difference between variables inside and outside of init

Question

Is there any difference at all between these classes besides the name   class WithClass         def   init   self           self value    Bob      def my func self           print self value   class WithoutClass         value    Bob       def my func self           print self value    Does it make any difference if I use or don t use the   init   method  for declaring the variable value   My main worry is that I ll be using it one way  when that ll cause me further problems down the road

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further to S Lott s reply  class variables get passed to metaclass new method and can be accessed through the dictionary when a metaclass is defined  So  class variables can be accessed even before classes are created and instantiated   for example   class meta type       def   new   cls name bases dicto               two chars missing in original of next line               if dicto  class var       A                print  There  class proxyclass object         class var    A          metaclass     meta

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I would like to add something to the responses that I read in this thread and this thread  which references this one    Disclaimer  this remarks come from the experiments I ran  Variables outside   init     These are  in fact  static class variables and are  therefore  accesible to all instances of the class   Variables inside   init     The value of these instance variables are only accesible to the instance at hand  through the self reference   My contribution   One thing that programmers must consider when using static class variables is that they can be shadowed by instance variables  if you are accessing the static class variables through the self reference    Explanation   Previously  I thought that both ways of declaring the variables were exactly the same  silly me   and that was partly because I could access both kind of variables through the self reference  It was now  when I ran into trouble  that I researched the topic and cleared it up   The problem with accessing static class variables through the  self reference is that it only references the static class variable if there is no instance variable with the same name  and to make things worse  trying to redefine a static class variable through the self reference does not work because an instance variable is created which then shadows the previously-accesible static class variable   To get around this problem  you should always reference static class variables through the name of the class   Example      usr bin env python  class Foo      static var    every instance has access       def   init   self name           self instance var    I am  s    name      def printAll self           print  self instance var    s    self instance var         print  self static var    s    self static var         print  Foo static var    s    Foo static var  f1   Foo  f1    f1 printAll    f1 static var    Shadowing static var   f1 printAll    f2   Foo  f2    f2 printAll    Foo static var    modified class   f1 printAll   f2 printAll     Output   self instance var   I am f1 self static var   every instance has access Foo static var   every instance has access self instance var   I am f1 self static var   Shadowing static var Foo static var   every instance has access self instance var   I am f2 self static var   every instance has access Foo static var   every instance has access self instance var   I am f1 self static var   Shadowing static var Foo static var   modified class self instance var   I am f2 self static var   modified class Foo static var   modified class   I hope this is helpful to someone

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Without Self  Create some objects   class foo object       x    original class   c1  c2   foo    foo     I can change the c1 instance  and it will not affect the c2 instance   c1 x    changed instance  c2 x  gt  gt  gt   original class    But if I change the foo class  all instances of that class will be changed as well   foo x    changed class  c2 x  gt  gt  gt   changed class    Please note how Python scoping works here   c1 x  gt  gt  gt   changed instance    With Self  Changing the class does not affect the instances   class foo object       def   init   self           self x    original self   c1   foo   foo x    changed class  c1 x  gt  gt  gt   original self

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This is very easy to understand if you track class and instance dictionaries   class C     one   42    def   init   self val           self two val ci C 50  print ci   dict    print C   dict        The result will be like this     two   50      module        main      one   42     init      lt function C   init   at 0x00000213069BF6A8 gt      dict      lt attribute    dict    of  C  objects gt      weakref      lt attribute    weakref    of  C  objects gt      doc     None    Note I set the full results in here but what is important that the instance ci dict will be just   two   50   and class dictionary will have the  one   42 key value pair inside   This is all you should know about that specific variables

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As noted by S Lott   Variable set outside init belong to the class  They re shared by all instances  Variables created inside init  and all other method functions  and prefaced with self  belong to the object instance   However  Note that class variables can be accessed via self  lt var gt  until they are masked by an object variable with a similar name This means that reading self  lt var gt  before assigning it a value will return the value of Class  lt var gt  but afterwards it will return obj  lt var gt    Here is an example In  20   class MyClass                elem   123                          def update self i                     self elem i                def print self                     print  MyClass elem  self elem                       c1   MyClass              c2   MyClass             c1 print             c2 print                                                                                                                                                                                                                                                                  123 123  123 123  In  21   c1 update 1            c2 update 42            c1 print             c2 print                                                                                                                                                                                                                                                                  123 1  123 42 In  22   MyClass elem 22           c1 print              c2 print                                                                                                                                                                                                                                                                  22 1  22 42  Second note  Consider slots  They may offer a better way to implement object variables

