slicing

stringsworksexactlythesameas

slicinglists,whichmakessense,becausestringsarejustsequencesof

characters.

120

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