What is the purpose of the word self

Question

What is the purpose of the self word in Python  I understand it refers to the specific object created from that class  but I can t see why it explicitly needs to be added to every function as a parameter  To illustrate  in Ruby I can do this   class myClass     def myFunc name           name   name     end end   Which I understand  quite easily  However in Python I need to include self   class myClass      def myFunc self  name           self name   name   Can anyone talk me through this  It is not something I ve come across in my  admittedly limited  experience

User · Answer

As well as all the other reasons already stated, it allows for easier access to overridden methods; you can call Class.some_method(inst).

An example of where it’s useful:

class C1(object):
    def __init__(self):
         print "C1 init"

class C2(C1):
    def __init__(self): #overrides C1.__init__
        print "C2 init"
        C1.__init__(self) #but we still want C1 to init the class too

>>> C2()
"C2 init"
"C1 init"

User · Answer

When objects are instantiated  the object itself is passed into the self parameter      Because of this  the object   s data is bound to the object  Below is an example of how you might like to visualize what each object   s data might look  Notice how    self    is replaced with the objects name  I m not saying this example diagram below is wholly accurate but it hopefully with serve a purpose in visualizing the use of self      The Object is passed into the self parameter so that the object can keep hold of its own data   Although this may not be wholly accurate  think of the process of instantiating an object like this  When an object is made it uses the class as a template for its own data and methods  Without passing it s own name into the self parameter  the attributes and methods in the class would remain as a general template and would not be referenced to  belong to  the object  So by passing the object s name into the self parameter it means that if 100 objects are instantiated from the one class  they can all keep track of their own data and methods   See the illustration below

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self is acting as like current object name or instance of class      Self explanation     class classname object        def   init   self name            self name name           Self is acting as a replacement of object name           self name object1 name     def display self         print  Name of the person is    self name        print  object name   object1 name     object1 classname  Bucky    object2 classname  ford     object1 display    object2 display           Output  Name of the person is   Bucky object name  Bucky Name of the person is   ford object name  Bucky

User · Answer

Is because by the way python is designed the alternatives would hardly work  Python is designed to allow methods or functions to be defined in a context where both implicit this  a-la Java C    or explicit    a-la ruby  wouldn t work  Let s have an example with the explicit approach with python conventions   def fubar x       self x   x  class C      frob   fubar   Now the fubar function wouldn t work since it would assume that self is a global variable  and in frob as well   The alternative would be to execute method s with a replaced global scope  where self is the object    The implicit approach would be  def fubar x      myX   x  class C      frob   fubar   This would mean that myX would be interpreted as a local variable in fubar  and in frob as well   The alternative here would be to execute methods with a replaced local scope which is retained between calls  but that would remove the posibility of method local variables   However the current situation works out well    def fubar self  x       self x   x   class C       frob   fubar   here when called as a method frob will receive the object on which it s called via the self parameter  and fubar can still be called with an object as parameter and work the same  it is the same as C frob I think

User · Answer

The following excerpts are from the Python documentation about self   As in Modula-3  there are no shorthands  in Python  for referencing the object   s members from its methods  the method function is declared with an explicit first argument representing the object  which is provided implicitly by the call  Often  the first argument of a method is called self  This is nothing more than a convention  the name self has absolutely no special meaning to Python  Note  however  that by not following the convention your code may be less readable to other Python programmers  and it is also conceivable that a class browser program might be written that relies upon such a convention   For more information  see the Python documentation tutorial on classes

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The use of the argument  conventionally called self isn t as hard to understand  as is why is it necessary  Or as to why explicitly mention it  That  I suppose  is a bigger question for most users who look up this question  or if it is not  they will certainly have the same question as they move forward learning python  I recommend them to read these couple of blogs   1  Use of self explained  Note that it is not a keyword      The first argument of every class method  including init  is always a reference to the current instance of the class  By convention  this argument is always named self  In the init method  self refers to the newly created object  in other class methods  it refers to the instance whose method was called  For example the below code is the same as the above code    2  Why do we have it this way and why can we not eliminate it as an argument  like Java  and have a keyword instead  Another thing I would like to add is  an optional self argument allows me to declare static methods inside a class  by not writing self   Code examples   class MyClass        def staticMethod            print  This is a static method       def objectMethod self           print  This is an object method which needs an instance of a class  and that is what self refers to    PS This works only in Python 3 x   In previous versions  you have to explicitly add  staticmethod decorator  otherwise self argument is obligatory

User · Answer

It   s there to follow the Python zen    explicit is better than implicit     It   s indeed a reference to your class object  In Java and PHP  for example  it s called this   If user type name is a field on your model you access it by self user type name