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Example code  class inside      def   init   self           self l           def insert self  element           self l append element    class outside      l                    static variable - the same for all instances      def insert self  element           self l append element    def main        x   inside       x insert 8      print x l          8      y   inside       print y l                  ----------------------------     x   outside       x insert 8      print x l          8      y   outside       print y l          8              here is the difference   if   name         main         main

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class foo object       mStatic   12      def   init   self           self x    OBj    Considering that foo has no access to x at all  FACT    the conflict now is in accessing mStatic   by an instance or directly by the class    think of it in the terms of Python s memory management    12 value is on the memory and the name mStatic  which accessible from the class   points to it       c1  c2   foo    foo      this line makes two instances   which includes the name mStatic that points to the value 12  till now     foo mStatic   99    this makes mStatic name pointing to a new place in the memory which has the value 99 inside it    and because the  babies  c1   c2 are still following  daddy  foo    they has the same name  c1 mStatic  amp  c2 mStatic   pointing to the same new value    but once each baby decides to walk alone   things differs    c1 mStatic   c1 Control  c2 mStatic   c2 Control    from now and later   each one in that family  c1 c2 foo  has its mStatica pointing to different value      Please  try use id   function for all of c1 c2 foo  in different sates that we talked about   i think it will make things better     and this is how our real life goes   sons inherit some beliefs from their father and these beliefs still identical to father s ones until sons decide to change it    HOPE IT WILL HELP

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Variable set outside   init   belong to the class   They re shared by all instances    Variables created inside   init    and all other method functions  and prefaced with self  belong to the object instance

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class User object       email    none      firstname    none      lastname    none       def   init   self  email None  firstname None  lastname None           self email   email         self firstname   firstname         self lastname   lastname       classmethod     def print var cls  obj           print   obj email obj firstname obj lastname           print obj email  obj firstname  obj lastname          print  cls email cls firstname cls lastname           print cls email  cls firstname  cls lastname   u1   User email  abc xyz   firstname  first   lastname  last   User print var u1    In the above code  the User class has 3 global variables  each with value  none   u1 is the object created by instantiating this class  The method print var prints the value of class variables of class User and object variables of object u1  In the output shown below  each of the class variables User email  User firstname and User lastname has value  none   while the object variables u1 email  u1 firstname and u1 lastname have values  abc xyz    first  and  last      obj email obj firstname obj lastname   abc xyz    first    last   cls email cls firstname cls lastname   none    none    none

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classes are like blueprints to create objects  Let s make a metaphor with building a house  You have the blueprint of the house so you can build a house  You can build as many houses as your resources allow    In this metaphor  the blueprint is the class and the house is the instantiation of the class  creating an object   The houses have common attributes like having a roof  living room  etc  This is where you init method goes  It constructs the object  house  with the attributes you want   Lets suppose you have    class house    roof   True   def   init   self  color     self wallcolor   color     gt  gt  create little goldlock s house    gt  gt  goldlock   house       invoke s class house  not function   gt  gt  goldlock roof   gt  gt  True  all house s have roofs  now let s define goldlock s wall color to white    gt  gt  goldlock wallcolor    white   gt  gt goldlock wallcolor  gt  gt   white

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In Python  a class comes with member functions  methods   class variables  attributes instance variables  and probably class methods too     class Employee         Class Variable     company    mycompany com       def   init   self  first name  last name  position             Instance Variables         self  first name   first name         self  last name   last name         self  position   position        Member function     def get full name self           return f  self  first name   self  last name       By creating an instance of the object  my employee   Employee  John    Wood    Software Engineer     we essentially trigger   init   that is going to initialise the instance variables of the newly created Employee  This means that  first name   last name and  position are explicit parameters of the specific my employee instance    Likewise  member functions return information or change the state of a specific instance      Now any variable defined outside the constructor   init   are considered to be class variables  Those variables are shared amongst all the instances of the class    john   Employee  John    Wood    Software Engineer   bob   Employee  Bob    Smith    DevOps Engineer0    print john get full name    print bob get full name    print john company  print bob company    gt  gt  gt  John Wood  gt  gt  gt  Bob Smith  gt  gt  gt  mycompany com  gt  gt  gt  mycompany com   You can also use class methods in order to change the class variable for all the instances of the class  For example     classmethod def change my companys name cls  name       cls company   name   and now change my companys name     bob change my companys name  mynewcompany com     will have effect on all the instances of class Employee   print bob company  print john company    gt  gt  gt  mynewcompany com  gt  gt  gt  mynewcompany com

[python] difference between variables inside and outside of init()

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