New-style classes were introduced in Python 2.2 to unify classes and types. A new-style class is neither more nor less than a user-defined ...
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typeofclassinpython
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whyifIdo:
classC():pass
type(C())
Igot:,butifIdo:
classC(object):pass
type(c())
Igot:?
Thefirstisnotveryuserfull
pythonclasstypesnew-style-class
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askedOct7,2010at21:44
RuggeroTurraRuggeroTurra
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Whatdoyouwanttohaveinstead?Whatisnotuseful?Thatitisdifferent?
– FelixKling
Oct7,2010at21:53
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Lookupthedifferencebetweenold-styleandnew-styleclasses.Theformerarethedefault,andthelatterinheritexplicitlyfromobject.
Allold-styleobjectswereimplementedwiththebuilt-intypeinstance.Thefactthattheyarestillthedefaultandtheirtyperemains'instance'isaresultofretro-compatibilityprecautions.
ThisisextractedfromthePythondocs(http://docs.python.org/reference/datamodel.html)
3.3.New-styleandclassicclassesClassesandinstancescomeintwo
flavors:old-style(orclassic)and
new-style.
UptoPython2.1,old-styleclasses
weretheonlyflavouravailabletothe
user.Theconceptof(old-style)class
isunrelatedtotheconceptoftype:
ifxisaninstanceofanold-style
class,thenx.classdesignatesthe
classofx,buttype(x)isalways
.Thisreflectsthe
factthatallold-styleinstances,
independentlyoftheirclass,are
implementedwithasinglebuilt-in
type,calledinstance.
New-styleclasseswereintroducedin
Python2.2tounifyclassesandtypes.
Anew-styleclassisneithermorenor
lessthanauser-definedtype.Ifxis
aninstanceofanew-styleclass,then
type(x)istypicallythesameas
x>.class(althoughthisisnot
guaranteed-anew-styleclass
instanceispermittedtooverridethe
valuereturnedforx.class).
Themajormotivationforintroducing
new-styleclassesistoprovidea
unifiedobjectmodelwithafull
meta-model.Italsohasanumberof
practicalbenefits,liketheability
tosubclassmostbuilt-intypes,or
theintroductionof“descriptors”,
whichenablecomputedproperties.
Forcompatibilityreasons,classesare
stillold-stylebydefault.New-style
classesarecreatedbyspecifying
anothernew-styleclass(i.e.atype)
asaparentclass,orthe“top-level
type”objectifnootherparentis
needed.Thebehaviourofnew-style
classesdiffersfromthatofold-style
classesinanumberofimportant
detailsinadditiontowhattype()
returns.Someofthesechangesare
fundamentaltothenewobjectmodel,
likethewayspecialmethodsare
invoked.Othersare“fixes”thatcould
notbeimplementedbeforefor
compatibilityconcerns,likethe
methodresolutionorderincaseof
multipleinheritance.
Whilethismanualaimstoprovide
comprehensivecoverageofPython’s
classmechanics,itmaystillbe
lackinginsomeareaswhenitcomesto
itscoverageofnew-styleclasses.
Pleasesee
http://www.python.org/doc/newstyle/
forsourcesofadditionalinformation.
Old-styleclassesareremovedin
Python3.0,leavingonlythesemantics
ofnew-styleclasses.ofnew-styleclasses.
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answeredOct7,2010at21:52
slezicaslezica
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youneedtoedityouranswerandchangeallyour"x.__class__"into"`x.__class__`"
– tzot
Oct9,2010at1:19
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