A Python decorator is a function that helps to add some additional functionalities to an already defined function. In Python, everything is an ...
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Howdoesthe@propertydecoratorworkinPython?
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Iwouldliketounderstandhowthebuilt-infunctionpropertyworks.Whatconfusesmeisthatpropertycanalsobeusedasadecorator,butitonlytakesargumentswhenusedasabuilt-infunctionandnotwhenusedasadecorator.
Thisexampleisfromthedocumentation:
classC:
def__init__(self):
self._x=None
defgetx(self):
returnself._x
defsetx(self,value):
self._x=value
defdelx(self):
delself._x
x=property(getx,setx,delx,"I'mthe'x'property.")
property'sargumentsaregetx,setx,delxandadocstring.
Inthecodebelowpropertyisusedasadecorator.Theobjectofitisthexfunction,butinthecodeabovethereisnoplaceforanobjectfunctioninthearguments.
classC:
def__init__(self):
self._x=None
@property
defx(self):
"""I'mthe'x'property."""
returnself._x
@x.setter
defx(self,value):
self._x=value
@x.deleter
defx(self):
delself._x
Howarethex.setterandx.deleterdecoratorscreatedinthiscase?
pythonpropertiesdecoratorpython-decoratorspython-internals
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editedJan13,2021at23:00
Boris
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askedJun26,2013at20:47
ashimashim
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Seealso:HowdoPythonpropertieswork?
– MartinThoma
Feb3,2015at8:24
3
propertyisactuallyaclass(notafunction),althoughitdoesprobablydoescallthe__init__()methodwhenyoumakeanobject,ofcourse.Usinghelp(property)fromtheterminalisinsightful.helpisalsoaclassforsomereason.
– Brōtsyorfuzthrāx
Dec8,2016at8:17
2
Ithinkthislinkprovidesagoodexample:[property](journaldev.com/14893/python-property-decorator)
– ShengBi
Feb7,2019at16:55
4
@Shule2-year-oldthread,butstill:Everythingisaclass.Evenclasses.
– Artemis
Apr23,2019at20:27
3
Thiswasconfusingtometoo.Ifinallyfoundanarticlethatwasabletobreakitdownforme.Ihopethishelpssomeoneelse.programiz.com/python-programming/propertyI'mnotaffiliatedinanywaywiththesite.
– jjwdesign
Jun14,2019at17:55
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14Answers
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1185
Theproperty()functionreturnsaspecialdescriptorobject:
>>>property()
Itisthisobjectthathasextramethods:
>>>property().getter
>>>property().setter
>>>property().deleter
Theseactasdecoratorstoo.Theyreturnanewpropertyobject:
>>>property().getter(None)
thatisacopyoftheoldobject,butwithoneofthefunctionsreplaced.
Remember,thatthe@decoratorsyntaxisjustsyntacticsugar;thesyntax:
@property
deffoo(self):returnself._foo
reallymeansthesamethingas
deffoo(self):returnself._foo
foo=property(foo)
sofoothefunctionisreplacedbyproperty(foo),[email protected](),whatyouaredoingiscallthatproperty().settermethodIshowedyouabove,whichreturnsanewcopyoftheproperty,butthistimewiththesetterfunctionreplacedwiththedecoratedmethod.
Thefollowingsequencealsocreatesafull-onproperty,byusingthosedecoratormethods.
Firstwecreatesomefunctionsandapropertyobjectwithjustagetter:
>>>defgetter(self):print('Get!')
...
>>>defsetter(self,value):print('Setto{!r}!'.format(value))
...
>>>defdeleter(self):print('Delete!')
...
>>>prop=property(getter)
>>>prop.fgetisgetter
True
>>>prop.fsetisNone
True
>>>prop.fdelisNone
True
Nextweusethe.setter()methodtoaddasetter:
>>>prop=prop.setter(setter)
>>>prop.fgetisgetter
True
>>>prop.fsetissetter
True
>>>prop.fdelisNone
True
Lastweaddadeleterwiththe.deleter()method:
>>>prop=prop.deleter(deleter)
>>>prop.fgetisgetter
True
>>>prop.fsetissetter
True
>>>prop.fdelisdeleter
True
Lastbutnotleast,thepropertyobjectactsasadescriptorobject,soithas.__get__(),.__set__()and.__delete__()methodstohookintoinstanceattributegetting,settinganddeleting:
>>>classFoo:pass
...
