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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))
461
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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
itertools.izip(a, b)
zip(a, b)
itertools.imap(a, b)
map(a, b)
itertools.ifilter(a, b)
filter(a, b)
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()
.
3.
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
class C(metaclass=PapayaMeta):
pass
unchanged
class Whip:
__metaclass__ = PapayaMeta
class Whip(metaclass=PapayaMeta):
pass
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
1
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
A
PPENDIX
B.S
PECIAL
M
ETHOD
N
AMES
Myspecialtyisbeingrightwhenotherpeople arewrong.
GeorgeBernardShaw
B.1.D
IVING
I
N
W
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…
toinitializeaninstance
x = MyClass()
x.__init__()
the“official”representationasa
string
repr(x)
x.__repr__()
the“informal”valueasastring
str(x)
x.__str__()
the“informal”valueasabyte
array
bytes(x)
x.__bytes__()
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
T
HAT
A
CT
L
IKE
I
TERATORS
In
theIteratorschapter,you sawhowtobuildaniteratorfromtheground upusingthe
__iter__()
and
__next__()
methods.
Notes YouWant…
SoYouWrite…
AndPython
Calls…
toiteratethroughasequence
iter(seq)
seq.__iter__()
togetthenextvaluefroman
iterator
next(seq)
seq.__next__()
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
x
.Whenthe
__next__()
methodraisesa
StopIteration
exception,the
for
loopends
gracefully.
468
B.4.C
OMPUTED
A
TTRIBUTES
Notes YouWant…
SoYouWrite…
AndPythonCalls…
togetacomputedattribute
(unconditionally)
x.my_property
x.__getattribute__('my_property')
togetacomputedattribute
(fallback)
x.my_property
x.__getattr__('my_property')
tosetanattribute
x.my_property = = value
x.__setattr__('my_property',
value)
todeleteanattribute
del x.my_property
x.__delattr__('my_property')
tolistallattributesand
methods
dir(x)
x.__dir__()
1. Ifyourclassdefinesa
__getattribute__()
method,Pythonwillcallitoneveryreferencetoanyattributeor
methodname(exceptspecialmethodnames,sincethatwouldcausean unpleasantinfiniteloop).
2. Ifyourclassdefinesa
__getattr__()
method,Pythonwillcallitonlyafterlookingfortheattributeinallthe
normalplaces.Ifan instance
x
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__()
or
__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':
return 'PapayaWhip'
else:
raise AttributeError
>>> dyn = Dynamo()
>>> dyn.color
'PapayaWhip'
>>> dyn.color = 'LemonChiffon'
>>> dyn.color
'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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