Python Class Variables With Examples - PYnative

In Python, Class variables are declared when a class is being ... Access class variable inside instance method by using either self of class ... Home»Python»PythonObject-OrientedProgramming(OOP)»PythonClassVariables InObject-orientedprogramming,whenwedesignaclass,weuseinstancevariablesandclassvariables. InClass,attributescanbedefinedintotwoparts: Instancevariables:Ifthevalueofavariablevariesfromobjecttoobject,thensuchvariablesarecalledinstancevariables.ClassVariables:Aclassvariableisavariablethatisdeclaredinsideofclass,butoutsideofanyinstancemethodor __init__() method. Afterreadingthisarticle,you’lllearn: HowtocreateandaccessclassvariablesModifyvaluesofaclassvariablesInstancevariablevs.classvariablesBehaviourofaclassvariableininheritance TableofcontentsWhatisanClassVariableinPython?CreateClassVariablesAccessingClassVariablesModifyClassVariablesClassVariablevsInstancevariablesClassVariablesInInheritanceWrongUseofClassVariables WhatisanClassVariableinPython? Ifthevalueofavariableisnotvariedfromobjecttoobject,suchtypesofvariablesarecalledclassvariablesorstaticvariables. Classvariablesaresharedbyallinstancesofaclass.Unlikeinstancevariable,thevalueofaclassvariableisnotvariedfromobjecttoobject, InPython,Classvariablesaredeclaredwhenaclassisbeingconstructed.Theyarenotdefinedinsideanymethodsofaclassbecauseofthisonlyonecopyofthestaticvariablewillbecreatedandsharedbetweenallobjectsoftheclass. Forexample,inStudentclass,wecanhavedifferentinstancevariablessuchasnameandrollnumberbecauseeachstudent’snameandrollnumberaredifferent. But,ifwewanttoincludetheschoolnameinthestudentclass,wemustusetheclassvariableinsteadofaninstancevariableastheschoolnameisthesameforallstudents.Soinsteadofmaintainingtheseparatecopyineachobject,wecancreateaclassvariablethatwillholdtheschoolnamesoallstudents(objects)canshareit. Wecanaddanynumberofclassvariablesinaclass. UnderstandClassVariables CreateClassVariables Aclassvariableisdeclaredinsideofclass,butoutsideofanyinstancemethodor __init__() method. Byconvention,typicallyitisplacedrightbelowtheclassheaderandbeforetheconstructormethodandothermethods. Example: classStudent: #Classvariable school_name='ABCSchool' def__init__(self,name,roll_no): self.name=name self.roll_no=roll_no #createfirstobject s1=Student('Emma',10) print(s1.name,s1.roll_no,Student.school_name) #accessclassvariable #createsecondobject s2=Student('Jessa',20) #accessclassvariable print(s2.name,s2.roll_no,Student.school_name) Output Emma10ABCSchool Jessa20ABCSchool Intheaboveexample,wecreatedtheclassvariableschool_nameandaccesseditusingtheobjectandclassname. Note:Likeregularvariables,classvariablescanstoredataofanytype.WecanusePythonlist,Pythontuple,andPythondictionaryasaclassvariable. AccessingClassVariables Wecanaccessstaticvariableseitherbyclassnameorbyobjectreference,butitisrecommendedtousetheclassname. InPython,wecanaccesstheclassvariableinthefollowingplaces Accessinsidetheconstructorbyusingeitherselfparameterorclassname.AccessclassvariableinsideinstancemethodbyusingeitherselfofclassnameAccessfromoutsideofclassbyusingeitherobjectreferenceorclassname. Example1:AccessClassVariableintheconstructor classStudent: #Classvariable school_name='ABCSchool' #constructor def__init__(self,name): self.name=name #accessclassvariableinsideconstructorusingself print(self.school_name) #accessusingclassname print(Student.school_name) #createObject s1=Student('Emma') Output ABCSchool ABCSchool Example2:AccessClassVariableinInstancemethodandoutsideclass classStudent: #Classvariable