>>>prop.__get__(Foo(),Foo)
Get!
>>>prop.__set__(Foo(),'bar')
Setto'bar'!
>>>prop.__delete__(Foo())
Delete!
TheDescriptorHowtoincludesapurePythonsampleimplementationoftheproperty()type:
classProperty:
"EmulatePyProperty_Type()inObjects/descrobject.c"
def__init__(self,fget=None,fset=None,fdel=None,doc=None):
self.fget=fget
self.fset=fset
self.fdel=fdel
ifdocisNoneandfgetisnotNone:
doc=fget.__doc__
self.__doc__=doc
def__get__(self,obj,objtype=None):
ifobjisNone:
returnself
ifself.fgetisNone:
raiseAttributeError("unreadableattribute")
returnself.fget(obj)
def__set__(self,obj,value):
ifself.fsetisNone:
raiseAttributeError("can'tsetattribute")
self.fset(obj,value)
def__delete__(self,obj):
ifself.fdelisNone:
raiseAttributeError("can'tdeleteattribute")
self.fdel(obj)
defgetter(self,fget):
returntype(self)(fget,self.fset,self.fdel,self.__doc__)
defsetter(self,fset):
returntype(self)(self.fget,fset,self.fdel,self.__doc__)
defdeleter(self,fdel):
returntype(self)(self.fget,self.fset,fdel,self.__doc__)
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editedOct20,2019at19:02
Boris
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answeredJun26,2013at20:54
MartijnPieters♦MartijnPieters
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Verygood.YoucouldaddthefactthatafterFoo.prop=propyoucandoFoo().prop=5;prontFoo().prop;delFoo().propwiththedesiredoutcome.
– glglgl
Jun27,2013at7:25
13
Methodobjectsarecreatedontheflyandcanreusethesamememorylocationifavailable.
– MartijnPieters
♦
Jun12,2014at19:15
1
@MarkusMeskanen:Iratherusetype()asaccessingdunderattributesandmethodsaremeanttobeusedasextensionpointsbythestandardfunctionsandoperators.
– MartijnPieters
♦
Mar10,2015at21:46
2
@MarkusMeskanen:becausetheobjectisimmutable,andifyoumutateditinplaceyoucouldnotspecialiseitinasubclass.
– MartijnPieters
♦
Jun13,2015at14:25
7
@MarkusMeskanen:seePythonoverridinggetterwithoutsetter;if@human.name.getteralteredthepropertyobjectin-placeratherthanreturnanew,thehuman.nameattributewouldbealtered,changingthebehaviourofthatsuperclass.
– MartijnPieters
♦
Jun13,2015at14:33
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293
Documentationsaysit'sjustashortcutforcreatingreadonlyproperties.So
@property
defx(self):
returnself._x
isequivalentto
defgetx(self):
returnself._x
x=property(getx)
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editedJul15,2016at20:45
λuser
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answeredJun26,2013at20:52
J0HNJ0HN
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Thefullcontext(most-upvotedanswer)isgood,butthisanswerwaspracticallyusefulforfiguringoutwhysomeoneelsehadused@propertyasadecoratorintheirclass.
– ijoseph
Apr16,2019at23:23
4
"...shortcutforcreatingreadonlyproperties.".Themilliondolahanswer!
– Saptadeep
Oct27,2021at18:00
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153
Hereisaminimalexampleofhow@propertycanbeimplemented:
classThing:
def__init__(self,my_word):
self._word=my_word
@property
defword(self):
returnself._word
>>>print(Thing('ok').word)
'ok'
Otherwisewordremainsamethodinsteadofaproperty.
classThing:
def__init__(self,my_word):
self._word=my_word
defword(self):
returnself._word
>>>print(Thing('ok').word())
'ok'
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answeredFeb15,2017at0:46
AlexGAlexG
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Howwouldthisexamplelookiftheword()function/propertyneededtobedefinedininit?
