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☞

types.StringType

bytes

types.DictType

dict

types.IntType

int

types.LongType

int

types.ListType

list

types.NoneType

type(None)

types.BooleanType

bool

types.BufferType

memoryview

types.ClassType

type

types.ComplexType

complex

types.EllipsisType

type(Ellipsis)

types.FloatType

float

types.ObjectType

object

types.NotImplementedType

type(NotImplemented)

types.SliceType

slice

types.TupleType

tuple

types.TypeType

type

types.XRangeType

range

types.StringType

getsmappedto

bytes

insteadof

str

becauseaPython 2“string”

(notaUnicodestring,justaregularstring)isreallyjustasequenceofbytesina

particularcharacterencoding.

A.36.isinstance()

GLOBALFUNCTION

The

isinstance()

functionchecks whetheranobjectisaninstanceofaparticularclass ortype.InPython2,

youcouldpass atupleoftypes,and

isinstance()

would return

True

iftheobjectwasanyofthosetypes.In

Python3,youcanstilldothis,butpassingthesametypetwiceisdeprecated.

Notes

Python2

Python3

isinstance(x, (int, float, , int))

isinstance(x, (int, float))

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A.37.basestring

DATATYPE

Python2hadtwostringtypes:Unicodeand non-Unicode.Buttherewas alsoanothertype,

basestring

.It

was anabstracttype,asuperclass forboththe

str

and

unicode

types.Itcouldn’tbecalledorinstantiated

directly,butyoucouldpassittotheglobal

isinstance()

functiontocheckwhetheranobjectwaseithera

Unicodeornon-Unicodestring.InPython3,thereis onlyonestringtype,so

basestring

hasnoreason to

exist.

Notes

Python2

Python3

isinstance(x, basestring)

isinstance(x, str)

A.38.itertools

MODULE

Python2.3introducedthe

itertools

module,whichdefinedvariantsoftheglobal

zip()

map()

,and

filter()

functionsthatreturnediteratorsinsteadoflists.InPython3,thoseglobalfunctionsreturniterators,sothose

functionsinthe

itertools

modulehavebeeneliminated.(Therearestill

lotsofusefulfunctionsinthe

itertools

module,justnotthese.)

Notes

Python2

Python3

①

itertools.izip(a, b)

zip(a, b)

②

itertools.imap(a, b)

map(a, b)

③

itertools.ifilter(a, b)

filter(a, b)

④

from itertools s import t imap, izip, foo

from itertools import foo

1. Insteadof

itertools.izip()

,justusetheglobal

zip()

function.

2. Insteadof

itertools.imap()

,justuse

map()

itertools.ifilter()

becomes

filter()

4. The

itertools

modulestillexistsinPython3,itjustdoesn’thavethefunctionsthathavemigratedtothe

globalnamespace.The

2to3

scriptissmartenoughtoremovethespecificimportsthatnolongerexist,while

leavingotherimportsintact.

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A.39.sys.exc_type,sys.exc_value,sys.exc_traceback

Python2hadthreevariablesinthe

sys

modulethatyoucouldaccesswhileanexceptionwasbeinghandled:

sys.exc_type

sys.exc_value

sys.exc_traceback

.(Actually,thesedateallthewaybacktoPython1.)Ever

sincePython1.5,thesevariables havebeendeprecatedinfavorof

sys.exc_info

,whichisatuplethat

contains allthreevalues.InPython3,theseindividualvariableshavefinallygoneaway;youmustuse

sys.exc_info

Notes

Python2

Python3

sys.exc_type

sys.exc_info()[0]

sys.exc_value

sys.exc_info()[1]

sys.exc_traceback

sys.exc_info()[2]

A.40.L

ISTCOMPREHENSIONSOVERTUPLES

InPython2,if youwantedtocodealistcomprehension thatiteratedoveratuple,youdid notneedtoput

parenthesesaroundthetuplevalues.InPython3,explicitparentheses arerequired.