school_name='ABCSchool' #constructor def__init__(self,name,roll_no): self.name=name self.roll_no=roll_no #Instancemethod defshow(self): print('Insideinstancemethod') #accessusingself print(self.name,self.roll_no,self.school_name) #accessusingclassname print(Student.school_name) #createObject s1=Student('Emma',10) s1.show() print('Outsideclass') #accessclassvariableoutsideclass #accessusingobjectreference print(s1.school_name) #accessusingclassname print(Student.school_name) Output Insideinstancemethod Emma10ABCSchool ABCSchool Outsideclass ABCSchool ABCSchool Inthisexample,weaccessedtheclassvariableschool_nameusingclassnameandaselfkeywordinsideamethod. ModifyClassVariables Generally,weassignvaluetoaclassvariableinsidetheclassdeclaration.However,wecanchangethevalueoftheclassvariableeitherintheclassoroutsideofclass. Note:Weshouldchangetheclassvariable’svalueusingtheclassnameonly. Example classStudent: #Classvariable school_name='ABCSchool' #constructor def__init__(self,name,roll_no): self.name=name self.roll_no=roll_no #Instancemethod defshow(self): print(self.name,self.roll_no,Student.school_name) #createObject s1=Student('Emma',10) print('Before') s1.show() #Modifyclassvariable Student.school_name='XYZSchool' print('After') s1.show() Output: Before Emma10ABCSchool After Emma10XYZSchool Note: Itisbestpracticetouseaclassnametochangethevalueofaclassvariable.Becauseifwetrytochangetheclassvariable’svaluebyusinganobject,anewinstancevariableiscreatedforthatparticularobject,whichshadowstheclassvariables. Example: classStudent: #Classvariable school_name='ABCSchool' #constructor def__init__(self,name,roll_no): self.name=name self.roll_no=roll_no #createObjects s1=Student('Emma',10) s2=Student('Jessa',20) print('Before') print(s1.name,s1.roll_no,s1.school_name) print(s2.name,s2.roll_no,s2.school_name) #Modifyclassvariableusingobjectreference s1.school_name='PQRSchool' print('After') print(s1.name,s1.roll_no,s1.school_name) print(s2.name,s2.roll_no,s2.school_name) Output: Before Emma10ABCSchool Jessa20ABCSchool After Emma10PQRSchool Jessa20ABCSchool Anewinstancevariableiscreatedforthes1object,andthisvariableshadowstheclassvariables.Soalwaysusetheclassnametomodifytheclassvariable. ClassVariablevsInstancevariables Thefollowingtableshowsthedifferencebetweentheinstancevariableandtheclassvariable. InPython,propertiescanbedefinedintotwoparts: Instancevariables:Instancevariable’svaluevariesfromobjecttoobject.Instancevariablesarenotsharedbyobjects.EveryobjecthasitsowncopyoftheinstanceattributeClassVariables:Aclassvariableisavariablethatisdeclaredinsideofclass,butoutsideofanyinstancemethodor __init__() method.Classvariablesaresharedbyallinstancesofaclass. ReadMore:InstancevariablesinPythonwithExamples InstanceVariableClassVariableInstancevariablesarenotsharedbyobjects.EveryobjecthasitsowncopyoftheinstanceattributeClassvariablesaresharedbyallinstances.Instancevariablesaredeclaredinsidetheconstructori.e.,the__init__()method.Classvariablesaredeclaredinsidetheclassdefinitionbutoutsideanyoftheinstancemethodsandconstructors.Itisgetscreatedwhenaninstanceoftheclassiscreated.Itiscreatedwhentheprogrambeginstoexecute.Changesmadetothesevariablesthroughoneobjectwillnotreflectinanotherobject.Changesmadeintheclassvariablewillreflectinallobjects.ClassVariablesvs.InstanceVariables Example: Let’sseetheexampletocreateaclassvariableandinstancevariable. classCar: #Classvariable manufacturer='BMW' def__init__(self,model,price): #instancevariable self.model=model self.price=price #createObject car=Car('x1',2500) print(car.model,car.price,Car.manufacturer) Output: x12500BMW ClassVariablesInInheritance Asyouknow,onlyonecopyoftheclassvariablewillbecreatedandsharedbetweenallobjectsofthatclass. Whenweuseinheritance,allvariablesandmethodsofthebaseclassareavailabletothechildclass.Insuchcases,Wecanalsochangethevalueoftheparentclass’sclassvariableinthechildclass. Wecanusetheparentclassorchildclassnametochangethevalueofaparentclass’sclassvariableinthechildclass. Example classCourse: #classvariable course="Python" classStudent(Course): def__init__(self,name): self.name=name defshow_student(self): #Accessingclassvariableofparentclass print('Before') print("Studentname:",self.name,"CourseName:",Student.course) #changingclassvariablevalueofbaseclass print('Now') Student.course="MachineLearning" print("Studentname:",self.name,"CourseName:",Student.course) #creatingobjectofStudentclass stud=Student("Emma") stud.show_student() Output Before Studentname:EmmaCourseName:Python Now Studentname:EmmaCourseName:MachineLearning Whatifbothchildclassandparentclasshasthesameclassvariablename.Inthiscase,thechildclasswillnotinherittheclassvariableofabaseclass.Soitisrecommendedtocreateaseparateclassvariableforchildclassinsteadofinheritingthebaseclassvariable. Example: classCourse: #classvariable course="Python" classStudent(Course): #classvariable course="SQL" def__init__(self,name): self.name=name defshow_student(self): #Accessingclassvariable print('Before') print("Studentname:",self.name,"CourseName:",Student.course) #changingclassvariable'svalue print('Now') Student.course="MachineLearning" print("Studentname:",self.name,"CourseName:",Student.course) #creatingobjectofStudentclass stud=Student("Emma") stud.show_student() #parentclasscoursename print('ParentClassCourseName:',Course.course) Output: Before Studentname:EmmaCourseName:SQL Now Studentname:EmmaCourseName:MachineLearning ParentClassCourseName:Python WrongUseofClassVariables InPython,weshouldproperlyusetheclassvariablebecauseallobjectssharethesamecopy.Thus,ifoneoftheobjectsmodifiesthevalueofaclassvariable,thenallobjectsstartreferringtothefreshcopy. Forexample, Example classPlayer: #classvariables club='Chelsea' sport='Football' def__init__(self,name): #Instancevariable self.name=name defshow(self): print("Player:",'Name:',self.name,'Club:',self.club,'Sports:',self.sport) p1=Player('John') #wronguseofclassvariable p1.club='FC' p1.show() p2=Player('Emma') p2.sport='Tennis' p2.show() #actualclassvariablevalue print('Club:',Player.club,'Sport:',Player.sport) Output Player:Name:JohnClub:FCSports:Football Player:Name:EmmaClub:ChelseaSports:Tennis Club:ChelseaSport:Football Intheaboveexample,theinstancevariablenameisuniqueforeachplayer.Theclassvariableteamandsportcanbeaccessedandmodifiedbyanyobject. Becausebothobjectsmodifiedtheclassvariable,anewinstancevariableiscreatedforthatparticularobjectwiththesamenameastheclassvariable,whichshadowstheclassvariables. Inourcase,forobjectp1newinstancevariableclubgetscreated,andforobjectp2newinstancevariablesportgetscreated. Sowhenyoutrytoaccesstheclassvariableusingthep1orp2object,itwillnotreturntheactualclassvariablevalue. Toavoidthis,alwaysmodifytheclassvariablevalueusingtheclassnamesothatallobjectsgetstheupdatedvalue.Likethis Player.club='FC' Player.sport='Tennis' Didyoufindthispagehelpful?Letothersknowaboutit.SharinghelpsmecontinuetocreatefreePythonresources. TweetF sharein shareP Pin AboutVishal FounderofPYnative.comIamaPythondeveloperandIlovetowritearticlestohelpdevelopers.FollowmeonTwitter.AllthebestforyourfuturePythonendeavors! 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