– J.J
Apr9,2017at9:51
14
CansomeonepleaseexplainwhyIwouldcreateapropertydecoratorhere,insteadofjusthavingself.word=my_word--whichwouldthenworkthesamewayprint(Thing('ok').word)='ok'
– SilverSlash
Jun9,2017at4:41
1
@SilverSlashThisisjustasimpleexample,arealuse-casewouldinvolveamorecomplicatedmethod
– AlexG
Jun9,2017at16:22
canyoupleaseexplainme,howprintingThing('ok').wordcallsthefunctioninternallyatruntime?
– Vicrobot
Dec27,2019at13:35
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90
Thefirstpartissimple:
@property
defx(self):...
isthesameas
defx(self):...
x=property(x)
which,inturn,isthesimplifiedsyntaxforcreatingapropertywithjustagetter.
Thenextstepwouldbetoextendthispropertywithasetterandadeleter.Andthishappenswiththeappropriatemethods:
@x.setter
defx(self,value):...
returnsanewpropertywhichinheritseverythingfromtheoldxplusthegivensetter.
x.deleterworksthesameway.
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answeredJun26,2013at20:53
glglglglglgl
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84
Belowisanotherexampleonhow@propertycanhelpwhenonehastorefactorcodewhichistakenfromhere(Ionlysummarizeitbelow):
ImagineyoucreatedaclassMoneylikethis:
classMoney:
def__init__(self,dollars,cents):
self.dollars=dollars
self.cents=cents
andanusercreatesalibrarydependingonthisclasswherehe/sheusese.g.
money=Money(27,12)
print("Ihave{}dollarand{}cents.".format(money.dollars,money.cents))
#printsIhave27dollarand12cents.
Nowlet'ssupposeyoudecidetochangeyourMoneyclassandgetridofthedollarsandcentsattributesbutinsteaddecidetoonlytrackthetotalamountofcents:
classMoney:
def__init__(self,dollars,cents):
self.total_cents=dollars*100+cents
Iftheabovementionedusernowtriestorunhis/herlibraryasbefore
money=Money(27,12)
print("Ihave{}dollarand{}cents.".format(money.dollars,money.cents))
itwillresultinanerror
AttributeError:'Money'objecthasnoattribute'dollars'
ThatmeansthatnoweveryonewhoreliesonyouroriginalMoneyclasswouldhavetochangealllinesofcodewheredollarsandcentsareusedwhichcanbeverypainful...So,howcouldthisbeavoided?Byusing@property!
Thatishow:
classMoney:
def__init__(self,dollars,cents):
self.total_cents=dollars*100+cents
#Getterandsetterfordollars...
@property
defdollars(self):
returnself.total_cents//100
@dollars.setter
defdollars(self,new_dollars):
self.total_cents=100*new_dollars+self.cents
#Andthegetterandsetterforcents.
@property
defcents(self):
returnself.total_cents%100
@cents.setter
defcents(self,new_cents):
self.total_cents=100*self.dollars+new_cents
whenwenowcallfromourlibrary
money=Money(27,12)
print("Ihave{}dollarand{}cents.".format(money.dollars,money.cents))
#printsIhave27dollarand12cents.
itwillworkasexpectedandwedidnothavetochangeasinglelineofcodeinourlibrary!Infact,wewouldnotevenhavetoknowthatthelibrarywedependonchanged.
Alsothesetterworksfine:
money.dollars+=2
print("Ihave{}dollarand{}cents.".format(money.dollars,money.cents))
#printsIhave29dollarand12cents.
money.cents+=10
print("Ihave{}dollarand{}cents.".format(money.dollars,money.cents))
#printsIhave29dollarand22cents.
Youcanuse@propertyalsoinabstractclasses;Igiveaminimalexamplehere.