Notes

Python2

Python3

[i for r i i in 1, 2]

[i for r i i in (1, , 2)]

A.41.os.getcwdu()

FUNCTION

Python2hadafunctionnamed

os.getcwd()

,whichreturnedthecurrentworkingdirectory as a(non-

Unicode) string.Becausemodernfilesystems canhandledirectorynamesinanycharacterencoding,Python

2.3introduced

os.getcwdu()

.The

os.getcwdu()

functionreturnedthecurrentworkingdirectoryasa

Unicodestring.InPython3,thereis

onlyonestringtype(Unicode),so

os.getcwd()

isallyouneed.

Notes

Python2

Python3

os.getcwdu()

os.getcwd()

463

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A.42.M

ETACLASSES

InPython2,youcouldcreatemetaclasseseitherby definingthe

metaclass

argumentintheclassdeclaration,

orbydefiningaspecialclass-level

__metaclass__

attribute.InPython3,theclass-levelattributehas been

eliminated.

Notes Python2

Python3

①

class C(metaclass=PapayaMeta):

pass

unchanged

②

class Whip:

__metaclass__ = PapayaMeta

class Whip(metaclass=PapayaMeta):

pass

③

class C(Whipper, Beater):

__metaclass__ = PapayaMeta

class C(Whipper, Beater, metaclass=PapayaMeta):

pass

1. Declaringthemetaclass s intheclassdeclarationworkedinPython2,and itstillworksthesameinPython3.

2. Declaringthemetaclass s inaclassattributeworkedinPython2,butdoesn’tworkinPython3.

3. The

2to3

scriptissmartenoughtoconstructavalidclassdeclaration,eveniftheclassisinheritedfromone

ormorebaseclasses.

A.43.M

ATTERSOFSTYLE

Therestofthe“fixes”listedherearen’treally fixesperse.Thatis,thethingstheychangearematters of

style,notsubstance.TheyworkjustaswellinPython3astheydoinPython2,butthedevelopersof

PythonhaveavestedinterestinmakingPythoncodeas uniformaspossible.Tothatend,thereisan

official

Pythonstyleguidewhichoutlines—inexcruciatingdetail—allsortsofnitpickydetails thatyoualmost

certainlydon’tcareabout.Andgiventhat

2to3

providessuchagreatinfrastructureforconvertingPython

codefromonethingtoanother,theauthorstookituponthemselvestoaddafewoptionalfeaturesto

improvethereadabilityofyourPythonprograms.

A.43.1.set()

LITERALS

(

EXPLICIT

)

InPython2,theonlywaytodefinealiteralsetinyourcodewastocall

set(a_sequence)

.Thisstillworksin

Python3,butaclearerwayofdoingitistousethenewsetliteralnotation:curlybraces.Thisworks for

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everythingexceptemptysets,becausedictionaries alsousecurlybraces,so

{}

is anemptydictionary,notan

emptyset.

The

2to3

scriptwillnotfix

set()

literals bydefault.Toenablethisfix,specify

-f

set_literal

onthecommandlinewhenyoucall

2to3

Notes

Before

After

set([1, 2, 3])

{1, 2, 3}

set((1, 2, 3))

{1, 2, 3}

set([i for i in a_sequence])

{i for r i i in a_sequence}

A.43.2.buffer()

GLOBAL FUNCTION

(

EXPLICIT

)

PythonobjectsimplementedinCcanexporta“bufferinterface,”whichallowsotherPythoncodetodirectly

readandwriteablockofmemory.(Thatisexactlyaspowerfulandscaryasitsounds.) InPython3,

buffer()

hasbeenrenamedto

memoryview()

.(It’salittlemorecomplicatedthanthat,butyoucanalmost

certainlyignorethedifferences.)

The

2to3

scriptwillnotfixthe

buffer()

functionbydefault.Toenablethisfix,

specify

-f buffer

onthecommandlinewhenyoucall

2to3

Notes

Before

After

x = buffer(y)

x = memoryview(y)

A.43.3.W

HITESPACE AROUND COMMAS

(

EXPLICIT

)

Despitebeingdraconianaboutwhitespaceforindentingandoutdenting,Python isactuallyquiteliberalabout

whitespaceinotherareas.Withinlists,tuples,sets,anddictionaries,whitespacecanappearbeforeandafter

commas withnoilleffects.However,thePythonstyleguidestatesthatcommasshould beprecededbyzero

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spacesandfollowedbyone.Althoughthisispurely anaestheticissue(thecodeworkseitherway,inboth

Python2andPython3),the

2to3

scriptcanoptionally fixthisforyou.