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Let   s take a simple vector class   class Vector      def   init   self  x  y           self x   x         self y   y   We want to have a method which calculates the length  What would it look like if we wanted to define it inside the class       def length self           return math sqrt self x    2   self y    2    What should it look like when we were to define it as a global method function   def length global vector       return math sqrt vector x    2   vector y    2    So the whole structure stays the same  How can me make use of this  If we assume for a moment that we hadn   t written a length method for our Vector class  we could do this   Vector length new   length global v   Vector 3  4  print v length new      5 0   This works because the first parameter of length global  can be re-used as the self parameter in length new  This would not be possible without an explicit self     Another way of understanding the need for the explicit self is to see where Python adds some syntactical sugar  When you keep in mind  that basically  a call like  v instance length     is internally transformed to  Vector length v instance    it is easy to see where the self fits in  You don t actually write instance methods in Python  what you write is class methods which must take an instance as a first parameter  And therefore  you   ll have to place the instance parameter somewhere explicitly

User · Answer

self is an object reference to the object itself  therefore  they are same  Python methods are not called in the context of the object itself  self in Python may be used to deal with custom object models or something

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from the docs       the special thing about methods is that the instance object is passed as the first argument of the function  In our example  the call x f   is exactly equivalent to MyClass f x   In general  calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting the method   s instance object before the first argument    preceding this the related snippet   class MyClass         A simple example class        i   12345      def f self           return  hello world    x   MyClass

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I will demonstrate with code that does not use classes   def state init state       state  field      init   def state add state  x       state  field      x  def state mult state  x       state  field      x  def state getField state       return state  field    myself      state init myself  state add myself   added   state mult myself  2   print  state getField myself     -- gt   initaddedinitadded    Classes are just a way to avoid passing in this  state  thing all the time  and other nice things like initializing  class composition  the rarely-needed metaclasses  and supporting custom methods to override operators    Now let s demonstrate the above code using the built-in python class machinery  to show how it s basically the same thing   class State object       def   init   self           self field    init      def add self  x           self field    x     def mult self  x           self field    x  s   State   s add  added        self is implicitly passed in s mult 2            self is implicitly passed in print  s field      migrated my answer from duplicate closed question

User · Answer

self is inevitable   There was just a question should self be implicit or explicit  Guido van Rossum resolved this question saying self has to stay   So where the self live   If we would just stick to functional programming we would not need self  Once we enter the Python OOP we find self there   Here is the typical use case class C with the method m1  class C      def m1 self  arg           print self    inside           pass  ci  C   print ci    outside   ci m1 None  print hex id ci      hex memory address     This program will output    lt   main   C object at 0x000002B9D79C6CC0 gt   outside  lt   main   C object at 0x000002B9D79C6CC0 gt   inside 0x2b9d79c6cc0   So self holds the memory address of the class instance  The purpose of self would be to hold the reference for instance methods and for us to have explicit access to that reference          Note there are three different types of class methods     static methods  read  functions    class methods   instance methods  mentioned

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it s an explicit reference to the class instance object

User · Answer

quot self quot  keyword holds the reference of class and it is upto you if you want to use it or not but if you notice  whenever you create a new method in python  python automatically write self keyword for you  If you do some R amp D  you will notice that if you create say two methods in a class and try to call one inside another  it does not recognize method unless you add self  reference of class   class testA  def   init   self       print  ads   def m1 self       print  method 1       self m2   def m2 self       print  method 2    Below code throws unresolvable reference error  class testA  def   init   self       print  ads   def m1 self       print  method 1       m2     throws unresolvable reference error as class does not know if m2 exist in class scope def m2 self       print  method 2    Now let see below example class testA  def   init   self       print  ads   def m1 self       print  method 1   def m2        print  method 2    Now when you create object of class testA  you can call method m1   using class object like this as method m1   has included self keyword obj   testA   obj m1    But if you want to call method m2    because is has no self reference so you can call m2   directly using class name like below testA m2    But keep in practice to live with self keyword as there are other benefits too of it like creating global variable inside and so on

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I would say for Python at least  the self parameter can be thought of as a placeholder  Take a look at this  class Person    def   init   self  name  age       self name   name     self age   age  p1   Person  quot John quot   36   print p1 name  print p1 age   Self in this case and a lot of others was used as a method to say store the name value  However  after that  we use the p1 to assign it to the class we re using  Then when we print it we use the same p1 keyword  Hope this helps for Python