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editedSep16,2019at21:42
answeredSep23,2018at11:58
ClebCleb
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yoursummaryisverygood,theexamplethatwebsitetakesisalittlebitstrange..Abeginnerwouldask..whycan'twejuststicktotheself.dollar=dollars?wehavedonesomuchwith@property,butitseemsnoextractfunctionalityisadded.
– ShengBi
Feb7,2019at16:39
4
@ShengBi:Donotfocusthatmuchontheactualexamplebutmoreontheunderlyingprinciple:If-forwhateverreason-youhavetorefactorcode,youcandosowithoutaffectinganyoneother'scode.
– Cleb
Feb7,2019at16:44
1
@clebyoudarealmvp.Everybodyelseusesthatgettersetterexamplelikethisone,programiz.com/python-programming/property.Butyouaretheonlyonethatactuallyexplainswhywewantproperty.It'sbecausewhenwewritessomethinguponwhichalotofpeoplearegoingtobuild,wewanttobeabletomodifythebaseclasseswithoutnorealimpactonhowthesuccessorsuseorbuilduponourwork,implementationwise.
– ChenLizi
Apr7,2021at0:28
1
@ClebIsuredid!:)
– ChenLizi
Apr7,2021at17:27
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58
Thisfollowing:
classC(object):
def__init__(self):
self._x=None
@property
defx(self):
"""I'mthe'x'property."""
returnself._x
@x.setter
defx(self,value):
self._x=value
@x.deleter
defx(self):
delself._x
Isthesameas:
classC(object):
def__init__(self):
self._x=None
def_x_get(self):
returnself._x
def_x_set(self,value):
self._x=value
def_x_del(self):
delself._x
x=property(_x_get,_x_set,_x_del,
"I'mthe'x'property.")
Isthesameas:
classC(object):
def__init__(self):
self._x=None
def_x_get(self):
returnself._x
def_x_set(self,value):
self._x=value
def_x_del(self):
delself._x
x=property(_x_get,doc="I'mthe'x'property.")
x=x.setter(_x_set)
x=x.deleter(_x_del)
Isthesameas:
classC(object):
def__init__(self):
self._x=None
def_x_get(self):
returnself._x
x=property(_x_get,doc="I'mthe'x'property.")
def_x_set(self,value):
self._x=value
x=x.setter(_x_set)
def_x_del(self):
delself._x
x=x.deleter(_x_del)
Whichisthesameas:
classC(object):
def__init__(self):
self._x=None
@property
defx(self):
"""I'mthe'x'property."""
returnself._x
@x.setter
defx(self,value):
self._x=value
@x.deleter
defx(self):
delself._x
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answeredMay24,2017at18:38
BimoBimo
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Thefirstandlastcodeexamplesarethesame(verbatim).
– AdomasBaliuka
Aug15,2019at2:13
2
Ithinkit'sintentional.Eitherway,thiswasthemostusefulexampletomebecauseIcangrokmeaningfromtheseexamples.Thanks@BillMoore
– Jérémie
Jun14,2020at16:20
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Let'sstartwithPythondecorators.
APythondecoratorisafunctionthathelpstoaddsomeadditionalfunctionalitiestoanalreadydefinedfunction.
InPython,everythingisanobject.FunctionsinPythonarefirst-classobjectswhichmeansthattheycanbereferencedbyavariable,addedinthelists,passedasargumentstoanotherfunction,etc.
Considerthefollowingcodesnippet.
defdecorator_func(fun):
defwrapper_func():
print("Wrapperfunctionstarted")
fun()
print("Givenfunctiondecorated")
#Wrapperfunctionaddsomethingtothepassedfunctionanddecorator
#returnsthewrapperfunction
returnwrapper_func
defsay_bye():
print("bye!!")
say_bye=decorator_func(say_bye)
say_bye()
#Output:
#Wrapperfunctionstarted
#bye!!
#Givenfunctiondecorated
Here,wecansaythatthedecoratorfunctionmodifiedoursay_byefunctionandaddedsomeextralinesofcodetoit.