The

2to3

scriptwillnotfixwhitespacearoundcommasbydefault.Toenablethisfix,

specify

-f wscomma

onthecommandlinewhenyoucall

2to3

Notes

Before

After

a ,b

a, b

{a :b}

{a: b}

A.43.4.C

OMMON IDIOMS

(

EXPLICIT

)

Therewereanumberofcommonidioms builtupinthePythoncommunity.Some,likethe

while 1:

loop,

datebacktoPython1.(Pythondidn’thaveatruebooleantypeuntilversion2.3,sodevelopers used

and0

instead.)ModernPythonprogrammersshouldtraintheirbrainstousemodernversionsoftheseidioms

instead.

The

2to3

scriptwillnotfixcommonidiomsbydefault.Toenablethisfix,specify

-f

idioms

onthecommand linewhenyoucall

2to3

Notes

Before

After

while 1:

do_stuff()

while True:

do_stuff()

type(x) == T

isinstance(x, T)

type(x) is T

isinstance(x, T)

a_list = list(a_sequence)

a_list.sort()

do_stuff(a_list)

a_list = sorted(a_sequence)

do_stuff(a_list)

466

PPENDIX

B.S

PECIAL

ETHOD

AMES

❝

Myspecialtyisbeingrightwhenotherpeople arewrong.

❞

—

GeorgeBernardShaw

B.1.D

IVING

e’vealreadycoveredafewspecialmethod names elsewhereinthisbook—“magic”methodsthat

Pythoninvokeswhenyouusecertainsyntax.Usingspecialmethods,yourclassescanactlikesequences,like

dictionaries,likefunctions,likeiterators,orevenlikenumbers!This appendixservesbothasareferencefor

thespecialmethodswe’veseenalready and abriefintroductiontosomeofthemoreesotericones.

B.2.B

ASICS

Ifyou’vereadthe

introductiontoclasses,you’vealreadyseenthemostcommonspecialmethod:the

__init__()

method.Themajorityofclasses Iwriteendupneedingsomeinitialization.Therearealsoafew

otherbasicspecialmethodsthatareespeciallyusefulfordebuggingyourcustomclasses.

Notes YouWant…

SoYouWrite…

AndPythonCalls…

①

toinitializeaninstance

x = MyClass()

x.__init__()

②

the“official”representationasa

string

repr(x)

x.__repr__()

③

the“informal”valueasastring

str(x)

x.__str__()

④

the“informal”valueasabyte

array

bytes(x)

x.__bytes__()

⑤

thevalueasaformattedstring

format(x, format_spec)

x.__format__(format_spec)

1. The

__init__()

methodiscalled aftertheinstanceiscreated.Ifyouwanttocontroltheactualcreation

process,use

the

__new__()

method.

2. Byconvention,the

__repr__()

method shouldreturnastringthatisavalidPython expression.

467

3. The

__str__()

methodisalsocalled whenyou

print(x)

4. NewinPython3,sincethe

bytes

typewasintroduced.

5. Byconvention,

format_spec

shouldconformtothe

FormatSpecification Mini-Language.

decimal.py

in the

Pythonstandardlibraryprovidesits own

__format__()

method.

B.3.C

LASSES

HAT

IKE

TERATORS

theIteratorschapter,you sawhowtobuildaniteratorfromtheground upusingthe

__iter__()

and

__next__()

methods.

Notes YouWant…

SoYouWrite…

AndPython

Calls…

①

toiteratethroughasequence

iter(seq)

seq.__iter__()

②

togetthenextvaluefroman

iterator

next(seq)

seq.__next__()

③

tocreateaniteratorinreverse

order

reversed(seq)

seq.__reversed__()

1. The

__iter__()

methodiscalled wheneveryoucreateanewiterator.It’sagood placetoinitializethe

iteratorwithinitialvalues.