User · Answer

In the   init   method  self refers to the newly created object  in other class methods  it refers to the instance whose method was called   self  as a name  is just a convention  call it as you want   but when using it  for example to delete the object  you have to use the same name    del   var   where var was used in the   init   var         You should take a look at cls too  to have the bigger picture  This post could be helpful

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Take a look at the following example  which clearly explains the purpose of self  class Restaurant object         bankrupt   False      def open branch self           if not self bankrupt             print  branch opened     create instance1  gt  gt  gt  x   Restaurant    gt  gt  gt  x bankrupt False   create instance2  gt  gt  gt  y   Restaurant    gt  gt  gt  y bankrupt   True     gt  gt  gt  y bankrupt True   gt  gt  gt  x bankrupt False     self is used needed to distinguish between instances   Source  self variable in python explained - Pythontips

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I m surprised nobody has brought up Lua  Lua also uses the  self  variable however it can be omitted but still used  C   does the same with  this   I don t see any reason to have to declare  self  in each function but you should still be able to use it just like you can with lua and C    For a language that prides itself on being brief it s odd that it requires you to declare the self variable

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my little 2 cents In this class Person  we defined out init method with the self and interesting thing to notice here is the memory location of both the self and instance variable p is same  lt   main   Person object at 0x106a78fd0 gt  class Person       def   init   self  name  age           self name   name          self age   age       def say hi self           print  quot the self is at  quot   self          print  f quot hey there  my name is  self name  and I am  self age  years old quot         def say bye self           print  quot the self is at  quot   self          print f quot good to see you  self name  quot    p   Person  quot john quot   78  print  quot the p is at quot  p  p say hi     p say bye     so as explained in above  both self and instance variable are same object

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First of all  self is a conventional name  you could put anything else  being coherent  in its stead   It refers to the object itself  so when you are using it  you are declaring that  name and  age are properties of the Student objects  note  not of the Student class  you are going to create   class Student       called each time you create a new Student instance     def   init   self name age    special method to initialize         self name name         self age age      def   str   self    special method called for example when you use print         return  Student  s is  s years old    self name self age       def call self  msg    silly example for custom method         return   Hey   s    msg   self name   initializing two instances of the student class bob Student  Bob  20  alice Student  Alice  19    using them print bob name print bob age print alice  this one only works if you define the   str   method print alice call  Come here     notice you don t put a value for self   you can modify attributes  like when alice ages alice age 20 print alice   Code is here

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Python is not a language built for Object Oriented Programming unlike Java or C      When calling a static method in Python  one simply writes a method with regular arguments inside it    class Animal        def staticMethod            print  This is a static method    However  an object method  which requires you to make a variable  which is an Animal  in this case  needs the self argument  class Animal        def objectMethod self           print  This is an object method which needs an instance of a class    The self method is also used to refer to a variable field within the class    class Animal         animalName made in constructor     def Animal self           self animalName             def getAnimalName self           return self animalName   In this case  self is referring to the animalName variable of the entire class  REMEMBER  If you have a variable within a method  self will not work  That variable is simply existent only while that method is running  For defining fields  the variables of the entire class   you have to define them OUTSIDE the class methods    If you don t understand a single word of what I am saying  then Google  Object Oriented Programming   Once you understand this  you won t even need to ask that question

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The reason you need to use self  is because Python does not use the   syntax to refer to instance attributes  Python decided to do methods in a way that makes the instance to which the method belongs be passed automatically  but not received automatically  the first parameter of methods is the instance the method is called on  That makes methods entirely the same as functions  and leaves the actual name to use up to you  although self is the convention  and people will generally frown at you when you use something else   self is not special to the code  it s just another object   Python could have done something else to distinguish normal names from attributes -- special syntax like Ruby has  or requiring declarations like C   and Java do  or perhaps something  yet more different -- but it didn t  Python s all for making things explicit  making it obvious what s what  and although it doesn t do it entirely everywhere  it does do it for instance attributes  That s why assigning to an instance attribute needs to know what instance to assign to  and that s why it needs self

User · Answer

Its use is similar to the use of this keyword in Java  i e  to give a reference to the current object

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Let s say you have a class ClassA which contains a method methodA defined as   def methodA self  arg1  arg2         do something   and ObjectA is an instance of this class   Now when ObjectA methodA arg1  arg2  is called  python internally converts it for you as   ClassA methodA ObjectA  arg1  arg2    The self variable refers to the object itself

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I like this example   class A       foo      a  b   A    A   a foo append 5  b foo ans   5   class A       def   init   self            self foo      a  b   A    A   a foo append 5  b foo ans

[python] What is the purpose of the word 'self'?

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