Pythonsyntaxfordecorator
defdecorator_func(fun):
defwrapper_func():
print("Wrapperfunctionstarted")
fun()
print("Givenfunctiondecorated")
#Wrapperfunctionaddsomethingtothepassedfunctionanddecorator
#returnsthewrapperfunction
returnwrapper_func
@decorator_func
defsay_bye():
print("bye!!")
say_bye()
Let'sgothrougheverythingwithacasescenario.Butbeforethat,let'stalkaboutsomeOOPprinciples.
Gettersandsettersareusedinmanyobject-orientedprogramminglanguagestoensuretheprincipleofdataencapsulation(whichisseenasthebundlingofdatawiththemethodsthatoperateonthesedata.)
Thesemethodsare,ofcourse,thegetterforretrievingthedataandthesetterforchangingthedata.
Accordingtothisprinciple,theattributesofaclassaremadeprivatetohideandprotectthemfromothercode.
Yup,@propertyisbasicallyapythonicwaytousegettersandsetters.
Pythonhasagreatconceptcalledpropertywhichmakesthelifeofanobject-orientedprogrammermuchsimpler.
LetusassumethatyoudecidetomakeaclassthatcouldstorethetemperatureindegreesCelsius.
classCelsius:
def__init__(self,temperature=0):
self.set_temperature(temperature)
defto_fahrenheit(self):
return(self.get_temperature()*1.8)+32
defget_temperature(self):
returnself._temperature
defset_temperature(self,value):
ifvalue1000:
self.__x=1000
else:
self.__x=x
Thisisgreat,isn'tit:Youcanstartwiththesimplestimplementationimaginable,andyouarefreetolatermigratetoapropertyversionwithouthavingtochangetheinterface!Sopropertiesarenotjustareplacementforgettersandsetters!
YoucancheckthisImplementationhere
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editedJul23,2021at9:25
Useless
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answeredSep17,2018at0:01
DivyanshuRawatDivyanshuRawat
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YourCelsiusclassisgoingtoinfinitelyrecursewhensetting(whichmeansuponinstantiation).
– TedPetrou
Sep23,2018at12:25
1
@TedPetrouIDidn'tgetyou?Howitwillinfinitelyrecursewhensetting?
– DivyanshuRawat
Sep23,2018at12:37
1
Peopleareaskinghowdoesitworknotwhyitworks?@ShengBi
– DivyanshuRawat
Feb7,2019at17:53
1
Itisjustacomment,mypersonalopinion.Youranswercouldbereallygood.soleaveit.
– ShengBi
Feb10,2019at1:14
2
comparedtothetopvotedanswers,thisoneisdesignedforhumans;thanks.
– Info5ek
May18,2020at0:06
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28
Ireadallthepostshereandrealizedthatwemayneedareallifeexample.Why,actually,wehave@property?
So,consideraFlaskappwhereyouuseauthenticationsystem.
YoudeclareamodelUserinmodels.py:
classUser(UserMixin,db.Model):
__tablename__='users'
id=db.Column(db.Integer,primary_key=True)
email=db.Column(db.String(64),unique=True,index=True)
username=db.Column(db.String(64),unique=True,index=True)
password_hash=db.Column(db.String(128))
...
@property
defpassword(self):
raiseAttributeError('passwordisnotareadableattribute')
@password.setter
defpassword(self,password):
self.password_hash=generate_password_hash(password)
defverify_password(self,password):
returncheck_password_hash(self.password_hash,password)
Inthiscodewe've"hidden"attributepasswordbyusing@propertywhichtriggersAttributeErrorassertionwhenyoutrytoaccessitdirectly,whileweused@property.settertosettheactualinstancevariablepassword_hash.
Nowinauth/views.pywecaninstantiateaUserwith:
...
@auth.route('/register',methods=['GET','POST'])
defregister():
form=RegisterForm()
ifform.validate_on_submit():
user=User(email=form.email.data,
username=form.username.data,
password=form.password.data)
db.session.add(user)
db.session.commit()
...