2. The

__next__()

methodiscalled wheneveryouretrievethenextvaluefromaniterator.

3. The

__reversed__()

methodisuncommon.Ittakesanexistingsequenceandreturnsaniteratorthatyields

theitemsinthesequenceinreverseorder,fromlasttofirst.

Asyousawin

theIterators chapter,a

for

loop canacton aniterator.Inthisloop:

for x in seq:

print(x)

Python3willcall

seq.__iter__()

tocreateaniterator,thencallthe

__next__()

methodonthatiteratorto

geteachvalueof

.Whenthe

__next__()

methodraisesa

StopIteration

exception,the

for

loopends

gracefully.

468

B.4.C

OMPUTED

TTRIBUTES

Notes YouWant…

SoYouWrite…

AndPythonCalls…

①

togetacomputedattribute

(unconditionally)

x.my_property

x.__getattribute__('my_property')

②

togetacomputedattribute

(fallback)

x.my_property

x.__getattr__('my_property')

③

tosetanattribute

x.my_property = = value

x.__setattr__('my_property',

value)

④

todeleteanattribute

del x.my_property

x.__delattr__('my_property')

⑤

tolistallattributesand

methods

dir(x)

x.__dir__()

1. Ifyourclassdefinesa

__getattribute__()

method,Pythonwillcallitoneveryreferencetoanyattributeor

methodname(exceptspecialmethodnames,sincethatwouldcausean unpleasantinfiniteloop).

2. Ifyourclassdefinesa

__getattr__()

method,Pythonwillcallitonlyafterlookingfortheattributeinallthe

normalplaces.Ifan instance

defines anattribute

color

x.color

willnotcall

x.__getattr__('color')

;itwill

simply returnthealready-definedvalueof

x.color

3. The

__setattr__()

methodiscalled wheneveryouassignavaluetoanattribute.

4. The

__delattr__()

methodiscalled wheneveryoudeleteanattribute.

5. The

__dir__()

methodisusefulifyoudefinea

__getattr__()

__getattribute__()

method.Normally,

calling

dir(x)

wouldonlylisttheregularattributesandmethods.Ifyour

__getattr()__

methodhandles a

color

attributedynamically,

dir(x)

wouldnotlist

color

asoneoftheavailableattributes.Overridingthe

__dir__()

methodallowsyoutolist

color

asanavailableattribute,whichishelpfulforotherpeoplewho

wishtouseyourclasswithoutdiggingintotheinternals ofit.

Thedistinctionbetweenthe

__getattr__()

and

__getattribute__()

methods issubtlebutimportant.Ican

explain itwithtwoexamples:

469

class Dynamo:

def __getattr__(self, key):

if key == 'color':

①

return 'PapayaWhip'

else:

raise AttributeError

②

>>> dyn = Dynamo()

>>> dyn.color

③

'PapayaWhip'

>>> dyn.color = 'LemonChiffon'

>>> dyn.color

④

'LemonChiffon'

1. Theattributenameispassedintothe

__getattr()__

methodasastring.Ifthenameis

'color'

,themethod

returnsavalue.(In this case,it’sjustahard-coded string,butyou would normallydosomesortof

computationandreturntheresult.)

2. Iftheattributenameisunknown,the

__getattr()__

methodneedstoraisean

AttributeError

exception,

otherwiseyourcodewillsilentlyfailwhenaccessingundefinedattributes.(Technically,ifthemethod doesn’t

raiseanexceptionorexplicitlyreturnavalue,itreturns

None

,thePythonnullvalue.Thismeansthatall

attributes notexplicitlydefinedwillbe

None

,whichisalmostcertainlynotwhatyouwant.)

3. The

dyn

instancedoesnothaveanattributenamed

color

,sothe

__getattr__()

methodiscalledtoprovide

acomputedvalue.

4. Afterexplicitlysetting

dyn.color

,the

__getattr__()

methodwillnolongerbecalledtoprovideavaluefor

dyn.color

,because

dyn.color

is alreadydefinedontheinstance.

Ontheotherhand,the

__getattribute__()

methodisabsoluteandunconditional.

470

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