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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
I
N
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'
>>> len(s)
9
>>> s[0]
'深'
>>> s + ' 3'
'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
S
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'],
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.
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
'''
if size < 0:
raise ValueError('number must be non-negative')
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)
raise ValueError('number too large')
1.
'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'
>>> "{0}'s password is {1}".format(username, password)
"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
F
IELD
N
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]
>>> si_suffixes
['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB']
>>> '1000{0[0]} = 1{0[1]}'.format(si_suffixes)
'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
ST
format()
argument.
{1} is
replaced by
the 2
ND
.
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
SI
suffixes:
['KB', 'MB', 'GB', 'TB', 'PB', 'EB',
'ZB', 'YB']
.Sothereplacementfield
{0.modules[humansize].SUFFIXES[1000]}
referstothatlist.
sys.modules['humansize'].SUFFIXES[1000][0]
is thefirstitemofthelistof
SI
suffixes:
'KB'
.Therefore,the
completereplacementfield
{0.modules[humansize].SUFFIXES[1000][0]}
is replacedbythetwo-character
string
KB
.
4.4.2.F
ORMAT
S
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“
.1
means“roundtothenearesttenth”(i.e.displayonlyonedigitafterthedecimalpoint).Theformatspecifier
f
”means“fixed-pointnumber”(as opposedtoexponentialnotationorsomeotherdecimalrepresentation).
Thus,givena
size
of
698.24
and
suffix
of
'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
C
OMMON
S
TRING
M
ETHODS
Besidesformatting,strings candoanumberofotherusefultricks.
118
>>> s = '''Finished files are the re-
... sult of years of scientif-
... ic study combined with the
... experience of years.'''
>>> s.splitlines()
['Finished files are the re-',
'sult of years of scientif-',
'ic study combined with the',
'experience of years.']
>>> print(s.lower())
finished files are the re-
sult of years of scientif-
ic study combined with the
experience of years.
>>> s.lower().count('f')
6
1. You u caninputmultilinestringsinthePythoninteractiveshell.Onceyoustartamultilinestringwithtriple
quotationmarks,justhit
ENTER
andtheinteractiveshellwillpromptyoutocontinuethestring.Typingthe
closingtriplequotationmarksendsthestring,andthenext
ENTER
willexecutethecommand(inthis case,
assigningthestringto
s
).
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('&')
>>> a_list
['user=pilgrim', 'database=master', 'password=PapayaWhip']
>>> a_list_of_lists = [v.split('=', 1) for v in a_list]
>>> a_list_of_lists
[['user', 'pilgrim'], ['database', 'master'], ['password', 'PapayaWhip']]
>>> a_dict = dict(a_list_of_lists)
>>> 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
a
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
AS
TRING
Onceyou’vedefinedastring,youcangetanypartofitas anewstring.Thisiscalledslicingthestring.Slicing
stringsworksexactlythesameas
slicinglists,whichmakessense,becausestringsarejustsequencesof
characters.
120
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