Noticeattributepasswordthatcomesfromaregistrationformwhenauserfillstheform.PasswordconfirmationhappensonthefrontendwithEqualTo('password',message='Passwordsmustmatch')(incaseifyouarewondering,butit'sadifferenttopicrelatedFlaskforms).
Ihopethisexamplewillbeuseful
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editedSep10,2019at15:24
answeredMar23,2018at14:47
LeoSkhrnkvLeoSkhrnkv
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ThispointisbeenclearedbymanypeopleuptherebuthereisadirectpointwhichIwassearching.
ThisiswhatIfeelisimportanttostartwiththe@propertydecorator.
eg:-
classUtilityMixin():
@property
defget_config(self):
return"Thisisproperty"
Thecallingoffunction"get_config()"willworklikethis.
util=UtilityMixin()
print(util.get_config)
IfyounoticeIhavenotused"()"bracketsforcallingthefunction.ThisisthebasicthingwhichIwassearchingforthe@propertydecorator.Sothatyoucanuseyourfunctionjustlikeavariable.
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editedNov26,2018at6:39
answeredNov9,2018at13:00
DevendraBhatDevendraBhat
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8
propertyisaclassbehind@propertydecorator.
Youcanalwayscheckthis:
print(property)#
Irewrotetheexamplefromhelp(property)toshowthatthe@propertysyntax
classC:
def__init__(self):
self._x=None
@property
defx(self):
returnself._x
@x.setter
defx(self,value):
self._x=value
@x.deleter
defx(self):
delself._x
c=C()
c.x="a"
print(c.x)
isfunctionallyidenticaltoproperty()syntax:
classC:
def__init__(self):
self._x=None
defg(self):
returnself._x
defs(self,v):
self._x=v
defd(self):
delself._x
prop=property(g,s,d)
c=C()
c.x="a"
print(c.x)
Thereisnodifferencehowweusethepropertyasyoucansee.
Toanswerthequestion@propertydecoratorisimplementedviapropertyclass.
So,thequestionistoexplainthepropertyclassabit.
Thisline:
prop=property(g,s,d)
Wastheinitialization.Wecanrewriteitlikethis:
prop=property(fget=g,fset=s,fdel=d)
Themeaningoffget,fsetandfdel:
|fget
|functiontobeusedforgettinganattributevalue
|fset
|functiontobeusedforsettinganattributevalue
|fdel
|functiontobeusedfordel'inganattribute
|doc
|docstring
Thenextimageshowsthetripletswehave,fromtheclassproperty:
__get__,__set__,and__delete__aretheretobeoverridden.ThisistheimplementationofthedescriptorpatterninPython.
Ingeneral,adescriptorisanobjectattributewith“bindingbehavior”,onewhoseattributeaccesshasbeenoverriddenbymethodsinthedescriptorprotocol.
Wecanalsousepropertysetter,getteranddeletermethodstobindthefunctiontoproperty.Checkthenextexample.Themethods2oftheclassCwillsetthepropertydoubled.
classC:
def__init__(self):
self._x=None
defg(self):
returnself._x
defs(self,x):
self._x=x
defd(self):
delself._x
defs2(self,x):
self._x=x+x
x=property(g)
x=x.setter(s)
x=x.deleter(d)
c=C()
c.x="a"
print(c.x)#outputs"a"
C.x=property(C.g,C.s2)
C.x=C.x.deleter(C.d)
c2=C()
c2.x="a"
print(c2.x)#outputs"aa"
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editedMay26,2019at21:10
answeredMay26,2019at16:27
prostiprosti
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Thebestexplanationcanbefoundhere:
Python@PropertyExplained–HowtoUseandWhen?(FullExamples)
bySelvaPrabhakaran|PostedonNovember5,2018
IthelpedmeunderstandWHYnotonlyHOW.
https://www.machinelearningplus.com/python/python-property/
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editedFeb25,2020at3:50
answeredFeb22,2020at1:28
VictorWangVictorWang
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Thisisthebestsourceasofnow
– thanos.a
Mar19at3:00
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5
Adecoratorisafunctionthattakesafunctionasanargumentandreturnsaclosure.Theclosureisasetofinnerfunctionsandfreevariables.Theinnerfunctionisclosingoverthefreevariableandthatiswhyitiscalled'closure'.Afreevariableisavariablethatisoutsidetheinnerfunctionandpassedintotheinnerviadocorator.
Asthenamesays,decoratorisdecoratingthereceivedfunction.
functiondecorator(undecorated_func):
print("callingdecoratorfunc")
inner():
print("Iaminsideinner")
returnundecorated_func
returninner
thisisasimpledecoratorfunction.Itreceived"undecorated_func"andpassedittoinner()asafreevariable,inner()printed"Iaminsideinner"andreturnedundecorated_func.Whenwecalldecorator(undecorated_func),itisreturningtheinner.Hereisthekey,indecoratorswearenamingtheinnerfunctionasthenameofthefunctionthatwepassed.
undecorated_function=decorator(undecorated_func)
nowinnerfunctioniscalled"undecorated_func".Sinceinnerisnownamedas"undecorated_func",wepassed"undecorated_func"tothedecoratorandwereturned"undecorated_func"plusprintedout"Iaminsideinner".sothisprintstatementdecoratedour"undecorated_func".
nowlet'sdefineaclasswithapropertydecorator:
classPerson:
def__init__(self,name):
self._name=name
@property
defname(self):
returnself._name
@name.setter
defname(self.value):
self._name=value
whenwedecoratedname()with@property(),thisiswhathappened:
name=property(name)#Person.__dict__youllseename
firstargumentofproperty()isgetter.thisiswhathappenedintheseconddecoration:
name=name.setter(name)
AsImentionedabove,thedecoratorreturnstheinnerfunction,andwenametheinnerfunctionwiththenameofthefunctionthatwepassed.
Hereisanimportantthingtobeawareof."name"isimmutable.inthefirstdecorationwegotthis:
name=property(name)
inthesecondonewegotthis
name=name.setter(name)
Wearenotmodifyingnameobj.Intheseconddecoration,pythonseesthatthisispropertyobjectanditalreadyhadgetter.Sopythoncreatesanew"name"object,addsthe"fget"fromthefirstobjandthensetsthe"fset".
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editedMar19at2:47
answeredNov30,2020at21:02
YilmazYilmaz
12.2k77goldbadges6464silverbadges9999bronzebadges
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Youranswerhasalotoftyposandsytaxmistakesthatpreventedmefromreadingit.
– thanos.a
Mar19at2:44
@thanos.aIamsosorryaboutit:)IdideditedsometyposbutIdonotseeanysyntaxerror
– Yilmaz
Mar19at2:48
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Apropertycanbedeclaredintwoways.
Creatingthegetter,settermethodsforanattributeandthenpassingtheseasargumenttopropertyfunction
Usingthe@propertydecorator.
YoucanhavealookatfewexamplesIhavewrittenaboutpropertiesinpython.
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answeredJul13,2017at9:20
nvdnvd
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canyouupdatedyouranswersayingthatpropertyisaclasssoIcanupvote.
– prosti
May26,2019at21:43
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Hereisanotherexample:
##
##PythonPropertiesExample
##
classGetterSetterExample(object):
##Setthedefaultvalueforx(wereferenceitusingself.x,setavalueusingself.x=value)
__x=None
##
##OnClassInitialization-dosomething...ifwewant..
##
def__init__(self):
##Setavalueto__xthroughthegetter/setter...Since__xisdefinedabove,thisdoesn'tneedtobeset...
self.x=1234
returnNone
##
##Definexasaproperty,ieagetter-Allgettersshouldhaveadefaultvaluearg,soIaddedit-itwillnotbepassedinwhensettingavalue,soyouneedtosetthedefaultheresoitwillbeused..
##
@property
defx(self,_default=None):
##Iaddedanoptionaldefaultvalueargumentasallgettersshouldhavethis-setittothedefaultvalueyouwanttoreturn...
_value=(self.__x,_default)[self.__x==None]
##Debugging-soyoucanseetheorderthecallsaremade...
print('[TestClass]Getx='+str(_value))
##Returnthevalue-weareagetterafterall...
return_value
##
##Definethesetterfunctionforx...
##
@x.setter
defx(self,_value=None):
##Debugging-soyoucanseetheorderthecallsaremade...
print('[TestClass]Setx='+str(_value))
##Thisistoshowthesetterfunctionworks....Ifthevalueisabove0,setittoanegativevalue...otherwisekeepitasis(0istheonlynon-negativenumber,itcan'tbenegativeorpositiveanyway)
if(_value>0):
self.__x=-_value
else:
self.__x=_value
##
##Definethedeleterfunctionforx...
##
@x.deleter
defx(self):
##Unloadtheassignment/dataforx
if(self.__x!=None):
delself.__x
##
##ToString/OutputFunctionfortheclass-thiswillshowthepropertyvalueforeachpropertyweadd...
##
def__str__(self):
##Outputthexpropertydata...
print('[x]'+str(self.x))
##Returnanewline-technicallyweshouldreturnastringsoitcanbeprintedwherewewantit,insteadofprintedearlyif_data=str(C())isused....
return'\n'
##
##
##
_test=GetterSetterExample()
print(_test)
##Forsomereasonthedeleterisn'tbeingcalled...
del_test.x
Basically,thesameastheC(object)exampleexceptI'musingxinstead...Ialsodon'tinitializein__init-...well..Ido,butitcanberemovedbecause__xisdefinedaspartoftheclass....
Theoutputis:
[TestClass]Setx=1234
[TestClass]Getx=-1234
[x]-1234
andifIcommentouttheself.x=1234ininitthentheoutputis:
[TestClass]Getx=None
[x]None
andifIsetthe_default=Noneto_default=0inthegetterfunction(asallgettersshouldhaveadefaultvaluebutitisn'tpassedinbythepropertyvaluesfromwhatI'veseensoyoucandefineithere,anditactuallyisn'tbadbecauseyoucandefinethedefaultonceanduseiteverywhere)ie:defx(self,_default=0):
[TestClass]Getx=0
[x]0
Note:Thegetterlogicistherejusttohavethevaluebemanipulatedbyittoensureitismanipulatedbyit-thesamefortheprintstatements...
Note:I'musedtoLuaandbeingabletodynamicallycreate10+helperswhenIcallasinglefunctionandImadesomethingsimilarforPythonwithoutusingpropertiesanditworkstoadegree,but,eventhoughthefunctionsarebeingcreatedbeforebeingused,therearestillissuesattimeswiththembeingcalledpriortobeingcreatedwhichisstrangeasitisn'tcodedthatway...IprefertheflexibilityofLuameta-tablesandthefactIcanuseactualsetters/gettersinsteadofessentiallydirectlyaccessingavariable...IdolikehowquicklysomethingscanbebuiltwithPythonthough-forinstanceguiprograms.althoughoneIamdesigningmaynotbepossiblewithoutalotofadditionallibraries-ifIcodeitinAutoHotkeyIcandirectlyaccessthedllcallsIneed,andthesamecanbedoneinJava,C#,C++,andmore-maybeIhaven'tfoundtherightthingyetbutforthatprojectImayswitchfromPython..
Note:Thecodeoutputinthisforumisbroken-Ihadtoaddspacestothefirstpartofthecodeforittowork-whencopy/pastingensureyouconvertallspacestotabs....IusetabsforPythonbecauseinafilewhichis10,000linesthefilesizecanbe512KBto1MBwithspacesand100to200KBwithtabswhichequatestoamassivedifferenceforfilesize,andreductioninprocessingtime...
Tabscanalsobeadjustedperuser-soifyouprefer2spaceswidth,4,8orwhateveryoucandoitmeaningitisthoughtfulfordeveloperswitheye-sightdeficits.
Note:Allofthefunctionsdefinedintheclassaren'tindentedproperlybecauseofabugintheforumsoftware-ensureyouindentitifyoucopy/paste
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answeredAug7,2018at11:08
AcecoolAcecool
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